Transcriber's notes:
In this transcription, italic text is denoted by _underscores_ and bold
text by =equal signs=. Superscripts are indicated by ^ (e.g. Fig. 509,
_I^a_).
The text contains numerous inconsistencies of hyphenation. A few have
been adjusted where there was clear evidence of a preferred style (e.g.
meso-colic-->mesocolic and meso-duodenum-->mesoduodenum) but most have
been left in their original format.
A few spelling typos have been corrected silently (e.g.
improtant-->important, mecocolon-->mesocolon) and missing letters have
been inserted inside square brackets (e.g. junct[i]on, t[r]ansverse).
Some spelling inconsistencies probably represent contemporarily
acceptable spelling alternatives (e.g. coati/coaiti, mesal/mesial,
praecava/precava, hyaena/hyena).
A small number of punctuation inconsistencies have been corrected
silently by insertion of missing punctuation or deletion of redundant
punctuation.
The abbreviation viz. appears inconsistently in both roman and italic
font.
Illustrations (not displayed in this transcription) were originally
grouped separately from the text. Their captions have been relocated
adjacent to their first mention in the text. Some illustrations
contained features labelled as X, A, B, 1, 2, etc. and when these are
cross referenced in the text they are sometimes inconsistently styled
(upper/lower case, roman/italic).
The numbering system and font styling of the TOC is not consistent
within the TOC nor with the corresponding headings in the text, and some
TOC entries correspond to in-line text rather than to true headings. No
attempt has been made to remedy these inconsistencies. One missing TOC
entry has been inserted.
The index is shown as it originally appeared - with some entries not in
correct alphabetic sequence.
Footnotes are located below the paragraph of reference.
THE ANATOMY
OF THE
HUMAN PERITONEUM
AND
ABDOMINAL CAVITY
CONSIDERED FROM THE STANDPOINT OF DEVELOPMENT
AND COMPARATIVE ANATOMY
BY
GEORGE S. HUNTINGTON, M.A., M.D.
PROFESSOR OF ANATOMY, COLLEGE OF PHYSICIANS AND SURGEONS,
COLUMBIA UNIVERSITY,
NEW YORK CITY
ILLUSTRATED WITH 300 FULL-PAGE PLATES CONTAINING
582 FIGURES, MANY IN COLORS
LEA BROTHERS & CO.
PHILADELPHIA AND NEW YORK
1903
Entered according to the Act of Congress, in the year 1903, by
LEA BROTHERS & CO.,
In the Office of the Librarian of Congress. All rights reserved
PREFACE.
In the following pages an attempt has been made to emphasize the value
of Embryology and Comparative Anatomy in elucidating the difficult and
often complicated morphological problems encountered in the study of
human adult anatomy.
Moreover, in addition to the direct advance in the method and scope of
anatomical teaching afforded by these aids, it is further hoped that the
broader interpretation, both of structure and function, obtained by
ontogenetic and phylogenetic comparison, will impart an interest to the
study of adult human morphology, such as the subject, considered solely
in the narrow field of its own limitations, could never arouse.
The book represents part of the course in visceral anatomy as developed
during the past fourteen years at Columbia University. The sections
dealing with the morphology of the vertebrate ileo-colic junction and
with the structural details of the human caecum and appendix are
considered somewhat more fully, as warranted by the extensive material
available. The illustrations are for the greater part taken from
preparations in the Morphological Museum of the University. Wherever
practicable the direct photographic reproduction of the actual
preparation is given. In the case of preparations not suitable for this
purpose, careful drawings have been made which offer in every instance a
faithful and correct interpretation of the conditions presented by the
actual object. A number of the embryonic illustrations are taken from
the standard text-books on the subject, due credit being given to their
source. I desire to express my sincere thanks to Dr. Edward Leaming, of
the Department of Photography and to Mr. M. Petersen, artist of the
Anatomical Department of the University, for their skilful and
thoroughly reliable work in the preparation of the illustrations.
=George S. Huntington.=
=Columbia University=, in the City of New York,
_December, 1902_.
CONTENTS.
PAGE.
INTRODUCTION 17
Development of Vertebrate Ovum 19
Development of Coelom and of Alimentary Canal 21
Development of Cloaca 24
Development and Divisions of the Peritoneum 32
Derivatives of Entodermal Intestinal Canal 34
Divisions of Alimentary Canal 38
PART I. ANATOMY OF THE PERITONEUM AND ABDOMINAL CAVITY 39
COMPARATIVE ANATOMY OF FOREGUT AND STOMACH 42
Morphological Types of Stomach 43
Development of the Intestine 51
I. Intestinal Rotation and Definition of Adult Segments of
the Intestinal Canal 58
Development of Aortal Arterial System 63
II. Demonstration of Intestinal Rotation in the Lower
Mammalia 67
Peritoneal and Visceral Relations in the Infra-colic
Compartment of the Abdominal Cavity in the Adult 74
Part II. ANATOMY OF THE PERITONEUM IN THE SUPRA-COLIC COMPARTMENT
OF THE ABDOMEN 99
1. STOMACH AND DORSAL MESOGASTRIUM 100
_a._ Changes in Position of Stomach 102
_b._ Changes in Direction and Extent of Dorsal Mesogastrium 103
_c._ Development of Spleen and Pancreas in the Dorsal
Mesogastrium and Changes in the Disposition of the Great
Omentum 108
1. Development of Spleen 108
2. Development of Pancreas 111
Development of Pancreas in Lower Vertebrates 115
Comparative Anatomy of Pancreas 116
Pyloric Caeca or Appendices 119
Peritoneal Relations of Pancreas 122
Comparison of Embryonal Stages during the Development of
the Human Dorsal Mesogastrium, Spleen and Pancreas with
the Permanent Adult Condition of the same Structures in
Lower Mammalia 126
1. Spleen, Pancreas and Great Omentum of Cat 127
2. Relation of Great Omentum to Transverse Colon,
Transverse Mesocolon and Third Part of Duodenum 129
2. VENTRAL MESOGASTRIUM AND LIVER 140
I. _A._ Development of Liver 141
_B._ Comparative Anatomy of Liver 144
_C._ Development of Vascular System of Liver 145
Comparative Anatomy of the Hepatic Venous Circulation 154
II. Ventral Mesogastrium 163
Peritoneal Relations of Liver 167
Relation of Hepatic Peritoneum to the "Lesser Sac" 174
Caudal Boundary of Foramen of Winslow 178
Pancreatico-gastric Folds 181
PART III. LARGE AND SMALL INTESTINE, ILEO-COLIC JUNCTION
AND CAECUM 189
I. GENERAL REVIEW OF MORPHOLOGY AND PHYSIOLOGY OF THE
VERTEBRATE INTESTINE 190
I. Midgut or Small Intestine 192
Intestinal Folds 193
Divisions of Small Intestine 194
Structure of Small Intestine 194
1. Secretory Apparatus 194
2. Absorbing Apparatus 195
Valvulae Conniventes 196
II. Endgut or Large Intestine 198
II. SERIAL REVIEW OF THE ILEO-COLIC JUNCTION AND CONNECTED
STRUCTURES IN VERTEBRATES 200
I. Fishes 200
II. Amphibia 201
III. Reptilia 201
IV. Birds 203
V. Mammalia 204
Monotremata 204
Marsupalia 204
Edentata 206
Sirenia 208
Cetacea 209
Ungulata 209
Rodentia 211
Carnivora 212
Cheiroptera 212
Insectivora 213
Primates 213
III. PHYLOGENY OF THE TYPES OF ILEO-COLIC JUNCTION AND CAECUM
IN THE VERTEBRATE SERIES 217
1. Symmetrical Form of Ileo-colic Junction; Mid- and End-gut
in Direct Linear Continuity 221
2. Asymmetrical Development of a Single Caecal Pouch, lateral
to the Ileo-colic Junction, Mid- and End-gut Preserving
their Linear Continuity 223
3. Rectangular Ileo-colic Junction, with Direct Linear
Continuity of Caecum and Colon 225
IV. STRUCTURE OF CAECAL APPARATUS AND SPECIALIZED MORPHOLOGICAL
CHARACTERS OF COLON IN RODENTS AND UNGULATES 229
1. Caecum Proper 229
2. Structural Modifications of Proximal Segment of Colon
analogous in their Functional Significance to the Caecal
Apparatus 230
V. CAECAL APPARATUS AND COLON IN HYRAX. 234
PART IV. MORPHOLOGY OF THE HUMAN CAECUM AND VERMIFORM APPENDIX 237
I. Development of the Caecum and Appendix 237
II. Changes in the Position of the Caecum and Appendix during
normal Development, depending upon the Rotation of the
Intestine and the subsequent Descent of the Caecum 239
III. Variations of Adult Caecum and Appendix 244
_A._ Shape of Caecum and Origin of Appendix. Types and
Variations of Adult Caecum and Appendix 245
_B._ Position and Peritoneal Relations of Appendix 250
_C._ Ileo-Caecal Folds and Fossae 260
INTRODUCTION.
In considering the anatomy of the human abdominal cavity and peritoneum
in the following pages the explanation of the adult conditions
encountered is based upon the development of the parts, and the
successive human embryonal stages are illustrated by the examination of
the lower vertebrates presenting permanent adult structural conditions
which appear as merely temporary embryonal stages in the development of
the higher mammalian alimentary tract.
For the sake of clearness and brevity all discussion of the _theories of
peritoneal development_ has been designedly omitted. The assumption of
peritoneal _adhesion_, and consequent obliteration of serous areas,
offers many advantages in considering the adult human abdominal cavity,
especially from the standpoint of comparative anatomy. The same has
consequently been adopted without reference to divergent views and
theories.
In studying the descriptive text and the diagrams the student should
remember that the volume offers in no sense a complete or detailed
account of the development of the abdominal cavity and its contents. The
purpose is not to present the embryology of this portion of the
vertebrate body, but to _utilize_ certain embryological facts in order
to _explain_ the complicated adult conditions encountered. To avoid
confusion, and to bring the salient points into strong relief, the
majority of the diagrams illustrating human embryonal stages are purely
schematic.
Moreover, in order to avoid confusing and unnecessary details it is
often desirable to disregard developmental chronology entirely. Many of
the diagrams combine several successive developmental stages, showing
different degrees of development in different portions of the same
drawing. Again it is frequently necessary, for the sake of brevity and
clearness, to actually depart from known embryological conditions. If,
for example, the stomach and liver are treated as if they were from
their inception abdominal organs, the student of systematic embryology
will recall the fact that this position is only _obtained after_ their
primitive differentiation by growth and migration.
Again the mesenteries are treated here as if they formed definite and
well-defined membranes from the beginning--without reference to the
abdominal organs with which they are associated. We speak of the liver
as growing into and between the layers of the ventral mesogastrium,
because this conception offers the opportunity of more clearly
explaining the adult condition. Actually, however, the membrane
develops, as a new structure, after the first differentiation of liver
and stomach, as these organs descend into the abdominal cavity.
Similar discrepancies between fact and schema are encountered
throughout. Consequently, while the purpose of the volume is to
facilitate the study and comprehension of the _adult_ peritoneal cavity
and its contents, the reader should guard against receiving the
developmental illustration as a correct successive and detailed account
of the _embryology_ of the parts concerned.
In like manner the comparative anatomical facts adduced form in no sense
even approximately a complete serial morphological account of the
vertebrate alimentary tract.
To the student of human anatomy the zoological position of the forms
which help him to understand complicated human structural conditions is
immaterial. He can draw on all the vertebrate classes independently of
their mutual relations. Hence neither ontogeny nor phylogeny are here
introduced, except as aids to the study of adult human anatomy. The
following pages offer neither an embryology nor a comparative anatomy of
the alimentary tract, but an attempt has been made in them to illustrate
the significance of the complicated anatomical details presented by the
adult human abdominal cavity by reference to the simpler antecedent
conditions encountered during the early developmental stages of the
higher forms and permanently in the structure of the lower vertebrates.
While, as just stated, a complete presentation of the development of the
abdominal cavity is not required, yet the student will find it of
advantage to rehearse the main facts of vertebrate embryology, for the
purpose of bringing a clear understanding of the manner in which the
vertebrate body is built up to bear upon the problems which the special
organs and structures of the body-cavity present for his consideration.
This purpose can be accomplished by a very brief and condensed
consideration of the cardinal facts.
The entire vertebrate body is the product of developmental changes
taking place after fertilization in a single primitive CELL, the EGG or
OVUM (Fig. 1).
[Illustration: FIG. 1.--Human ovum, from a mature follicle, a sphere of
about 0.2 mm. diameter. x 25. (Kollmann.)]
In structure the ovum corresponds to other animal cells. On account of
their special significance during development the different component
parts of the egg-cell have received special distinctive names. The
_cell-body_ is known as the _vitellus_ or _yolk_. It is composed of two
substances, the _protoplasm_ or formative yolk and the _deuteroplasm_ or
nutritive yolk, which vary in their relative proportions in the ova of
different animals.
The protoplasm represents the material from which in the course of
development the cells forming the body of the individual are derived,
while the deuteroplasm serves for the nutrition of the ovum during the
earliest stages of development.
The _nucleus_ of the egg-cell is distinguished as the _germinal
vesicle_, and its _nucleolus_ as the _germinal spot_.
The _cell-body_ or _vitellus_ is surrounded by a condensed portion of
the cell contents to which the name of the vitelline membrane has been
applied, which in turn is enclosed by a transparent and elastic cover,
the _zona pellucida_, presenting a radially striated appearance.
The ovum is contained in the cortical portion of the ovary, enclosed in
the _Graafian follicle_, a vesicle 4-8 mm. in diameter, whose fibrous
walls are lined by several layers of epithelial cells, which surround
the ovum, forming the _discus proligerus_.
After impregnation the egg-cell, by a process of repeated division or
cleavage, undergoes _segmentation_, the cell-body being divided
successively into two, four, eight, sixteen, thirty-two, etc., _cells_,
called _blastomeres_ (Figs. 2 and 3). The mass of cells finally
resulting from this process of segmentation forms the ground work of the
future body. A vertebrate ovum in this stage of complete segmentation is
called the _morula_ from its resemblance to a mulberry (Fig. 4).
[Illustration: FIG. 2.--Segmentation of mammalian ovum (bat). (After E.
von Beneden.) Two blastomeres, each with a nucleus, shown in lighter
color. The dark bodies are yolk-granules.]
[Illustration: FIG. 3.--Segmentation of mammalian ovum. Four
blastomeres. (After E. von Beneden.)]
[Illustration: FIG. 4.--Ovum of rabbit, from terminal portion of
oviduct. The zona pellucida appears thickened, and contains many
spermatozoa which failed to penetrate the ovum. (After Bischoff.)]
After segmentation is completed a cavity filled with fluid and
surrounded by the developing cells is gradually formed in the interior
of the mass. This cavity is known as the _segmentation-cavity_. The egg
is now called the _blastula_, _blastosphere_ or _blastodermic vesicle_
and the cellular membrane enclosing the segmentation-cavity forms the
_germinal membrane_ or _blastoderm_ (Figs. 5 and 6). The cells of the
blastoderm become aggregated at one point on the circumference of the
vesicle (dorsal pole of blastosphere) forming, when viewed from above, a
thickened biscuit or disk-shaped opaque area. This is known as the
_germinal area_, or _primitive blastoderm_ or _embryonic shield_ (Figs.
7 and 12).
[Illustration: FIG. 5.--Blastodermic vesicle of rabbit. (After E. von
Beneden.)]
[Illustration: FIG. 6.--Blastodermic vesicle of _Triton taeniatus_.
(Hertwig.)]
[Illustration: FIG. 7.--Embryonic area of rabbit embryo. (Heisler, after
E. von Beneden.) The primitive streak beginning in the
cell-proliferation known as the "node of Hensen."]
[Illustration: FIG. 12.--Oval embryonic area of rabbit's egg, detached
with part of wall of blastodermic vesicle. x 30. (Kollmann.)]
This is the first indication of the coming division of the entire
egg-cell into the _embryo proper_ and the _vitelline_ or _yolk-sac_
(Figs. 8 and 9). The entire future individual develops from the cells of
the germinal area. This area comprises both the embryo proper and the
region immediately surrounding it.
[Illustration: FIG. 8.--Blastodermic vesicle of mammal. (E. von
Beneden.) The layer of cells lining the interior of the vesicle next to
the zona pellucida forms Rauber's "Deckschichte" or prochorion. This is
not the true ectoderm, since it does not participate in the formation of
the embryo, which is entirely derived from the cells of the germinal
area.]
[Illustration: FIG. 9.--Human embryo with yolk-sac, amnion, and
belly-stalk of fifteen to eighteen days. (Heisler, after Coste.)]
The remainder of the ovum, serving temporary purposes of nutrition and
respiration, gradually becomes absorbed and disappears.
[Illustration: FIG. 10.--Embryonal area of sheep, composed of ectoderm
and entoderm. (After Bonnet.)]
[Illustration: FIG. 11.--Blastodermic vesicle of rabbit. Section through
embryonic area at caudal limit of node of Hensen. (Rabl.)]
Transverse sections at right angles to the long axis of the embryonic
area show that the single layer of cells composing the primitive
germinal membrane becomes differentiated first into two (Fig. 10) and
subsequently into three layers of cells (Fig. 11). At the margins of the
germinal area these layers are of course continuous with the rest of
yolk-sac wall. From their position in reference to the center of the
cell the three layers of the blastoderm are described as--
1. The outer, Epiblast or Ectoderm.
2. The middle, Mesoblast or Mesoderm.
3. The inner, Hypoblast or Entoderm.
The central nervous system (brain and spinal cord) is derived from the
ectoderm by the development of a groove in the long axis of the
embryonic area (Figs. 13, 14, 16 and 17), and by the subsequent union in
the dorsal midline of the ridges bounding the groove to form a closed
tube (Fig. 18). (Medullary groove, plates and canal.)
[Illustration: FIG. 13.--Transverse section of embryonic area of ovum of
sheep of fourteen and a half days. (Heisler, after Bonnet.)]
[Illustration: FIG. 14.--Germinal area of rabbit's ovum. (Kollmann.)]
[Illustration: FIG. 15.--Surface-view of area pellucida of an
eighteen-hour chick-embryo. (Balfour.)]
[Illustration: FIG. 16.--Transverse section of human embryo before
development of protovertebrae or chorda dorsalis. (Keibel.)]
[Illustration: FIG. 17.--Transverse section of a sixteen and a half day
sheep embryo. (Heisler, after Bonnet.)]
[Illustration: FIG. 18.--Embryo of bird, at beginning of third day, with
four blastodermic layers, resulting from the division of the mesoderm
into parietal and visceral layers, separated by the coelom cavity.
Transverse section. x 170. (Kollmann.)]
The following changes in the ventral aspect lead to the formation of the
alimentary canal and body-cavity:
The developing embryo at first lies flat on the subjacent yolk-mass, and
subsequently becomes gradually separated more and more from the rest of
the blastoderm by grooves or furrows which develop along the sides and
at the cephalic and caudal extremity of the embryo. The folds resulting
from these furrows indent the yolk more and more as development proceeds
and tend to approach each other at a central point, the future
_umbilicus_.
In the meanwhile changes in the region of the mesoderm have led to
conditions which produce a differentiation of the ventral portion of the
embryo into two tubes or cylinders, the _alimentary_ or _intestinal
canal_ and the _general body-cavity_, the former being included within
the latter.
Early in the course of development a number of spaces appear in the
mesoderm on each side of the axial line of the embryo. These spaces soon
unite to form two large cavities, one on each side. Taken together these
cavities constitute the _coelom_ or _body-cavity_, which becomes
subdivided in the adult mammal into the pleural, pericardial and
abdominal cavities.
As these coelom cavities develop in the mesoderm the cells lining them
become distinctly epithelial. This mesodermic epithelium lining the
coelom is called the _mesothelium_.
The development of the coelom space divides the mesoderm on each side
into an outer leaf, the _somatic_ or _parietal mesoderm_, and an inner
leaf, the _splanchnic_ or _visceral mesoderm_ (Figs. 18 and 19). The
former is closely applied to the ectoderm, forming with it the
_somatopleure_ or _body-wall_. The latter, in close contact with the
entoderm, forms with it the _splanchnopleure_ or wall of the alimentary
canal. In the dorsal median line both somatic and splanchnic mesoderm
become continuous with each other and with the axial mesoderm (Fig. 20).
[Illustration: FIG. 19.--Transverse section of a seventeen and a half
day sheep embryo. (Bonnet.)]
[Illustration: FIG. 20.--Curves of blastodermic layers and division of
mesoderm in amniote embryo. (Kollmann.)]
The folds of the splanchnopleure, indenting the yolk-sac, form a gutter
directly connected with the yolk, the _primitive intestinal groove_ or
_furrow_, whose margins gradually approach each other (Fig. 20). In this
way the primitive alimentary canal becomes separated from the yolk. At
first this separation is ill-defined, and the channel of communication
between the primitive intestine and the yolk is wide (Figs. 13, 16, 17
and 19). The folding of the splanchnopleure completes, at an early
period, the dorsal and lateral walls of the embryonic gut, but
ventrally, toward the yolk, the tube is incomplete and widely open.
By union and coalescence of the splanchnopleural folds, proceeding from
the caudal and cephalic ends towards the center, this primitive wide
channel gradually becomes narrowed down, until the communication between
the yolk-sac and the intestine is reduced to a canal, the
_vitello-intestinal_ or _omphalo-mesenteric duct_. The intestinal gutter
is thus converted into a closed tube except at the point of implantation
of the vitelline duct during the persistence of this structure. In the
meanwhile the somatopleural folds forming the body-walls grow more and
more together from the sides, approaching the vitello-intestinal duct.
Finally touching each other they coalesce to form the ventral body wall,
in the same manner as the splanch[n]opleural folds met and united to
form the alimentary tube.
At the same time the vitello-intestinal duct and the remnant of the
yolk-sac, to which it was attached ("umbilical vesicle"), normally
become obliterated and disappear.
After the intestinal tube and the body cavity have thus become closed
the embryo straightens out and the alimentary canal appears as a nearly
straight cylindrical tube extending from the cephalic to the caudal end
of the embryo. This primitive alimentary tube at first terminates at its
cephalic extremity in a blind pouch, while at the caudal end in the
early stages the intestine is connected with the nerve-tube by a channel
called the _neuro-enteric canal_, forming in the earliest embryos a
communication between the ectoderm lining the bottom of the medullary
groove and the entoderm (Figs. 22 and 26). In man this stage is
encountered very early, in embryos of 2 mm. before the formation of
either heart or provertebrae.
[Illustration: FIG. 22.--Caudal half of human blastoderm measuring 3
mm., with open medullary groove. Dorsal view. x 30. (After Spee.)]
[Illustration: FIG. 26.--Neuro-enteric canal in section of human embryo
of 2 mm. (After Spee.)]
At the point where the canal develops the primitive groove presents a
thickened circumvallate spot, marking the beginning perforation of the
medullary plate from the ectoderm to the entoderm. The canal exists only
for a short period during the earliest stages of embryonal life. It
becomes rapidly closed, the neural and intestinal tubes henceforth
remaining permanently separated from each other.
The embryonal caudal end of the primitive alimentary canal is not the
final adult termination of the tube. When the anal aperture is formed in
a manner to be presently detailed, the opening is situated cephalad of
the portion connected with the nerve-tube by the neuro-enteric canal.
Hence this terminal portion of the early embryonic alimentary canal is
called the "post-anal gut" (Fig. 21).
[Illustration: FIG. 21.--Sagittal section of caudal extremity of cat
embryo of 6 mm. (Tourneux.)]
The post-anal gut and the neuro-enteric canal are better developed in
the embryos of the lower than in those of the higher vertebrates. But in
all vertebrates of the present day both of these structures undergo
regressive changes and finally disappear altogether. They serve to
recall conditions which existed in bygone ages, and, while they have a
long and significant phylogenetic history, they have lost among living
vertebrates all physiological importance.
After closure of the neuro-enteric canal and obliteration of the
post-anal gut the alimentary tube ends, during a short period, both
cephalad and caudad in a blind pouch. Very soon, however, the ectoderm
becomes invaginated at both extremities and finally perforates into the
lumen of the intestine, thus establishing the oral and anal
communications with the exterior. The anal ectodermal invagination
(proctodaeum) (Fig. 21), is smaller than the oral (stomadaeum) (Fig. 27),
but the intestinal tube forms an extensive pouch in the anal region
which descends to meet the ectodermal invagination of the proctodaeum.
The details of the embryonic processes leading to the final
establishment of the adult condition are of great interest on account of
the pathological importance of abnormal or arrested development in these
parts. Failure of the caudal intestinal pouch to establish a
communication with the anal invagination, or failure of development in
either anal invagination or intestinal pouch, leads to the condition
known as atresia ani or imperforate anus, of which there are several
varieties.
[Illustration: FIG. 27.--Median section through head of embryo rabbit of
6 mm. (Mihulkovics.)]
Before the anal opening forms the primitive caudal intestine receives
from above the stalk of the _allantois_, while the Wolffian duct, the
canal of the embryonic excretory apparatus, also opens into it. The
renal bud on the Wolffian duct in Fig. 28 indicates the beginning
development of the permanent kidney (metanephros), and the proximal
portion of the allantoic stalk is destined to form by a spindle-shaped
enlargement the future urinary bladder (Fig. 28). The caudal gut has as
yet no anal opening. Ventrad of the tail end of the embryo the ectoderm
presents at this time a depression (Fig. 21). The ectoderm lining the
bottom of this anal fossa or depression is separated by a little
mesoderm tissue from the entodermal lining of the blind pouch of the
caudal gut. Ectoderm and entoderm in this region with the intervening
mesodermal layer form the _cloacal membrane_ (Fig. 21).
[Illustration: FIG. 28.--Reconstruction of caudal end of human embryo of
11.5 mm. (four and a half weeks), showing pelvic structures. x 40.
(After Keibel.)]
=Development of Cloaca.=--The entodermal pouch or prolongation sent down
from the end-gut to meet the anal invagination enlarges and dilates to
form a short wide piece of the intestinal tube into which open on the
one hand the urinary and sexual ducts of the genito-urinary system,
while it receives on the other the termination of the _end-gut proper_
(Figs. 28 and 29).
[Illustration: FIG. 29.--Reconstruction of caudal end of human embryo of
14 mm. (five weeks). x 20 (After Keibel.)]
This is the permanent condition of the terminal openings of the
alimentary and genito-urinary tracts in the lower vertebrates. It is
found in certain fishes, in all amphibia, reptiles and birds, and occurs
also in one order of mammals, the monotremes. In man and mammals
generally the anal orifice is separated from the genito-urinary opening,
lying dorsad of the same and provided with special sphincters. Only in
the monotremes do the anus and the genito-urinary tract open into a
common cloaca surrounded by a sphincter common to the anal and
genito-urinary openings (sphincter cloacae). In birds, reptiles, amphibia
and many fishes (especially the Plagiostomata) this cloacal formation is
the rule. In many fishes, especially the Teleosts, the anus and the
genito-urinary openings are separate, as in mammals, but their position
is reversed, the anus being ventral, while the genito-urinary opening is
placed dorsally.
[Illustration: FIG. 23.--Genito-urinary tract and cloaca of _Iguana
tuberculata_, female. (Columbia University Museum, No. 1846.)]
Fig. 23 shows the cloaca in a female specimen of _Iguana tuberculata_.
The ventral wall of the cloaca has been divided to the left of the
median line and turned over to the right, carrying with it the cloacal
opening of the bladder. The termination of the alimentary canal opens
into the cloaca from above.
A transverse fold of the mucosa separates this upper compartment of the
cloaca (_coprodaeum_) from a lower space (_urodaeum_) which receives in
its dorsal wall the openings of the two oviducts and immediately above
them--upon two papillae--the openings of the ureters, while the ventral
wall contains the cloacal opening of the bladder.
The right ovary has been removed--to show the abdominal opening of the
right oviduct--by dividing the mesovarian peritoneal fold.
[Illustration: FIG. 24.--Genito-urinary tract and cloaca of the hen,
_Gallus bankiva_. (Columbia University Museum, No. 1208.)]
Fig. 24--taken from a preparation of the hen--shows the typical
arrangement of the female genito-urinary tract and cloaca in the birds.
The terminal portion of the alimentary canal, in entering the cloaca,
forms an expanded upper cloacal compartment for the accumulation of the
excreta, called the _coprodaeum_.
It is separated by a prominent mucous fold from the central compartment,
or _urodaeum_ which receives the terminations of the two ureters and of
the single (left) oviduct. A second fold forms the distal limit of the
urodaeum and separates it from the lowest cloacal compartment, the
_proctodaeum_.
[Illustration: FIG. 25.--Genito-urinary tract and cloaca of _Platypus
anatinus_, duck-billed platypus. (Columbia University Museum, No.
1802.)]
Fig. 25 shows the male genito-urinary tract and the cloaca in the
monotreme, _Platypus anatinus_. The cloaca is a spacious sac formed by
the confluence of the rectum and the genito-urinary sinus.
The penis, consisting of two large cavernous bodies, is contained in a
fibrous sac which arises from the junction of the genito-urinary sinus
and the cloaca, and is continued into the ventral wall of the cloaca
near its termination by an opening through which the penis can pass into
the cloaca and beyond the external cloacal aperture.
The semen enters the penis at its root through a narrow opening situated
close to the junction of genito-urinary sinus and cloaca.
For a short period, therefore, the human embryo and the embryos of the
higher mammalia present conditions which correspond to the permanent
structure of the parts in these lower vertebrates. In human embryos of
11.5 mm. cervico-coccygeal measure (32-33 days) (Fig. 28), the cloaca
appears as a short sac continuous dorsad with the intestine, ventrad
with the rudiment of the urinary bladder. The larger portion of the
caudal gut (postanal gut) has disappeared, having been reduced to a thin
epithelial strand which gradually becomes entirely absorbed. Only the
proximal portion of the end-gut is used for the development of the
cloaca, which, however, at first has no external opening (Fig. 28).
The tail end of the embryo becomes more extended and between it and the
umbilical cord an interval appears in which the genital protuberance
develops. Behind this point the ventral cloacal wall is formed by the
cloacal membrane.
A considerable interval also develops between the points of entrance
into the cloaca of the intestine proper and of the allantoic stalk
(urinary bladder). The growth of the mesoderm pushes the intestine
against the sacral vertebrae, while the stalk of the allantois with the
rudimentary urinary bladder is forced against the ventral abdominal
wall. These changes prepare the way for the first appearance of the
_genito-urinary sinus_. The neck of the embryonic bladder elongates and
receives the ducts of the urinary and genital glands (Fig. 29). In
embryos of 14 mm. cervico-coccygeal measure (36-37 days) (Figs. 29 and
30), the genito-urinary sinus perforates the cloacal membrane on the
ventral aspect of the genital protuberance, forming the _uro-genital
cleft_. The rectum remains closed for a few days longer. The perforation
is preceded by the formation of a transverse ectodermal reduplication,
producing a depression called the _transverse anal fissure_. This
depression increases in depth until a distinct anal invagination
results, known as the _proctodaeum_, which grows as a funnel-shaped fossa
toward the blind termination of the endgut. In embryos of 25 mm.
cervico-coccygeal measure (81/2-9 weeks) the intestine still ends in a
blind pouch. The anus is, therefore, independent of the end-gut in its
development. It is derived from the ectoderm and its production is
analogous to the formation of the oral cavity by means of the ectodermal
invagination called the _stomadaeum_.
[Illustration: FIG. 30.--Human female foetus, 3.4 cm. long,
vertex-coccygeal measure. The external perineal folds separate the anal
invagination from the uro-genital opening. (Kollmann.)]
Finally the cloaca is converted into a ventral tube from which part of
the urinary bladder, the urethra and genito-urinary sinus develop, and a
dorsal tube from which the _rectum_ is derived. This double disposition
of the cloaca is accomplished by gradual changes in the entoderm and
mesoderm. The entoderm proliferates until a partition is formed which
separates the two divisions of the cloacal tube from each other, and the
mesoderm likewise increases, surrounding the newly formed entodermal
tubes with tissue from which the muscles, connective tissue and blood
vessels of the parts are derived (Figs. 28 and 29).
This partition, the _septum uro-rectale_, develops symmetrically on each
side, appearing first as paired folds on the right and left sides called
the _internal perineal folds_ (Figs. 28 and 29). When these folds have
reached the cloacal membrane they complete the separation of the cloaca
into two adjacent canals. Each of these canals is still closed caudad by
its respective portion of the cloacal membrane, now divided into an
_anal_ and _uro-genital_ segment. These two portions of the original
cloacal membrane become perforated separately, the uro-genital before
the anal. Hence the external opening of the uro-genital sinus is the
first to appear, to be followed by the anal perforation. The internal
perineal folds are supplemented by the formation of similar external
folds, ridges of mesoderm tissue which surround the anal orifice in the
form of a low wall and thus deepen the anal ectodermal invagination into
the fossa of the proctodaeum.
These developmental stages in the formation of the end-gut are of
importance because they offer the explanation of the pathological
conditions which result from an arrest of development and from the
failure of either the uro-genital or anal opening to form in the usual
manner. These malformations must date back to an early stage, and
probably have their inception in disturbances occurring in the normal
development between the 15th and 23d day (embryos of 3-6 mm.). Perhaps
in some cases of atresia there may be a secondary obliteration of a
previously formed opening. In Fig. 31 the proctodaeum persists but the
perforation of the anal membrane into the end-gut has not occurred. The
ectoderm of the anal fossa and the intestinal entoderm remain separated
by a transverse mesodermal partition. Different degrees of this
malformation are observed. The layer separating the skin from the blind
end of the rectum may be so thin that the meconium contained in the
latter can be felt through it. On the other hand the rectum may
terminate high up in a blind pouch, which is separated from the skin by
a distance of several centimeters.
[Illustration: FIG. 31.--Section of pelvis of human foetus, showing
atresia recti. (Esmarch.)]
We may now briefly consider the genetic, histological and mechanical
conditions which the above-outlined course of development imposes on the
alimentary tract.
The ectoderm forms the superficial covering of the embryo and in the
dorsal axial line develops the medullary groove which subsequently
becomes converted into the cerebro-spinal axis by closure of the
medullary plates and inclusion of the neural tube within the surrounding
mesoblast (Fig. 18). The entoderm forms the epithelial lining of the
interior of the alimentary canal and its appendages and derivatives
(Fig. 19). The mesoderm furnishes the skeletal, muscular and vascular
systems. At first single, like the two remaining layers of the
blastoderm, the mesoderm splits early on each side of the chorda
dorsalis into two layers, including between them spaces which after
coalescence form the _primitive pleuro-peritoneal_ or _body-cavity_
(Fig. 20). One of these mesodermal layers bounding this space becomes
closely connected with the ectoderm, forming the _somatopleure_ or body
wall, while the other joins the entoderm to complete the wall of the
alimentary canal, forming the _splanchnopleure_. In the course of
further development the edges of these two layers approach each other
ventrally in the median line and finally fuse.
The products of this fusion are two epithelial tubes, one included
within the other, with walls reinforced by tissue derived from the two
layers of the mesoderm. The internal or entodermal tube is of much
smaller diameter than the outer or ectodermal tube, but much longer. The
walls of the two tubes are placed in contact with each other by their
mesodermal elements dorsally in the axial line, but elsewhere are
separated from each other by the body-cavity (except in the region of
the ventral mesogastrium).
The splanchnopleure is not so wide as the somatopleure. As it closes in
the ventral median line it includes the deepest or entodermal layer. It
now forms a tube whose walls are composed superficially of mesoderm
(splanchnopleure) while the lumen is lined by epithelium derived from
the entoderm. This tube is the _primitive enteric_ or _alimentary
canal_. The somatopleuric layers bounding the body cavity take a wider
sweep and after they have united ventrally in the median line they
embrace a much more extensive space, the _primitive body cavity_ or
_coelom_. The walls of this space are largely made up of the skeletal and
muscular elements developed from the mesoderm of the somatopleure,
covered superficially by the common ectodermal investment of the body.
It will be seen that the enteric tube thus becomes included within the
wider and more capacious coelom cavity.
Both the somatic and the splanchnic leaf of the mesoderm consist at
first solely of a layer of flattened epithelial cells, the mesothelium.
But very early this tissue is increased to form a massive layer by
direct development from the mesothelium. The new mesodermal cells thus
produced constitute the _mesenchyma_, which includes the whole of the
mesoderm of the embryo except the mesothelial lining of the coelom. The
cells of the mesenchyma, connected with each other and with the
mesothelial cells by protoplasmic processes, are not as close together
as in an epithelium and do not form a continuous membrane. By migration
and multiplication a large mass of mesodermal tissue is produced which
fills the entire space between the mesothelium and the primary germ
layers. The mesenchymal tissue between the mesothelium and the ectoderm
forms the mass of the skeletal, muscular and vascular systems. The
mesenchymal tissue between the mesothelium and the entoderm forms an
important constituent of the alimentary canal and of its appendages. The
entoderm furnishes the internal epithelial lining of the tube upon which
the performance of the specific physiological function of the entire
apparatus depends. This epithelial tube is covered from without by the
splanchnic mesoderm. The mesodermal elements thus added to the enteric
entodermal tube consist of connective tissue and muscular fibers. The
latter, arranged in the form of circular and longitudinal layers,
control the contractility of the tube and regulate the propulsion of the
contents. The connective tissue of the splanchnic mesoderm appears as an
intermediate layer uniting the epithelial lining and the muscular walls.
Situated thus between the mucous and muscular coats of the intestine
this layer is known as the _submucosa_. It contains, imbedded in its
tissue, the glandular elements of the intestine derived from the
entodermal epithelium, and the blood vessels, lymphatics and nerves. The
second chief function of the splanchnic and somatic mesoderm is the
production of the serous membrane investing the body cavity and its
contents from the mesothelium lining the primitive coelom. This
mesothelial tissue, differentiated as a layer of flattened cells, lines
the interior of the body cavity and covers the superficial aspect of the
enteric tube. By subsequent partition of the common coelom the great
serous membranes of the adult, the pleurae, pericardium and peritoneum,
are developed from it.
The entodermal enteric tube is, as already stated, closely attached at
an early period along its dorsal surface to the axial rod of mesoderm
containing the chorda dorsalis immediately ventrad of the neural canal.
In the earliest stages, just after the splanchnopleure and somatopleure
have closed to complete the alimentary tube and body cavity, the remnant
of these layers extends between the ventral abdominal wall and the
ventral surface of the intestine forming a partition which divides the
body into a right and left half. (Fig. 32, _A._) For the most part this
primitive connection between the ventral abdominal wall and the
intestinal tube is lost very early. The stomach, however, is always
connected by a ventral mesogastrium, from which the lesser omentum is
derived, to the ventral body wall. The disappearance of the ventral
mesentery caudad of this point establishes the condition indicated in
Fig. 32, _B._ The entodermal tube and the surrounding splanchnic
mesoderm forming the intestinal canal is attached along its dorsal
surface to the axial mesoderm of the dorsal mid-line. The primitive
mesothelial peritoneum is reflected along this line from the internal
surface of the body wall upon the ventral and lateral surfaces of the
intestine. The coelom of one side communicates ventrad of the intestine
with the coelom of the opposite side. Hence by the disappearance of the
ventral mesentery caudad of the stomach the paired body-cavities have
become fused into a single abdominal cavity--while cephalad the original
division into right and left halves is maintained by the portion of the
ventral mesentery which attaches the stomach to the ventral abdominal
wall. The mesodermal tissue which at this time attaches the alimentary
tube along its entire extent to the dorsal wall of the coelom carries the
primitive embryonic arterial vessel, the aorta. This vessel supplies a
series of small branches to the intestine, which reach the same by
passing ventrad imbedded in the mesoderm connecting the tube to the
dorsal body wall.
[Illustration: FIG. 32.--Schematic diagrams, illustrating the vertebral
mesentery. _A._ earlier; _B._ later condition. (Minot.)]
With the further development of the alimentary canal a gradual
elongation of this connecting band of mesoderm and of the contained
vessels is observed, the tube itself gradually receding from the
vertebral axis. The early broad attachment is replaced by a narrower
stalk into which the mesoderm is drawn out. With this narrowing in the
transverse and elongation in the sagittal direction the connecting
tissue assumes the character of a thin membrane with two free serous
surfaces, including the intestinal vessels imbedded between them.
Coincident with this elongation of the enteric attachment and its
narrowing in the transverse direction the primitive intestine becomes
more completely invested by the serous lining membrane of the coelom
cavity. In this stage we can speak of the double-layered membrane
attaching the tube to the dorsal body wall and carrying the intestinal
blood-vessels as the primitive dorsal mesentery. The intestinal canal
itself is invested by serous membrane except along a narrow strip of its
dorsal border where the mesentery is attached and where the vessels
reach the intestine. We can now distinguish the serous lining membrane
of the abdominal cavity, derived from the mesothelium of the splanchnic
and somatic mesoderm as the _peritoneum_. The membrane presents the
following topographical subdivisions:
1. _Parietal Peritoneum_, lining the inner surface of the abdominal
walls.
2. _Visceral Peritoneum_, investing the external surface of the
intestine and its derivatives.
3. _Mesenteric Peritoneum_, connecting these two, carrying the
intestinal blood vessels and lymphatics and acting as a suspensory
support to the alimentary canal.
The dorsal mesentery in fishes, amphibia and reptiles contains smooth
muscular fibers derived from the mesoderm. These bands of smooth muscle
fibers are also encountered, though less well developed, in the
mesentery of birds and mammals. The so-called "suspensory muscle of the
duodenum" belongs to this category. It consists of a few strands of
unstriped muscular and fibrous tissue which passes from the praeaortal
tissue around the origin of the superior mesenteric artery and coeliac
axis to the duodeno-jejunal angle. Fasciculi from this band may
penetrate into the root of the mesentery (Gegenbaur).
Similar muscular fasciculi have been observed in the peritoneal folds of
the ileo-caecal junction (Luschka) and in the mesorectum--forming in the
latter situation the recto-coccygeal muscles of Treitz, and in the
female the recto-uterine muscles.
In its earlier stages the primitive common mesentery forms a membrane
which carries the intestinal blood vessels between its two layers,
surrounds the embryonic alimentary canal and attaches the same to the
ventral aspect of the chorda dorsalis and aorta. This is the permanent
condition in many of the lower vertebrates in which the intestinal tube
is suspended by a simple dorsal mesentery, a condition which is repeated
by the embryos of man and the higher vertebrates. From this primitive
common mesentery are derived, by further development, displacement and
adhesion, all the other mesenteries, omenta and peritoneal folds of the
adult. The character and degree of these subsequent changes is
determined by the increase in length and change in position of the
intestine and the growth of large organs, like liver, spleen and
pancreas. Many portions of the intestinal canal, at first suspended by
the mesentery and freely movable within the abdominal cavity, become
later, by secondary adhesion, firmly connected with adjacent portions of
the tube or with the abdominal parietes.
In certain of the lower vertebrates (fishes) large sections of the
intestine lie entirely free within the abdomen, their only connection
with the parietes being afforded by the blood vessels. This condition
depends upon _absorption_ of the original mesentery. A similar process,
though much more circumscribed, is observed in the omenta of many
mammals, which appear perforated at several points.
=Derivatives of the Entodermal Intestinal Tube.=--The entodermal
epithelium is physiologically the characteristic element of the
alimentary canal. Besides lining the entire internal surface of the tube
it gives rise by budding and protrusion from the intestinal canal to a
series of organs which from the mode of their development must be
regarded as diverticular or derivatives of the alimentary canal (Figs.
33, 34, and 35). These organs, proceeding in order cephalo-caudad, are
the following:
The salivary glands.
Thymus and thyroid.
The lungs.
Pancreas.
Liver.
[Illustration: FIG. 33.--Schema of alimentary canal and accessory
organs, derived from same. (After Bonnet.)]
[Illustration: FIG. 34.--Reconstruction of alimentary canal of human
embryo of 4.2 mm. x 24. (After His.)]
[Illustration: FIG. 35.--Reconstruction of alimentary canal of human
embryo of 7 mm. (twenty-eight days). x 12. (After His.)]
The epithelium of all these structures is derived from the primitive
entoderm of the intestinal tube, except the epithelium of the salivary
glands, which, being derived from the stomadaeal invagination, is
ectodermal in character. We have previously noted the general history
and appearance of the yolk-sac and its connection by means of the
vitello-intestinal duct with the intestine. In contradistinction to the
adult organs just noted the yolk-sac or umbilical vesicle is merely a
temporary embryonal appendage to the alimentary canal. It also differs
from them in the fact that it is not an extension or budding from the
completed intestinal tube, like the liver and pancreas, but indicates,
by the implantation of the duct (Fig. 21), the last point at which
closure of the intestinal canal takes place, when after obliteration of
the duct the separation of the intestine from the yolk-sac is completed.
The segment of the primitive alimentary canal cephalad of the attachment
of the vitello-intestinal duct gives rise to the pharynx, oesophagus,
stomach, proximal portion of small intestine proper and its derivatives,
the liver and pancreas.
The portion situated caudad of the duct produces the rest of the small
and all of the large intestine (Figs. 33 and 35). At times in man and
other mammals (cat) the vitello-intestinal duct does not become
absorbed, but persists and continues to develop as a part of the small
intestine, forming the blind pouch or appendage known as _Meckel's
diverticulum_ (Figs. 37 and 38). This diverticulum may vary in length
from 1.5 to 15 cm. It either projects freely into the abdominal cavity
as a pouch arising from the convex border of the small intestine
opposite to the mesenteric attachment, or else it reaches the abdominal
wall at the umbilicus and is attached to the same. In a few instances it
has not terminated in a blind pouch, but has remained open at the
umbilicus, in which case the aperture discharges intestinal contents.
Sometimes the process of obliteration which normally leads to the
absorption of the vitello-intestinal duct extends to the adjoining
segment of the small intestine, resulting in obliteration of the
intestinal lumen and consequent obstruction at this point.
[Illustration: FIG. 36.--Reconstruction of alimentary canal of human
embryo of thirty-five days (13.8 mm.). x 8. (After His.)]
[Illustration: FIG. 37.--Human adult ileum with Meckel's diverticulum.
Ileo-diverticular serous fold and persistent omphalo-mesenteric artery.
(Columbia University Museum, No. 1803.)]
[Illustration: FIG. 38.--Human adult ileum, with Meckel's diverticulum.
(Columbia University Museum, No. 745.)]
The intestinal opening of the diverticulum is situated at a varying
distance above the ileo-colic junction, ranging from 27.5 cm. to 290
cm., with an average of 107 cm.
While the obliteration and complete absorption of the duct is normal in
nearly all vertebrates, a remnant persists in some birds, in which a
short caecal pouch (_diverticulum caecum vitelli_) is found at about the
middle of the small intestine. A portion of the vitello-intestinal duct
thus persists throughout life in some wading and swimming birds. Figs.
39 and 40 show this condition in the small intestine of _Urinator lumme_
and _imber_, the red-throated loon and the great northern diver. In
other birds, however, such as birds of prey, song birds, etc., the duct
is absorbed and disappears completely.
[Illustration: FIG. 39.--Small intestine of the red-throated loon,
_Urinator lumme_, showing persistent caecal pouch, the remnant of the
vitelline duct. (Columbia University Museum, No. 997.)]
[Illustration: FIG. 40.--Small intestine of great northern diver,
_Urinator imber_, with caecal pouch, the remnant of the vitelline duct.
(Columbia University Museum, No. 77, 1578.)]
In order to complete the embryological history of the alimentary canal
it is necessary to take brief account of another structure derived from
it, namely the _allantois_. Its significance to the adult organism is
seen in connection with the genito-urinary tract, the urinary bladder
being formed by its persistent portion. In the embryo, however, it has
important nutritive and respiratory functions. In the embryos of the
higher vertebrates nutrition depends only in the earliest stages upon
the yolk-sac of the ovum, over which a vascular network extends.
Very soon the caudal portion of the primitive intestine develops a
vascular sac-like outgrowth (Figs. 21 and 41). This pouch forms the
_allantois_. It is intimately connected with embryonal respiration, and
probably also forms a reservoir which receives the secretion of the
primitive kidney. This foreshadows the final destiny of the proximal
intra-abdominal portion of the allantoic sac which persists and is
converted into the urinary bladder of the adult.
[Illustration: FIG. 41.--Diagram illustrating the later stages in the
formation of the mammalian foetal membranes. (Heisler, modified from
Roule.)]
The allantois is present in Amphibia but is very small. In Amniota[1] it
is large and grows around the embryo. In those of the higher vertebrates
which are developed within an egg (reptiles, birds and monotremes) the
sac of the allantois comes to lie beneath the egg-shell and acts as a
respiratory organ. In the higher mammalia, developed within the uterus,
the allantois becomes attached by vascular villi to the uterine wall and
establishes a vascular connection between the foetal and maternal blood
vessels. In this way the _allantoic placenta_ is formed (Fig. 41). The
placenta, as just stated, is absent in the monotremes and is only
slightly developed in marsupials, in which animals the foetus develops to
maturity in the marsupial pouch after leaving the uterus. These animals
are therefore distinguished as _Aplacentalia_ from the remaining higher
mammals in which the allantoic placenta develops and which are hence
called the _Placentalia_.
[1] In the embryos of reptiles, birds and mammals folds of the
somatopleure arise externally to the constricting furrows by means of
which the embryo is gradually separated from the yolk-sac, with the
resulting formation of the intestinal and abdominal walls. These folds,
situated at the head, tail and on the sides, grow upwards and finally
meet and unite to form a membranous sac called the _amnion_. Hence these
higher vertebrates (reptiles, birds and mammals) are called _Amniota_,
in contradistinction to fishes and amphibia who have no amnion and are
hence known as _Anamnia_.
=Summary.=--To recapitulate, therefore, the intestinal tube gives origin
to two kinds of appendages or derivatives:
1. Organs of the adult body, derived by budding from the alimentary
entodermal epithelium, in the form of pouch-like diverticula which
follow the glandular type of development and become secondarily
associated with mesodermal elements. These organs are again of two
kinds:
(_a_) _Organs which retain their original connection with the lumen of
the digestive canal:_
The salivary glands,}
The liver, } Connected by their ducts with the digestive
The pancreas, } canal.
The lungs, }
which open by means of the trachea and the laryngeal aperture into the
pharyngeal cavum.
(_b_) _Organs which lose their primitive connection with the alimentary
canal._
Thymus and Thyroid Gland.
2. Embryonic appendages of the alimentary tract.
(_a_) The vitello-intestinal or omphalo-mesenteric duct and the yolk-sac
or umbilical vesicle. This structure does not form as an extension from
the intestinal tube after the same has been closed by coalescence of the
splanchnopleure in the ventral mid-line, but is the result of the
folding in of the layers of the embryonic germinal area, by means of
which the body-rudiment is constricted off from the yolk-sac. The
reduced channel of communication forms the vitello-intestinal duct. In
the vast majority of vertebrates this disappears completely by
absorption in the course of further development. It may persist in part
abnormally as Meckel's diverticulum. In a few birds its proximal portion
remains normally as a small blind pouch attached to the free border of
the small intestine.
(_b_) The allantois. This is a hollow outgrowth from the embryonic
intestinal canal of the higher vertebrates, performing important
functions in connection with the early nutrition of the embryo. In the
course of subsequent development its proximal portion, situated within
the abdominal cavity, becomes converted into the urinary bladder. In
mammals it loses its original connection with the intestinal canal and
is assigned entirely to the genito-urinary tract. In some of the lower
vertebrates, amphibia and reptiles it retains its connection with the
ventral wall of the cloaca throughout life. (See Fig. 42, genito-urinary
tract of _Iguana tuberculata_.)
[Illustration: FIG. 42.--Genito-urinary tract and cloaca of _Iguana
tuberculata_, female. (Columbia University Museum, No. 1846.)]
After the intestinal canal has become separated from the yolk-sac it
forms at first a straight tube, running cephalo-caudad beneath the
chorda dorsalis. In most forms, however, the intestine grows much more
rapidly in length than the body-cavity of the embryo in which it is
contained. Hence the intestine is forced to form coils or convolutions.
The entire alimentary canal, from the mouth to the anus, can be
separated into the following divisions and subdivisions:
=I. Foregut=, including
1. The oral cavity.
2. The pharynx.
3. The oesophagus.
4. The stomach.
=II. Midgut=, closely associated at its beginning with the liver and
pancreas.
It extends between the pyloric extremity of the stomach and the
beginning of the last segment, the endgut, frequently separated from
both by ring-like aggregations of the circular muscular fibers and
corresponding projections of the mucous membrane (pyloric and ileo-colic
valves).
The midgut is usually the longest portion of the intestinal tube.
=III. Endgut=, the last segment of the intestinal canal, courses through
the pelvic portion of the body cavity. From this short end-piece are
developed: (1) The colon, sigmoid flexure and rectum; (2) the cloaca
with the uro-genital sinus and the duct of the allantois.
PART I.
ANATOMY OF THE PERITONEUM AND ABDOMINAL CAVITY.
For the purpose of studying the adult human peritoneum it is in the
first place absolutely necessary to obtain a correct appreciation of the
disposition of the chief viscera within the abdominal cavity and of
their mutual relations. In the second place the visceral vascular supply
of the abdomen must be carefully considered in order to correctly
appreciate certain important relations of the peritoneal membrane.
A review of the visceral contents of the abdomen shows that we have to
deal chiefly with the divisions of the alimentary tract below the
oesophagus and the structures directly derived from the same, as liver
and pancreas, or associated topographically with the alimentary canal,
as the spleen. Portions of the urinary and reproductive systems situated
within the abdominal and pelvic cavities will also require
consideration.
The digestive apparatus as a whole presents, in the first place, a
segment designed to convey the food to the stomach, the
oesophagus--supplemented in mammalia by the special apparatus of the
mouth and pharynx, in which the food is mechanically prepared for
digestion by chewing and mixed with the secretion of the salivary
glands.
The _digestive apparatus proper_, succeeding to the oesophagus, is
usually divisible into two sections differing in function and structure.
1. The STOMACH, a short sac-like dilatation, in which chiefly
nitrogenous material is digested.
2. The SMALL INTESTINE, a long and usually much convoluted narrow tube,
chiefly devoted to the digestion of starches, fats and sugars, and to
the absorption of the digested matters.
In some of the lower vertebrates, as the _Cyclostomata_ (Fig. 43),
_Esox_, _Belone_, etc., among fishes (Fig. 48), _Necturus_ and _Proteus_
among amphibians (Figs. 50 and 51), the separation of the digestive
portion of the alimentary tract into stomach and small intestine is not
clearly defined (vide infra, p. 43).
[Illustration: FIG. 43.--Entire alimentary canal of the lamprey,
_Petromyzon marinus_, below the pericardium. (Columbia University
Museum, No. 1575.)]
[Illustration: FIG. 48.--Alimentary canal of _Belone_, pickerel.
(Nuhn.)]
[Illustration: FIG. 50.--_Necturus maculatus_, mud-puppy. Alimentary
canal and appendages. (Columbia University Museum, No. 1454.)]
[Illustration: FIG. 51.--Alimentary canal of _Proteus anguineus_.
(Nuhn.)]
A distinct digestive segment may even be entirely wanting, owing to its
failure to differentiate from the oesophagus on the one hand and from the
endgut on the other. In such forms the entire digestive canal appears as
a tube of uniform caliber extending from mouth to anus. It is necessary
to begin with these simple structural conditions in order to obtain a
clear conception of the disposition of the viscera in the adult human
abdomen. Such simple arrangement of the alimentary tract is found in the
embryo of man and of the higher vertebrates, and similar rudimentary
types are encountered, as the permanent condition, in some of the lower
forms. These latter are especially valuable for purposes of study,
because they afford an opportunity of examining directly, as macroscopic
objects, structural conditions which are found only as temporary
embryonal stages during the development of the higher mammalia (Fig.
43).
In the early stages the alimentary tract of the mammalian embryo
consists of a straight tube of nearly uniform caliber (Fig. 44, _A_),
extending from the pharynx to the cloaca, along the median line in the
dorsal region of the body cavity, connected with the ventral aspect of
the axial mesoderm by a membranous fold forming the primitive common
dorsal mesentery. Subsequently differentiation of this simple tube into
successive segments takes place, marked by differences in shape and
caliber and in histological structure.
[Illustration: FIG. 44.--Schematic diagram representing three stages in
the differentiation of the mammalian digestive tract: A. Early
undifferentiated stage, in which the entire canal appears as a tube of
uniform calibre. B. Spindle-shaped gastric dilatation. C. Typical
mammalian gastric dilatation.]
[Illustration: FIG. 45.--Reconstruction of human embryo. 1, 2, 3, 4,
Gill-pouches. (After Fol.)]
The first indication of the future stomach appears early, in human
embryos of from 5-6 days (Figs. 44, _B_, and 45; for later embryonal
stomach forms compare also Figs. 33, 35 and 36), as a small
spindle-shaped dilatation of a portion of the primitive entodermal tube,
placed in the median plane, dorsad of the embryonic outgrowth of the
liver, between it and the oesophagus. The appearance of this
dilatation marks the separation of the proximal cephalic part (pharynx
and oesophagus) from the distal caudal (intestinal) portion of the
primitive alimentary canal.
Further growth of the stomach takes place chiefly along the dorsal
margin of the dilatation, rendering the same more convex. The ventral
border develops to a less degree and in the course of further and more
complete differentiation the dorsal margin of the future stomach assumes
even at this period the character of the greater curvature, while the
opposite ventral margin, the future lesser curvature, following the
dilatation of the tube dorsad, becomes in turn concave (Fig. 44, _C_).
The early spindle-shaped dilatation has therefore assumed the general
shape of the adult organ. This differentiation of greater and lesser
curvature begins to appear in embryos of 5 mm. (Fig. 46) and is very
well marked in embryos of 12.5 mm., Fig. 36, of an embryo of five weeks,
indicates the adult form of the stomach clearly.
[Illustration: FIG. 46.--Alimentary canal of human embryo of 5 mm. x 15.
(Reconstruction after His.)]
It will, however, be noted that the oesophageal entrance is still at the
cephalic extremity of the rudimentary stomach, while the pyloric
transition to the intestine occupies the distal caudal point, under
cover of the liver, and turns with a slight bend dorsad and to the right
to pass into the duodenum. The future greater curvature is directed
dorsad and a little to the left toward the vertebral column, while the
concave lesser curvature is turned ventrad and a little to the right
toward the ventral abdominal wall. At this time there is but little
indication of the subsequent extension of the organ to the left of the
oesophageal entrance to form the great cul-de-sac or fundus of the adult
stomach.
In this stage of its development the stomach therefore presents ventral
and dorsal borders, and right and left surfaces, while the continuity of
its lumen with the adjacent segments of the alimentary canal appears as
a proximal or cephalic oesophageal and a distal or caudal intestinal
opening.
COMPARATIVE ANATOMY OF FOREGUT AND STOMACH.
A serial review of this portion of the alimentary tract in vertebrates
forms one of the most interesting and instructive chapters in
comparative anatomy.
Not only is every embryonal stage in the development of the higher
mammalia represented permanently in the adult structure of some of the
lower types, but the far-reaching influence of function and of the
physiological demands on the structure of this portion of the digestive
tract is strikingly illustrated by the numerous and marked modifications
which are encountered.
The foregut, strictly speaking, is in mammals separated from the oral
cavity by the musculo-membranous fold of the soft palate and uvula. In
all other vertebrates except the crocodile, the oral cavity and foregut
pass into each other without sharp demarcation (Fig. 47). In some of the
lower vertebrates the alimentary canal never advances beyond the
condition of a simple straight tube of nearly uniform caliber. There is
no gastric dilatation and hence no differentiation of a stomach properly
speaking. Such for example is the case in some teleost fishes, as the
pickerel (Fig. 48). In these forms we have to deal with the persistence
of the early embryonic pregastric stage of the higher types, before the
simple alimentary tube is differentiated by the appearance of the
distinct gastric dilatation.
[Illustration: FIG. 47.--_Gallus canis_, dog-shark, male. Genito-urinary
tract and cloaca _in situ_. The foregut has been divided just caudad of
the communication with the oral cavity. (Columbia University Museum, No.
1694.)]
In the _Cyclostomata_ (Fig. 43) the intestinal canal passes through the
body in a perfectly straight line and the three segments (mid-, fore-
and hindgut) are not clearly differentiated.
In the _Ammocoetes_ the foregut begins behind the wide branchial basket,
dorsad of the heart, with a narrow entrance, which is succeeded by a
dilated segment. The entrance of the hepatic duct separates fore- and
midgut.
In _Amphioxus_ the branchial pouch passes with a slight constriction
directly into the gut which extends through the body-cavity in a
straight line.
The narrow segment is usually regarded as the "oesophagus." This is
followed by a slightly dilated segment, the "stomach," into which a
blind pouch enters. This caecal pouch is usually considered as a
_hepatic_ diverticulum (Fig. 49).
[Illustration: FIG. 49.--_Amphioxus_, dissected from the ventral side.
The relatively enormous pharynx occupies more than half the length of
the body. The walls are separated by the gill-clefts, and the parallel
gill-bars abut at the midventral line on the _endostyle_. (Willey, after
Rathke.)]
But even in these rudimentary forms the point where the liver develops
from the entodermal intestinal tube marks the separation of fore- and
midgut. The stomach, when it develops, is situated cephalad of the
entrance of the hepatic duct into the intestine. The section cephalad of
the duct opening may be very short, and the food digested further on in
the intestinal tube. Consequently a function which in these lower
vertebrates is assigned to the midgut becomes transferred in the higher
forms to a specialized segment of the foregut, situated cephalad of the
hepato-enteric duct. This segment is the
STOMACH.
The distribution of the vagus nerve finds its explanation in this
derivation of the stomach. The primitive foregut is formed by the
passage between the branchial cavity and the midgut, and is within the
area supplied by the vagus. Hence when the stomach develops from the
foregut, as a specialized segment of the same, it is supplied by vagus
branches. The vertebrate stomach varies greatly in size and shape.
The type-form is presented by a longitudinal spindle-shaped dilatation
of the foregut, which retains its foetal vertical position in the long
axis of the body. An example of this form, which is encountered among
fishes and amphibia, is presented by the alimentary tube of _Proteus
anguineus_ and _Necturus maculatus_ (Figs. 50 and 51). Since this
condition is common to all vertebrates in the earliest foetal period it
can be designated as the foetal or primitive stomach form. All others
appear as secondary derivatives from this typical early condition.
The influences which bring about such derivations and modifications may
be enumerated as follows:
1. The habitual amount of food required by the animal.
2. The volume and digestible character of the food.
3. The size and shape of the abdominal cavity in which the stomach is
contained.
4. Structural modifications designed to increase the action of the
gastric juice on the food contained in the stomach.
5. The assumption, on part of the stomach, of functions which are
usually relegated to other organs.
Most of the individual stomach forms encountered among vertebrates owe
their production to several of these influences acting in conjunction.
We may group the main types as follows:
=1. Stomach Forms Depending on the Influence exerted by the Habitual
Amount of Food required by the Animal.=--The greater the activity of
tissue changes is, the greater will be the amount of food required and
the more pronounced will be the gastric dilatation of the alimentary
canal. Hence in the higher vertebrates generally the stomach appears as
a large and more sac-like dilatation than in lower forms, such as fishes
and amphibia and some reptilia, in which the stomach is usually smaller
and foetal in shape, forming a slight longitudinal dilatation situated in
the long axis of the body. An example is seen in the stomach of _Coluber
natrix_ (Fig. 52). Frequently this slight dilatation is scarcely
differentiated from the oesophagus at the cephalic and from the small
intestine at the caudal end. Many batrachians and perennibranchiates
possess this form among the amphibia. It is also encountered in the
pickerels, the _Cyprini_, and in _Labrus_ among fishes, and in some
saurians and ophidia among reptiles. It constitutes a slight advance in
development over the earliest stage represented, as we have seen, by the
nearly uniform and undifferentiated alimentary tube of amphioxus and the
cyclostomata.
[Illustration: FIG. 52.--Alimentary canal of _Coluber natrix_. (Nuhn.)]
This transition of the foetal form to the more advanced secondary types
of the stomach is marked by the development of two important structural
features:
(_a_) The separation in the interior of the canal of the stomach from
the intestine by the appearance of a ring-shaped valve, the _pyloric
valve_. This is produced by an aggregation of the circular muscular
fibers of the intestine at this point, and causes a projection of the
mucous membrane into the lumen of the canal. It begins to appear in the
fishes (pickerel, sturgeon, etc.), is found in most amphibia and is
regularly present in the stomach of the higher vertebrates. (Figs. 54
and 55.) A good example of the ring-shaped plate of the pylorus with
central circular opening produced by the aggregation of the circular
muscular fibers is afforded by the view of the interior of the
cormorant's stomach given in Fig. 69. The opposite or oesophageal
extremity of the stomach is less well differentiated from the afferent
tube of the oesophagus.
[Illustration: FIG. 54.--Human adult. Pyloro-duodenal junction and
pyloric valve in section. (Columbia University Museum, No. 1842.)]
[Illustration: FIG. 55.--Series of sections showing human pyloric valve
and gastro-duodenal junction:
1. Stomach of foetus at term in section.
2. Adult pyloric valve, gastric surface.
3. Adult pyloric valve and gastro-duodenal junction in section.
4. Foetal gastro-duodenal junction in section. Entrance of biliary and
pancreatic ducts on summit of papilla of duodenum. (Columbia University
Museum, No. 1851.)]
There is no aggregation of muscular circular fibers in this situation
and no valve. Superficially the external longitudinal muscular fibers of
the oesophagus pass continuously and without demarcation into the
superficial gastric muscular layer. The separation between oesophagus and
stomach is, however, marked on the mucous surface by a well-defined line
along which the flat, smooth and glistening oesophageal tesselated
epithelium passes into the granular cuboidal epithelium of the gastric
mucous membrane. The oesophageo-gastric junction in the adult human
subject is shown in Fig. 53.
[Illustration: FIG. 53.--Human adult. Mucous surface of
oesophageo-gastric junction. (Columbia University Museum, No. 1842.)]
(_b_) The pyloric end of the stomach makes an angular bend, while the
rest of the organ remains in the original vertical position in the long
axis of the body. An example of this condition is presented by the
stomach of _Scincus ocellatus_ (Fig. 56; cf. also Fig. 202).
[Illustration: FIG. 56.--Alimentary canal of _Scincus ocellatus_.
Pyloric extremity of the slightly marked gastric dilatation presents an
angular bend. (Nuhn.)]
The purpose of both of these provisions is to retain the gastric
contents for a longer time within the stomach. Hence this form is
encountered especially in those fishes and amphibians in which the
nutritive demands require a more complete digestion of the food taken.
This is the case, for example, in _Gobius_ (Fig. 57), the plagiostomata
(Fig. 58), and many saurians. The same transitory stomach form is even
found in some mammals, as the seals. Fig. 59 shows the stomach in _Phoca
vitulina_, the harbor seal. With the further increase in the demand for
complete digestion of the food the entire stomach assumes a transverse
position to the long axis of the body. This may occur while the stomach
still retains its primitive tubular form, as in most chelonians (Fig.
60). In others the change in position occurs after the gastric
dilatation has assumed the sac-like form, as in many land-turtles,
crocodiles, some batrachians and all higher vertebrates (Figs. 61 and
62). This transverse position, at right angles to the long axis of the
body, forms the starting point for the derivation of all secondary types
of stomach.
[Illustration: FIG. 57.--Alimentary canal of _Gobius niger_. (Nuhn.)]
[Illustration: FIG. 58.--Alimentary canal of shark. (Nuhn.)]
[Illustration: FIG. 59.--Stomach of _Phoca vitulina_, harbor seal.
(Columbia University Museum, No. 600.)]
[Illustration: FIG. 60.--Stomach of _Pseudemys elegans_, pond turtle.
(Columbia University Museum, No. 1710.)]
[Illustration: FIG. 61.--Stomach of _Chelydra serpentina_, snapping
turtle. (Columbia University Museum, No. 1852.)]
[Illustration: FIG. 62.--Same in section.]
=2. Stomach Forms Depending on the Influence Exerted by the Volume and
Digestible Character of the Foods.=--Vegetable substances usually have a
large volume in proportion to the amount of nutritive material which
they contain. Meat, on the other hand, contains considerable nutriment
in a comparatively small bulk. Hence carnivora (Fig. 63) usually have a
smaller stomach than herbivora (Fig. 64).
[Illustration: FIG. 63.--Stomach of _Lutra vulgaris_, otter. (Nuhn.)]
[Illustration: FIG. 64.--Stomach of _Equus caballus_, horse. (Nuhn.)]
=3. Stomach Forms Influenced by Size and Shape of the Abdominal Cavity
in which they are Contained.=--In animals whose bodies are long and
slender, as in snakes (Fig. 52), most saurians (Fig. 56), many tailed
batrachians and perennibranchiates (Figs. 50 and 51), many teleosts
(Fig. 48), the stomach is likewise usually long and slender in shape,
unless special modifying conditions exist. When on the other hand the
body is broad and short, as in Lophius (Fig. 65), Pipa (Fig. 66), and
most higher vertebrates, the stomach is also broader and more sac-like.
[Illustration: FIG. 65.--Stomach of _Lophius piscatorius_, angler.
(Nuhn.)]
[Illustration: FIG. 66.--Stomach of _Pipa verucosa_. (Nuhn.)]
=4. Stomach Forms Depending on Structural Modifications Designed to
Increase the Action of the Gastric Juice on the Food.=--This purpose is
accomplished:
(_a_) By increasing the source of supply of the gastric juice.
(_b_) By increasing the length of time during which the food remains in
the stomach.
(_a_) The source of supply of the gastric juice is increased by adding
to the usual gastric glands of the stomach a special accessory glandular
compartment, either placed at the cardia, where the oesophagus enters, as
in _Myoxus_ or _Castor_ (Fig. 67) or attached to the body of the stomach
to the left of the cardia, as in the manatee (Fig. 68). The first
arrangement is similar to the universal position of the glandular
stomach of birds (Fig. 69). In birds, however, the glandular
proventriculus is the _only_ source of the gastric juice, while in the
above-mentioned mammalia (myoxus and beaver) the accessory glandular
stomach is merely an addition to the supply derived from the usual
gastric glands situated in the body of the organ.
[Illustration: FIG. 67.--Stomach of _Castor fiber_, beaver. (Nuhn.)]
[Illustration: FIG. 68.--Stomach of _Manatus americanus_, manatee.
(Nuhn.)]
[Illustration: FIG. 69.--Stomach of _Phalacrocorax dilophus_,
double-crested cormorant; section. (Columbia University Museum, No.
67/1804.)]
(_b_) The increase of the length of time during which the food remains
in the stomach subject to the action of the gastric juice can be
accomplished in one of several ways.
1. The stomach, while it retains its general tubular form increases
considerably in length and assumes the shape and structure found in the
human large intestine. It is partially subdivided by folds projecting
into the interior and separating compartments resembling the colic cells
of the human large intestine. The time required for the passage of food
through the stomach is thus increased and the action of the gastric
juice is prolonged and rendered more intense.
Such modifications of the structure of the stomach are encountered in
_Semnopithecus_ among the monkeys and in the kangaroo, among marsupials
(Figs. 70 and 71).
[Illustration: FIG. 70.--Stomach of _Halmaturus derbyanus_, rock
kangaroo. (Columbia University Museum, No. 582.)]
[Illustration: FIG. 71.--Stomach of _Semnopithecus entellus_, entellus
monkey. (Columbia University Museum, No. 62/1805.)]
2. The same purpose is accomplished by the development of diverticula
from the stomach, in which the food is retained and acted on by the
gastric juice for longer periods.
The herbivora, omnivora and such carnivora as live on animal food
difficult of digestion furnish examples of this type of stomach. The
same is also found in most teleosts. In the latter the caecal gastric
pouch lies in the long axis of the body, opposite the entrance of the
oesophagus. A marked example of this arrangement is seen in the stomach
of the eel, _Anguilla anguilla_ (Fig. 72).
[Illustration: FIG. 72.--Alimentary canal of _Anguilla anguilla_, eel.
(Columbia University Museum, No. 1271.)]
In other forms, and in the mammalia especially, the blind pouch is
developed from the portion of the stomach lying to the left of the
oesophageal entrance at the cardia, and is hence placed transversely to
the long axis of the body.
This difference in the position of the cul-de-sac is explained by the
small transverse measure of the body in teleosts, while the greater
amount of available space in the abdominal cavity of mammalia permits
of the transverse position of the entire stomach and of the development
of the diverticulum from its left extremity.
Most mammals have only a single pouch, whose size varies with the
digestibility of the food habitually taken. It is greater in herbivora
(Figs. 64 and 73) than in omnivora and carnivora (Figs. 74 and 75). In
some of the latter, as _Lutra_ (Fig. 63), the cul-de-sac is almost
wanting.
[Illustration: FIG. 73.--Stomach of _Lepus cuniculus_, rabbit. (Nuhn.)]
[Illustration: FIG. 74.--Stomach of _Nasua rufa_, coati. (Nuhn.)]
[Illustration: FIG. 75.--Stomach of _Felis leo_, lion. (Nuhn.)]
[Illustration: FIG. 76.--Stomach of _Erethizon dorsatus_, American
porcupine. (Columbia University Museum, No. 358.)]
[Illustration: FIG. 77.--Stomach of _Cercopithecus cephus_, moustache
monkey. (Columbia University Museum, No. 158.)]
In some forms, as the pig, the left extremity of the stomach carries a
caecal appendix with a spiral valve in the interior separating its lumen
from the general gastric cavity (Fig. 78). Others have two such caecal
appendices added to the left end of the stomach (Peccary, Fig. 79).
These caecal pouches may arise from the _body_ of the stomach, instead of
from the left extremity. An example of this condition is furnished by
the American manatee (Fig. 68).
[Illustration: FIG. 78.--Stomach of _Sus scrofa_, pig. The fundus of the
stomach carries a caecal appendage separated in the interior by a spiral
fold of the mucous membrane from the gastric cavity.]
[Illustration: FIG. 79.--Stomach of _Dicotyles torquatus_, peccary. The
fundus is a capacious pouch prolonged ventrally and dorsally into two
caecal appendages resembling the single appendage of the pig's stomach.
(Columbia University Museum, No. 1806.)]
=5. Variations in the Form of the Stomach Depending upon the Assumption
by the Stomach of Special Functions, which are Usually Relegated to
other Organs.=--These functions are the following:
(_a_) Storage of food in special receptacles or compartments for
subsequent use.
(_b_) Mastication of the food is in some animals accomplished only
partly or not at all in the mouth, and is then performed in the stomach.
A portion of the stomach is thus converted into an apparatus for
mastication.
(_c_) The provisions for these two accessory functions may be combined
in the same stomach.
(_a_) Many of the higher vertebrates possess in connection with the
alimentary tract additional reservoirs for the storage of food until
used. Such reservoirs are found in mammals and birds connected with the
oral cavity, as cheek-pouches, or with the oesophagus, such as the crop
of the birds (Fig. 88). Fig. 80 shows the development of the
cheek-pouches in one of the primates, _Macacus nemestrinus_.
[Illustration: FIG. 80.--_Macacus nemestrinus_, pig-tail macaque monkey;
cheek-pouches. (From a fresh dissection.)]
In many mammals reservoirs of similar import are added
directly to the stomach and form an integral part of
the organ. Examples are furnished by the compound stomachs of many
rodents, ruminants, cetaceans and herbivorous edentates. The peculiar
appearance of these stomachs is explained if the additional reservoirs
are in imagination removed and the digestive stomach proper restored so
to speak to the type-form. The proximal or cardiac portion of the
stomach in many rodents is devoid of gastric glands and must be
interpreted as a storage chamber for food (Fig. 81). The same
significance attaches to the corresponding portion of the manatee's
stomach (Fig. 68).
[Illustration: FIG. 81.--Stomach of _Cricetus vulgaris_, hamster.
(Nuhn.)]
Similar contrivances are found in the ruminant stomach. The first
and second divisions (rumen and reticulum) are nothing but sac-like
gastric reservoirs or pouches, in which the food is collected, to be
subsequently returned to the mouth for mastication. When swallowed for
the second time the bolus is carried, by the closure of the so-called
oesophageal gutter, past the first and second stomach into the digestive
apparatus proper (the abomasum) (Figs. 82 and 83). Many ruminants
(_e. g._, _Moschus_) only have these three compartments. Most, however,
have four, the leaf stomach or psalterium being intercalated between
the retinaculum and the abomasum. The psalterium contains no digestive
glands. It may possibly serve for the absorption of the liquid portions
of the foods.
[Illustration: FIG. 82.--Stomach of _Ovis aries_, sheep. (Columbia
University Museum, No. 1807.)]
[Illustration: FIG. 83.--Scheme of ruminant compound stomach. (Nuhn.)]
The rumen or first stomach of the camels and llamas is provided with
so-called "water-cells," for the storage of water. These cells are
diverticula lined by a continuation of the gastric mucous membrane. The
entrance into these compartments can be closed by a sphincter muscle
after they are filled with water (Fig. 84).
[Illustration: FIG. 84.--Mucous membrane of stomach of _Camelus
dromedarius_, dromedary, showing water-cells. (Columbia University
Museum, No. 1123.)]
[Illustration: FIG. 85.--Stomach of _Phocaena_, porpoise. (Nuhn.)]
The three stomachs of the cetaceans are similar to those of the
ruminants (Fig. 85). The first is a crop-like reservoir for the
reception of the food when swallowed. The mucous membrane is entirely
devoid of digestive glands. In the dolphins the mucous membrane is
provided with a hard horny covering, which serves to break up the food
mechanically by trituration. The second stomach and the gut-like pyloric
prolongation constituting the third stomach contain gastric glands and
are hence digestive in function.
(_b_) Stomach forms, in which a portion of the organ is converted into
an apparatus for mastication, are seen especially in birds, in which
animals, on account of the absence of teeth, mastication cannot be
performed in the mouth.
The stomach of the bird is usually composed of two segments, one placed
vertically above the other.
The first appears like an elongated dilatation of the oesophagus, forming
the _Proventriculus_ or glandular stomach.
The second is larger, round in shape, with very strong and thick
muscular walls (Figs. 86 and 87).
[Illustration: FIG. 86.--Stomach of _Urinator imber_, red-throated loon.
(Columbia University Museum, No. 1808.)]
[Illustration: FIG. 87.--Scheme of stomach of granivorous bird. (Nuhn.)]
The proventriculus furnishes the gastric juice exclusively.
The second or muscular stomach, devoid of gastric glands, functions
merely as a masticating apparatus for the mechanical division of the
food. The thick muscular walls of this compartment may measure several
inches in diameter and carry on the opposed mucous surfaces lining the
cavity a hard horny plate with corrugated and roughened surface (Fig.
88). These hard plates are designed to crush the food between them, as
between two mill stones. The muscle stomach is best developed in
herbivorous birds, while both the muscular wall and the horny plate are
much weaker and thinner in carnivore wading and swimming birds (Fig.
89).
[Illustration: FIG. 88.--OEsophagus and stomach of _Gallus bankiva_, hen.
(Columbia University Museum, No. 1809.)]
[Illustration: FIG. 89.--Stomach of _Botaurus lentiginosus_, bittern.
(Columbia University Museum, No. 23/1810.)]
In birds of prey, especially in the owls, the stomach walls are scarcely
more massive than in other animals, and the mucous membrane is soft and
devoid of a horny covering. The glandular and masticatory stomachs are
less sharply divided from each other in these forms, and the entire
organ conforms more to the general vertebrate type (Fig. 90).
[Illustration: FIG. 90.--Stomach of owl sp. (Nuhn.)]
In some birds (herons, storks, etc.) a small rounded third stomach, the
so-called pyloric stomach, is placed between the muscle stomach and the
pylorus (Fig. 91). It contains no gastric glands, and possibly may
function as an additional absorbing chamber.
[Illustration: FIG. 91.--Stomach of _Ardea cinerea_, heron. (Nuhn.)]
[Illustration: FIG. 92.--Stomach of crocodile. (Nuhn.)]
Among reptiles the stomach of the crocodile resembles the organ in birds
(Fig. 92). It is flat and rounded in shape, the muscle wall carries a
tendinous plate, and there is a pyloric stomach. There is, however, no
glandular stomach or proventriculus, as in birds, and the mucous
membrane is not covered by a horny plate, but is soft and contains the
peptic glands. Figs. 93 and 94 show the stomach of _Alligator
mississippiensis_, in the ventral view and in section.
[Illustration: FIG. 93.--Stomach of _Alligator mississippiensis_.
(Columbia University Museum, No. 1811.)]
[Illustration: FIG. 94.--Same in section. Thin-walled cardiac segment
continues into cavity of pyloric ventriculus.]
[Illustration: FIG. 95.--Stomach of _Bradypustridactylus_, three-toed
sloth. I. First stomach, devoid of gastric glands, corresponding to
rumen of ruminants.
II. Second stomach, the homologue of the ruminant reticulum.
III. Digestive stomach proper, provided with gastric glands connected by
a gutter with the oesophagus.
IV. Muscular stomach, the walls formed by a thick muscular plate and
provided on the mucous surface with a dense corneous covering for
purposes of trituration.]
(_c_) The combination of the two accessory functions just described in
the same stomach is found in the three-toed sloth (Fig. 95).
There are here two large reservoirs, which correspond to the rumen and
retinaculum of the ruminants, and a digestive compartment containing
gastric glands, which corresponds to the ruminant abomasum, and is
connected by an oesophageal gutter directly with the oesophagus. At the
pyloric extremity the muscle wall is greatly increased and the mucous
membrane of this portion carries a thick horny covering, forming a
masticatory stomach greatly resembling the corresponding structure in
the bird. Its function is evidently to complete the mechanical division
of the food which has only been partly masticated in the mouth.
The same significance is probably to be attached to the thickened
muscular walls which the pyloric segment of the stomach in _Tamandua
bivittata_, another edentate, presents (Fig. 96), in strong contrast
with the thinner walled cardiac segment and fundus.
[Illustration: FIG. 96.--Stomach of _Tamandua bivittata_, collared
ant-eater, (Columbia University Museum, No. 68/1485.)]
INTESTINE.
Continuing our consideration of the development of the alimentary canal
we find that changes from the simple primitive straight tube below the
stomach depend upon two factors:
1. The increase in the length of the intestinal tube, which exceeds
relatively the increase in the length of the body cavity in which it is
contained.
2. The differentiation into small and large intestine, the development
of the caecum and ileo-caecal junction, and the development of the
accessory digestive glands, liver and pancreas, by budding from the
proximal portion of the primitive entodermal intestinal tube.
1. In embryos up to 5 mm. cervico-coccygeal measure (Fig. 97) the
intestinal tube follows the body curve without deviation. Subsequently
the elongation of the intestine causes a small bend, with the convexity
directed ventrad, to appear in the umbilical region. This bend gradually
increases until the gut forms a single long loop, beginning a short
distance below the pylorus and directed ventro-caudad. The apex of the
loop, to which the vitello-intestinal duct is attached (Fig. 98) (cf. p.
34) projects beyond the abdominal cavity into the hollow of the
umbilical cord, constituting the so-called "umbilical or embryonal
intestinal hernia." This entrance of the apex of the intestinal
umbilical loop into the umbilical cord begins in embryos of about 10 mm.
During the succeeding weeks--up to the tenth--the segment of the
intestine thus lodged within the hollow of the umbilical cord increases.
After this period the intestinal coils are gradually withdrawn within
the abdomen. The explanation of this temporary extrusion of the
intestine into the umbilical cord is probably to be found in the strain
produced by the yolk-sac which is attached by the vitello-intestinal
duct to the apex of the umbilical loop. As we have seen (p. 35) the site
of the original apex of the loop may still be indicated in the adult by
the persistence of a portion of the vitello-intestinal duct as a
"Meckel's diverticulum."
[Illustration: FIG. 97.--Alimentary canal of human embryo of 5 mm. x 15.
(Reconstruction after His.)]
[Illustration: FIG. 98.--Schema of human embryonic intestinal canal,
with intestinal umbilical loop, but before differentiation of the large
and small intestine.]
In its simplest primitive condition the loop presents a proximal,
descending or efferent limb, an apex, and an ascending, returning or
afferent limb (Fig. 98). In the human embryo these segments of the loop
furnish the jejuno-ileum and portions of the large intestine, in a
manner to be subsequently detailed.
This stage in the development of the higher vertebrate intestine is well
illustrated by the alimentary tract of the mud-puppy, _Necturus
maculatus_, shown in Fig. 99, which represents the entire situs viscerum
of an adult female animal.
[Illustration: FIG. 99.--Viscera of _Necturus maculatus_, mud-puppy, _in
situ_. (Columbia University Museum, No. 1175.)]
The stomach is tubular, not distinctly differentiated from the
oesophagus, placed vertically in the long axis of the body. The pyloric
end is marked by a constriction separating stomach from midgut and
immediately beyond this point the pancreas is applied to the intestine.
The rest of the intestinal canal forms a simple loop, the descending
limb presenting one or two primitive convolutions. There is no marked
differentiation between large and small intestine, the canal possessing
a nearly uniform caliber from pylorus to cloaca.
2. The differentiation of the small from the large intestine, marked by
the appearance of the caecal bud or protrusion (Fig. 100), takes place in
the ascending segment of the umbilical loop a short distance from the
apex. In the human embryo the caecal bud appears in the 6th week as a
plainly marked protuberance, which grows very slowly in length and
circumference. It shows very early an unequal rate of development; the
terminal piece, not keeping pace in growth with the proximal portion, is
converted into the vermiform appendix, while the proximal segment
develops into the caecum proper. The increase in the length of the loop,
which begins to be marked in the 7th week, is not uniform. The apex is
the first portion to present the evidences of this growth. Subsequently
the descending limb grows in length very rapidly and is early thrown
into numerous coils of the future mobile portion of the small intestine
(jejuno-ileum). Even before the withdrawal of the apex of the loop
within the abdominal cavity a prominent coil of these convolutions is
found protruding in the umbilical region (Fig. 544). The ascending limb
of the loop from which a portion of the large intestine is developed,
grows comparatively slowly at this time.
[Illustration: FIG. 100.--Schema of human embryonic intestinal canal
after differentiation of the large and small intestine.]
The future portions of the human adult alimentary tract below the
stomach may be referred, in reference to their derivation, to this
primitive condition of the tube as follows:
1. The segment of small intestine situated between the pylorus and the
beginning or point of departure of the proximal or descending limb of
the umbilical loop, develops into the _duodenum_. This portion of the
small intestine is indicated early in embryos of 2.15 mm. (Fig. 101),
by the origin of the hepatic duct from the intestinal tube. Somewhat
later, in embryos of 4.10-5 mm. length, (Fig. 102) it becomes
additionally marked by the origin of the pancreatic diverticulum. The
duodenum, at first straight, now begins to curve, forming a short
_duodenal loop_ or _bend_. In embryos of 6 weeks the duodenum forms a
simple loop placed transversely below the pyloric extremity of the
stomach (Figs. 103 and 104).
[Illustration: FIG. 101.--Human embryo of 2.15 mm., twelve days old.
Seessel's sac is the cephalic blind termination of the embryonic
foregut before the communication with the ectodermal invagination of
the stomadaeum has been formed. (Reconstruction after His.)]
[Illustration: FIG. 102.--Representation of alimentary canal and
appendages of human embryo of 4.1 mm.; isolated. x 15. (Kollmann, after
His.)]
[Illustration: FIG. 103.--Alimentary canal and appendages of human
embryo of 12.5 mm. x 12. (Kollmann, after His.)]
[Illustration: FIG. 104.--A. Schematic representation of alimentary
canal, with umbilical loop and mesenteric attachments in human embryo of
about six weeks. B and C, stages in the intestinal rotation.]
2. The descending limb, the apex and a small part of the ascending limb
of the umbilical loop form the jejuno-ileum.
3. The remainder of the ascending limb forms the caecum and appendix, the
ascending and transverse colon.
4. The distal straight portion of the primitive tube forms the terminal
portion of the transverse colon (the splenic flexure), the descending
colon, sigmoid flexure and rectum.
The primitive condition of the embryonal mammalian alimentary tract,
after differentiation of the large intestine is well illustrated by some
of the lower vertebrates in which development never proceeds beyond this
stage. Fig. 112 shows the entire alimentary canal of a teleost fish, the
conger eel (_Echelus conger_) isolated.
[Illustration: FIG. 112.--Alimentary canal, isolated and in section, of
_Echelus conger_, the conger eel. (Columbia University Museum, No.
1812.)]
The preparation forms a good illustration of the embryonal stage of the
higher vertebrates in which development has not proceeded beyond the
formation of the simple umbilical loop, about corresponding to the
schematic Fig. 98. The stomach is differentiated both by its caliber and
by the formation of a pyloric ring valve.
The midgut forms a simple loop with a descending and ascending limb
closely bound together by mesenteric attachment. Different from the
course of development followed in the human embryo is the situation of
the ileo-colic junction. The same appears in the terminal straight
segment of the canal--corresponding to the human descending colon--while
in the human embryo the differentiation of small and large intestine
takes place in the course of the ascending limb of the loop. This
condition depends upon the relatively much shorter extent of the
teleost endgut compared with the human large intestine. Other examples
are afforded by the alimentary tract of some of the Amphibia and
Reptilia. Fig. 105 shows the alimentary canal of _Rana catesbiana_, the
common bull frog. The stomach, fairly well differentiated, is succeeded
by the small intestine of considerable length and uniform caliber. The
proximal portion of the small intestine is characterized as duodenum by
its connection with liver and pancreas. In the remaining portion of the
intestinal canal it is not difficult to recognize the elements of the
umbilical loop of the higher mammalian embryo. The larger mass of the
jejuno-ileal coils is developed from the descending limb of the loop; a
smaller number of convolutions belong to the returning or ascending
limb, which also includes the ileo-colic junction. The very short large
intestine of the frog passes straight down to enter the cloaca. Another
example, in which the early embryonal stages of the higher mammalia are
illustrated by the permanent structure of one of the lower vertebrates,
is given in Fig. 106, which shows the alimentary tract of a chelonian,
_Pseudemys elegans_, the pond turtle. The bilobed liver fits over the
well-differentiated stomach in the manner of a saddle. The stomach
itself, as in chelonians generally, has a markedly transverse position
and passes under cover of the right lobe of the liver into the duodenum.
The coils of small intestine form a prominent mass, which, however, when
unravelled as shown in the figure, permits us to recognize its identity
with the mammalian embryonic umbilical loop. The well-marked ileo-colic
junction is situated at the termination of the returning limb of the
loop, close to the beginning of the descending limb. This close
approximation of the duodenum and colon (duodeno-colic isthmus) forms
one of the most important factors in the further development of the
mammalian intestinal canal and will again be referred to below.
[Illustration: FIG. 105.--_Rana catesbiana_, bull-frog. Alimentary canal
and appendages. (Columbia University Museum, No. 1454.)]
[Illustration: FIG. 106.--_Pseudemys elegans_, pond turtle. Alimentary
canal. (Columbia University Museum, No. 1437.)]
From the ileo-colic junction the large intestine of the turtle continues
caudad to the cloaca in a nearly straight line. The same primitive
condition of the intestinal canal may be observed in some members of
man's own class, the mammalia--as in certain edentates. Figs. 107 and
108 show the entire abdominal portion of the alimentary tract in
_Tamandua bivittata_, the little ant-eater of Brazil. The stomach is
turned cephalad and the great omentum elevated. The intestines are
turned over to the right side.
[Illustration: FIG. 107.--Abdominal viscera of _Tamandua bivittata_, the
little ant-eater, seen from the left, with the intestines turned to the
right. (From a fresh dissection.)]
[Illustration: FIG. 108.--The same view, from another specimen. Figures
107 and 108 should be studied and compared together, as each supplements
the other.]
It will be observed that in spite of the numerous coils of the small
intestine the general arrangement of the alimentary canal corresponds to
the primitive scheme shown in Fig. 98. The entire intestinal canal is
attached by a continuous vertical mesentery to the dorsal median line of
the abdominal cavity ventrad of the vertebral column and aorta. The
growth in length of the small intestine has necessitated a corresponding
lengthening of the attached border of the mesentery--consequently the
membrane presents a pleated or crenated appearance. The caecum is well
developed, the ileo-caecal junction being situated within the returning
limb of the loop, a little distance from the apex.
In Figs. 109 and 110, taken from the same specimens, the entire mass of
the small intestines has been turned to the left so as to exhibit the
right leaf of the common dorsal mesentery and the mesoduodenum, the
latter containing the head of the pancreas. It will be noted that the
mesentery, expanding beyond the duodeno-colic isthmus, is common to the
small and to the proximal portion of the large intestine, _i. e._, to
those segments of the alimentary canal which are developed from the two
limbs of the umbilical loop. Figs. 107-110 should be studied and
compared together, as each supplements the others.
[Illustration: FIG. 109.--Abdominal viscera of _Tamandua bivittata_, the
little ant-eater, seen from the right, with the intestines turned to the
left. (From a fresh dissection.)]
[Illustration: FIG. 110.--The same view, from another specimen.]
It will be observed, in reference to the change from the primitive loop
to the subsequent increase in the length of the tube and the resulting
arrangement of the mesentery, that three successive stages are to be
considered, represented schematically in Fig. 111. In the earliest stage
(Fig. 111, I.) the two segments of the loop are of equal length,
parallel to one another, the distance between the beginning and
termination of the loop (1-2) being maintained throughout its
extent. Hence the mesentery is of equal width in all its parts within
the loop, only drawn out, _i. e._, away from the vertebral column, in
accordance with the length of the loop. In the next stage (Fig. 111,
II.) the increase in the length of the intestine is accompanied by a
corresponding widening of the mesentery. The points 1 and 2 are still
approximately the same distance apart as in the earlier stage, but the
increase in the length of the tube between these points forces the two
limbs of the loop to abandon their early parallel course, and to form
curved lines with the concavity turned toward the mesenteric attachment.
In this condition the mesentery consequently forms a widely expanded
membrane framed by the intestine and narrowing between the points 1 and
2 to a neck or isthmus which effects the transition between the expanded
segment surrounded by the intestine and the rest of the dorsal primitive
mesentery. Finally in the stage represented in Fig. 111, III., the
increase in the length of the small intestine has reached a point where
a single curve is no longer sufficient for the accommodation of the
growth. Consequently the tube now appears coiled and convoluted, and the
mesentery, as it is attached to the gut, of necessity follows all the
twists and appears fluted or pleated in its distal attached portion.
[Illustration: FIG. 111.--Schematic representation of the development of
the mesentery of the umbilical loop.]
If we now carefully examine the conditions presented by the intestine
and mesentery in a form like _Tamandua_ (Figs. 107 and 108) we will find
that they correspond to the developmental facts thus far considered. The
termination of the duodenum (1) and the bend in the colon (2) mark the
two points at which in the primitive schema (Fig. 111, I.) the umbilical
loop begins and terminates. The proximal of these two points (1)
corresponds to the termination of the duodenum, which segment extends
from here cephalad to the pyloric extremity of the stomach. The distal
point (2) is placed on the colon where the returning limb of the loop
resumes the original median vertical course of the large intestine.
These two points mark the neck of the loop, which we can describe as the
_duodeno-colic neck_ or _isthmus_.
The same condition is well shown in the intestinal canal of the snapping
turtle (Fig. 113). The duodenum and colon approach each other very
closely at the isthmus and between these points the convolutions of the
intestine extend in a wide circle. We will find this approximation of
duodenum and colon a feature which persists throughout all the later
developmental stages of the higher vertebrates and has an important
bearing on the final arrangement of the intestinal canal in the human
adult.
[Illustration: FIG. 113.--_Chelydra serpentina_, snapping turtle;
intestinal canal, pancreas, and spleen, isolated. (Columbia University
Museum, No. 1369)]
=Further Changes in the Development of the Human Alimentary Canal.
Rotation of the Intestine. Formation of the Segments of the Colon. Final
Permanent Relations of the Segments of the Intestinal Tube.=--The next
important stage leading up to the final adult disposition of the
intestine in man and the higher mammals is the _rotation_ of the
portions developed from the two limbs of the primitive loop around an
oblique axis drawn from the duodeno-colic isthmus to the apex of the
loop. The portion of the large intestine, developed from the ascending
limb of the loop, moves in the third month to the middle line, coming
into contact with the ventral abdominal wall. From here the large
intestine passes, ventrad of the jejuno-ileal coils, toward the cephalic
end of the abdominal cavity and lies transversely along the greater
curvature of the stomach. The growing coils of the small intestine crowd
the colon more and more cephalad. In the fourth month the caecum turns to
the right, coming into contact with the caudal surface of the liver,
ventrad of the duodenum, and subsequently reaches the ventral surface of
the right kidney. As the result of this rotation the ileo-colic
junction, caecum and succeeding portion of the colon are carried from the
original position in the distal and left part of the abdomen cephalad
and to the right across the proximal (duodenal) portion of the small
intestine, while the coils of the jejuno-ileum, developed from the
descending limb and apex of the loop, are turned in the opposite
direction, caudad and to the left underneath the preceding (Figs. 114
and 115). This change in the relative position of the parts of the
intestinal tract and the resulting altered bearing of the colon to the
duodenum will be best appreciated by considering in the first place the
effect of the change on the arrangement of the primitive mesentery and
the intestinal vessels, and secondly by repeating actually the rotation
in the intestinal tract of a mammal (cat) in which the adult arrangement
of the intestine and peritoneum permits us to perform the manipulations
and note the result.
[Illustration: FIG. 114_A_.--Intestinal canal in stage of umbilical
loop--before rotation.]
[Illustration: FIG. 114_B_.--First stage in rotation, colon crossing
duodenum.]
[Illustration: FIG. 115_A_.--Second stage in rotation--rotation of small
intestine.]
[Illustration: FIG. 115_B_.--Schema of intestinal canal after complete
rotation and descent of caecum.]
=I. Effect of Rotation on the Disposition of the Primitive Mesentery and
on the Relative Position of Duodenum and Colon, and Consequent
Arrangement of the Intestinal Blood Vessels.=--It will be appreciated
that in Fig. 111, representing a profile view of the original
arrangement, or in Figs. 107 and 108, showing the intestinal canal of
_Tamandua_, the left layer of the primitive mesentery is turned toward
the observer. The membrane is seen to pass from the ventral aspect of
the vertebral column and aorta, through the narrow neck of the
duodeno-colic isthmus, to expand in the manner already indicated toward
its intestinal attachment. In the rotation of the intestine the twist
takes place at the duodeno-colic neck, carrying, as already stated, the
large intestine cephalad and to the right, while the jejuno-ileum is
turned in the opposite direction caudad and to the left. During this
rotation the duodeno-jejunal angle (Figs. 114, _B_ and 115, _A_) passes
to the left underneath the proximal segment of the colon, which now lies
ventrad and to the right of the duodenal portion of the small intestine.
The mesenteric peritoneum, occupying the bight of the umbilical loop,
will, after the rotation, in the left profile view shown in Fig. 104,
_A_ and _B_, turn its original right leaf toward the beholder, _i. e._,
toward the left, while the original left leaf is turned toward the
right.
Observation of the difference in the position of the ileo-colic junction
will still further accentuate the change in the relative position of the
parts which has been effected by the rotation. In the primitive
condition shown in Fig. 104, _A_, the ileum enters the large intestine
from right to left, and the concavity of the caecal bud turns its
crescentic margin ventrad and to the right.
After rotation is accomplished (Fig. 104, _B_ and _C_, and Fig. 115) the
ileo-colic entrance takes place in the opposite direction, from left to
right and the caecum turns its concave margin caudad and to the left.
Figs. 116 and 117 show the intestinal tract of _Tamandua bivittata_
arranged so as to correspond to the human embryonic condition after
rotation. The caecum has been brought up and to the right across the
proximal duodenal portion of the small intestine, while the jejuno-ileal
coils have been turned down and to the left. The rotation has been
accomplished by a twist at the duodeno-colic isthmus, and the original
right leaf of the mesentery has become the left and _vice versa_.
Comparison with Figs. 107 and 108, representing the condition before
rotation in the same animal, will indicate the changes which have been
accomplished by imitating the course of development followed in the
higher mammals.
[Illustration: FIG. 116.--Abdominal viscera of _Tamandua bivittata_,
with the intestine rotated to correspond to the development in the human
subject. (From a fresh dissection.)]
[Illustration: FIG. 117.--The same view as Fig. 116, from another
specimen.]
Failure of rotation and arrest of development at the primitive stage,
with consequent persistent embryonic condition of the mesentery, occurs
occasionally in man. Such cases have been reported by W. J. Walsham, in
St. Barthol. Hosp. Rep., London, Vol. 16. The following four instances
of this condition, taken from the Columbia University museum, will
illustrate the disposition of the abdominal contents.
Fig. 118 shows the arrangement of the abdominal viscera in an adult
female body. Beginning at the pyloric extremity of the stomach the
entire course of the duodenum can be overlooked and its continuation
into the jejuno-ileal division traced. The small intestines occupy the
ventral and right part of the cavity. The ileo-colic junction is placed
in the lower left-hand corner of the abdomen and the small intestine
enters the large from right to left, the ascending colon is situated to
the left of the median line and at its point of transition into the
segment representing the transverse colon is connected by several
adhesions with the ventral surface of the duodenum. The transverse
colon, folded into several coils bound together by adhesion, occupies
the upper left portion of the abdomen.
[Illustration: FIG. 118.--Abdominal viscera of adult human female, in a
case of arrested rotation of the intestines. (Columbia University
Museum, Study Collection.)]
[Illustration: FIG. 119.--The same preparation with the intestinal coils
displaced upward and to the left.]
Fig. 119, taken from the same specimen, shows the entire mass of
intestines lifted up and turned to the left, exposing the background of
the abdominal cavity lined by parietal peritoneum. The duodenum is still
entirely free and non-adherent to the parietal peritoneum. The
continuity of the mesoduodenum with the jejuno-ileal mesentery is well
shown. The primitive right leaf of the mesentery is turned to the
observer. This layer after completed rotation would form the left layer
of the adult mesentery of the jejuno-ileum.
Fig. 120 illustrates another instance of the same condition in the
adult. In this case the duodenum was coiled twice upon itself and
adherent to the prerenal parietal peritoneum.
[Illustration: FIG. 120.--Abdominal viscera of adult human male;
non-rotation of intestine. (Columbia University Museum, Study
Collection.)]
Fig. 121, presenting the same adhesion of the duodenum, illustrates very
perfectly the persistence of the narrow duodeno-colic isthmus in cases
of non-rotation, as well as the development of the different segments of
the adult tract from the limbs of the embryonal umbilical intestinal
loop.
[Illustration: FIG. 121.--Abdominal viscera of adult human male;
non-rotation of intestine. (Columbia University Museum, Study
Collection.)]
It will be observed that beyond the duodeno-colic isthmus the coils of
the jejuno-ileum have resulted from the increase in length of the
descending limb, the apex and the proximal part of the ascending or
recurrent limb, carrying the ileo-colic junction and caecum. The
remainder of the ascending limb, terminating in the embryonic condition
at the splenic flexure by passing into the descending colon, has in the
course of further development in this individual produced a straight
segment--the misplaced ascending colon--and a convoluted and bent
representative of the normal transverse colon.
The same disposition of the large intestine may be noted in the other
preparations.
Fig. 122 shows an instance of non-rotation observed in the human infant
at two years of age.
[Illustration: FIG. 122.--Abdominal viscera of child, two years old;
non-rotation of intestine. (Columbia University Museum, Study
Collection.)]
[Illustration: FIG. 123.--Human foetus at term; abdominal viscera,
hardened _in situ_; non-rotation of caecum. (Columbia University Museum,
No. 1813.)]
Fig. 123, taken from a foetus at term, shows the result of failure to
completely rotate in the region of the caecum and ileo-colic junction.
The rest of the large intestine has rotated as usual and assumed the
normal position. The terminal ileum, however, passes behind the caecum
and enters the large intestine on its right side; the caecum is turned
upwards and to the right and the appendix lies ventrad of the beginning
of the ascending colon. In order to produce the normal arrangement,
shown in Fig. 124, taken from another foetus at term, it would be
necessary to turn the caecum and ileo-colic junction in Fig. 123 through
half a circle. The caecum would then turn upwards and to the left, the
ileum entering the large intestine from left to right, and the appendix
would be placed behind the caecum and ileo-colic junction. Figs. 125 and
126 show the normal and abnormal arrangement presented by these two
preparations diagrammatically. The instances in which in the adult the
ileo-colic entrance is placed on the right side of the large intestine
and in which the appendix is situated laterad of the ascending colon
unquestionably find their explanation in the failure of the intestine to
completely rotate at the ileo-colic junction.
[Illustration: FIG. 124.--Human foetus at term; abdominal viscera,
hardened _in situ_; normal position of completely rotated caecum and
appendix. (Columbia University Museum, No. 1814.)]
[Illustration: FIG. 125.--Just before final rotation of caecum and
terminal ileum. Concavity of caecum directed cephalad and to right.
Terminal ileum enters colon from right to left.]
[Illustration: FIG. 126.--Rotation completed. Concavity of caecum turns
caudad and to left. Terminal ileum enters colon from left to right.]
[Illustration: FIGS. 125, 126.--Schematic representation of final stages
in rotation of caecum and large intestine.]
The resulting conditions are shown in Figs. 127 and 128, taken from
adult human subjects in which the final stage of rotation of the large
intestine has not taken place.
[Illustration: FIG. 127.--Adult human subject with non-rotated caecum.
The terminal ileum turns caudad from right to left to enter right side
of colon.]
[Illustration: FIG. 128.--Adult human subject with non-rotated caecum,
the ileum entering large intestine from the right and behind, and the
appendix placed to the right of the ascending colon. (From a fresh
dissection.)]
In Fig. 127 the terminal ileum is sharply bent on itself and adherent to
the prerenal parietal peritoneum. It passes from right to left and
downwards to enter the right posterior circumference of the large
intestine. The caecum is turned cephalad and the appendix is in contact
with the right lobe of the liver. The caecum passes with a sharp bend
into the obliquely directed ascending colon.
In Fig. 128 the ileum enters the colon from the right and below. The
apex of the caecum is turned cephalad and to the right and the appendix
extends beneath peritoneal adhesions along the lateral border of the
proximal segment of the colon.
In the next place it is desirable to clearly understand the vascular
supply of the intestine before and after rotation and the final relation
of the superior mesenteric artery to the transverse portion of the
duodenum.
Development of Aortal Arterial System.
The thoracic and abdominal aortae are at first double, the first aortic
arches continuing as so-called "primitive aortae" ventrad of the
vertebral column to the caudal end of the body.
The cephalic portions of the two vessels unite in the chick on the third
day and from this point fusion into a single vessel proceeds slowly
caudad.
In the rabbit the fusion of the primitive aortae begins on the ninth day
in the region of the lung-buds and progresses from here caudad until by
the sixteenth day a single aorta is formed (Fig. 129).
[Illustration: FIG. 129.--Diagrams illustrating the arrangement of the
primitive heart and aortic arches. (After Heisler, modified from Allen
Thompson.)]
That the entire descending aorta in man results from the fusion of two
vessels is shown by the rare cases in which the aorta is divided
throughout its entire length by a septum.
The arteries of the allantois are originally the terminations of the
primitive aortae. After fusion of the primitive aortae to form the
abdominal aorta the allantoic arteries, now passing as the umbilical
arteries to the placenta, appear as the branches of bifurcation of the
abdominal aorta, in the same way as the common iliacs do in the adult.
They furnish branches, which at first are very small, to the budding
posterior extremities and the pelvic viscera. In time these rudiments of
the future external and internal iliac arteries become larger, but as
the umbilical arteries continue to develop throughout the entire
intra-uterine period they appear even in the foetus at term as end
branches of the aorta, a condition which is only changed after birth by
the obliteration of the umbilical arteries and their conversion into the
lateral ligaments of the bladder, while the iliac vessels now appear as
the terminal aortic branches. The statement that the umbilical arteries
appear as the terminal branches of the embryonal aorta requires to be
modified in the following respect:
When the allantois develops its arteries are in fact end-branches of the
two primitive aortae. After their fusion and after the formation of the
single aorta this vessel is continued beyond the umbilical arteries as
a small trunk, the caudal artery or rudiment of the adult sacralis
media. Consequently the umbilical arteries are really lateral branches
of a median vessel, viz., aorta abdominalis and arteria sacralis media.
But as the umbilical vessels are very large and the caudal aorta very
small, the former, even under these conditions, appear as the real
terminal branches of the abdominal aorta.
The arteries supplying the yolk-sac and subsequently the intestinal
canal are the vitelline or omphalo-mesenteric. At first they are
branches derived from the two primitive aortae, and after the fusion of
these vessels they arise from the resulting single abdominal aorta. The
omphalo-mesenteric arteries are at first multiple and later are reduced
to two. When the primitive intestine loses its original close contact
with the vertebral column and the common dorsal mesentery develops, the
two omphalo-mesenteric arteries unite to form a single vessel, running
between the layers of the mesentery. After a short course this artery
divides again into two branches, passing one on each side, around the
intestinal tube, which has in the meanwhile become closed. Ventrad of
the intestine these branches reunite so that the gut is surrounded by a
vascular circle. The left half of this loop becomes obliterated and the
trunk of the omphalo-mesenteric artery now passes on the right side of
the intestine to the umbilicus. The peripheral segment of the
omphalo-mesenteric artery disappears with the cessation of the vitelline
circulation. The proximal portion, situated between the layers of the
mesentery, gives numerous anastomosing branches to the intestine and is
converted into the main trunk of the superior mesenteric artery.
The derivation of the superior mesenteric as the fully developed
proximal segment of the embryonic omphalo-mesenteric artery passing to
the yolk-sac is responsible for the rare anomaly in the adult of a
branch of the superior mesenteric artery continuing beyond the intestine
to the umbilicus. I have encountered one instance of this persistence of
the intra-abdominal portion of the omphalo-mesenteric artery in a male
subject 54 years of age. A connective strand, containing a
small artery derived from the superior mesenteric vessels, extended
between the right layer of the mesentery, some distance from its
attached border, and the ventral abdominal wall at the umbilicus. The
vessel which was pervious throughout, was the size of one of the digital
arteries.
Hyrtl has observed the same variation. An example of partial persistence
of the omphalo-mesenteric artery in the adult is well seen in the case
of Meckel's diverticulum shown in Fig. 37, where the arterial vessel
continued upon the diverticulum represents the embryonic
omphalo-mesenteric artery.
The remaining intestinal arteries are at first more numerous and paired.
In man and most mammals they are early reduced in number, passing from
the abdominal aorta to the dorsal or attached border of the intestine,
between the two peritoneal layers of the primitive dorsal mesentery
(Fig. 104). The arterial blood supply of the intestinal canal then
presents three general divisions:
1. Vessels pass from the proximal part of the abdominal aorta to the
stomach and pyloric portion of the duodenum. This set of vessels forms
the rudiment of the future coeliac axis. With the development of the
liver and pancreas by budding from the duodenum, and with the appearance
of the spleen in the mesoderm of the dorsal mesentery, branches
corresponding to these organs (hepatic and splenic arteries) are added
to the gastric and duodenal vessels and the adult arrangement of the
coeliac axis is thus obtained (Figs. 130, 131, 132 and 133).
[Illustration: FIG. 130.--Diagrammatic representation of the arteries
proceeding to the alimentary canal and appendages prior to rotation of
intestine (stage of simple umbilical loop).]
[Illustration: FIG. 131.--Diagrammatic representation of the arteries of
the alimentary canal in the first stage of intestinal rotation, showing
relation of superior mesenteric artery to the transverse portion of the
duodenum.]
[Illustration: FIG. 132.--Arteries of alimentary canal in the later
stages of intestinal rotation.]
[Illustration: FIG. 133.--Final arrangement of arteries of alimentary
canal after completed rotation of the intestines.]
These vessels have an important bearing on the formation of the adult
peritoneal cavity in the retro-gastric space, and will be considered in
detail below with that portion of the subject.
2. The next vessel in order derived from the aorta and supplying the
duodenum, pancreas, the small and a part of the large intestine is the
above-mentioned superior mesenteric artery, which arises from the aorta
a short distance caudad of the coeliac axis (Figs. 130, 131, 132 and
133).