ANATOMY OF THE PERITONEUM IN THE SUPRA-COLIC COMPARTMENT OF THE ABDOMEN.
We have already seen that the transverse colon and mesocolon effect a
general division of the adult human abdominal cavity into a cephalic
supra-colic compartment, situated between the diaphragm and the level of
the transverse colon and mesocolon, comprising in general the
hypochondriac and epigastric regions, and a larger caudal infra-colic
space which includes the entire rest of the abdominal cavity and is
continued caudad into the pelvic cavity. The arrangement of the
peritoneum and viscera in this latter space has just been considered.
The fact will be recalled that the second or descending portion of the
duodenum, passing dorsad of the hepatic colic flexure, forms so to speak
the visceral connection between the portions of the alimentary tube
situated in the supra-colic compartment and those situated in the
infra-colic space. The fixation of this segment of the duodenum and its
consequent secondary retroperitoneal position in the adult human subject
masks this continuity of the alimentary canal to a certain extent so
that it requires more than a superficial examination in order to trace
correctly the course of the duodenum from the pylorus to the
duodeno-jejunal angle, dorsad of the colon, root of transverse mesocolon
and mesentery, and under cover of the secondary parietal peritoneum.
We have now to turn our attention to the viscera contained in the
cephalic or supra-colic compartment of the abdomen and to consider the
disposition of the serous membrane investing them and connecting them
with each other and with the abdominal parietes.
The visceral contents of the supra-colic compartment comprise the
liver, pancreas, spleen, stomach and the proximal portion of the
duodenum, including the hepatic angle and the supra-colic part of the
descending duodenum. Less directly the cephalic portions of the right
and left kidney and the corresponding suprarenal capsules belong to this
visceral group.
In this region of the abdomen we meet with the most extensive
modifications of the primitive dorsal peritoneal membrane, producing
conditions which, considered without reference to development and
comparative anatomy, are complex and difficult of comprehension. These
changes lead to the formation of the so-called "lesser sac," a term
which in some respects is unfortunate as it implies a more complete
degree of separation from the general peritoneal cavity or "greater sac"
than actually exists.
In order to clearly understand the adult arrangement of the peritoneum
in this region it is advisable to consider the subject in two distinct
subdivisions, dealing successively with the two cardinal facts which
contribute to effect the change from the simple primitive to the
complicated adult condition.
These two main elements are:
1. Developmental changes in the position of the stomach, alterations in
the disposition of the proximal part of the primitive dorsal mesentery
attached to the stomach, and the development of pancreas and spleen in
connection with this membrane.
2. The development of the liver and the successive stages in the
production of the final adult vascular and serous relations of this
organ.
=1. Stomach and Dorsal Mesogastrium.=--We have already considered the
early stages in the differentiation of the stomach from the primitive
intestinal tube of uniform caliber (p. 40). It will be recalled that the
stomach at a certain period, while it already presents the main
structural features familiar in the adult organ, occupies a vertical
position in the abdominal cavity, turning its concave margin (lesser
curvature) ventrad, while the convex dorsal border (greater curvature)
is directed toward the vertebral column, being attached to the same by
the layers of the proximal part of the primitive dorsal mesentery. At
this time the stomach presents right and left surfaces, and the
oesophageal entrance is at the highest or cephalic point of the organ,
while the pyloric transition to the small intestine occupies the distal
caudal extremity.
The primitive dorsal mesentery, as already stated, passes as a thin
double-layered membrane between the ventral surface of the vertebral
column and the dorsal border of the stomach, which, as we will presently
see, becomes during the later stages of development the caudal (lower)
margin or greater curvature.
It will be seen that the embryonic differentiation of the intestinal
tract into successive segments justifies the application of a
terminology based on this differentiation to the corresponding portions
of the primitive common dorsal mesentery.
Thus the proximal portion extending between the vertebral column and the
dorsal border or greater curvature of the stomach becomes the
_mesogastrium_; we differentiate this portion still further as the
"_dorsal mesogastrium_" to distinguish it from a "_ventral
mesogastrium_" which we will presently encounter in considering the
development of the liver and the connected peritoneum.
In the same way the section of the primitive common dorsal mesentery
attached to the duodenal loop becomes the _mesoduodenum_, that connected
with the mobile part of the small intestine (jejuno-ileum) the
_mesentery_ proper, while the portion passing to the colon forms the
_mesocolon_, to be subsequently still further subdivided, after the
different segments of the large intestine have become mapped out, as the
_ascending_, _transverse_ and _descending mesocolon_, the
_mesosigmoidea_ and the _mesorectum_.
In tracing the development of the adult human peritoneum it is well to
consider certain stages, which we will find illustrated by the permanent
conditions presented by some of the lower vertebrates:
These stages comprise:
(_a_) Changes in the position of the stomach.
(_b_) Changes in the direction and extent of the dorsal mesogastrium.
(_c_) Development of the pancreas and spleen in connection with the
mesogastrium.
A. Changes in the Position of the Stomach.
The primitive position of the organ above outlined (p. 41) is changed
during the course of further development by a twofold rotation.
1. The primitive vertical position, in which the oesophageal entrance
occupies the highest cephalic extremity, while the pyloric opening is at
the opposite caudal end, is exchanged for one directed more
transversely, approximating the two gastric orifices to the same
horizontal level. In human embryos of 13.9 mm. the fundus has already
descended, the pylorus moving cephalad and to the right, while the
cardia becomes shifted more to the left. At the same time the greater
growth and prominence of the convex border or greater curvature becomes
marked in comparison with the relatively short extent of the opposite
margin or lesser curvature.
2. Coincident with this change in position is a rotation around the
vertical axis, by means of which the original left side of the stomach
is turned ventrad, becoming the ventral or "anterior" surface, while the
original right surface of the organ now looks dorsad toward the
vertebral column, becoming the dorsal or "posterior" surface of human
anatomy. The oesophageal or cephalic end is placed to the left of the
median line, while the caudal or pyloric end is situated on the right
side (Figs. 169 and 170).
[Illustration: FIGS. 169, 170.--Two front views of the entodermal canal.
(Minot, after His.)]
[Illustration: FIG. 169.--Embryo Sch. 1 of His.]
[Illustration: FIG. 170.--Embryo Sch. 2 of His.]
The original ventral border, now the "lesser curvature" or "upper
border," looks cephalad and to the right, toward the caudal surface of
the liver, while the original dorsal border, as the "greater curvature"
or "lower border" is directed in the main caudad and to the left.
The prominence of this border is still further increased by the greater
development of the stomach to the left of the oesophageal entrance
resulting in the formation of the "fundus" or "great cul-de-sac."
This rotation of the stomach explains the asymmetrical position of the
vagus nerve in the adult, the left side of the embryonic stomach,
innervated by the left vagus, becoming the "anterior" surface of adult
descriptive anatomy and _vice versa_.
It will be readily appreciated that a comparatively flat organ like the
stomach, will, as long as it occupies a sagittal position, with right
and left surfaces, help to divide the upper part of the abdominal cavity
to a certain extent into a right and left half, even if the peritoneal
connections of the organ are left out of consideration. As soon,
however, as the above-described changes in position take place and the
surfaces of the stomach are directed ventrad and dorsad, the relative
arrangement and extent of this right and left abdominal space becomes
altered by the different disposition of the septum, _i. e._, the
stomach. The original right side of the organ is now directed dorsad,
and the rotation of the organ has created a space between this dorsal or
"posterior" surface of the stomach and the background of the abdominal
cavity, which is the inception of the "lesser peritoneal cavity" or
retrogastric space. We will find that this space becomes well defined
and circumscribed by the peritoneal connections of the stomach, but we
will realize, even at this stage, that the _dorsal_ surface of the
stomach will form a part of the general _ventral_ wall of the lesser
peritoneal space.
On the other hand, the partial division of the abdomen into a right and
left half, effected by the stomach in its primitive sagittal position,
disappears after rotation of the organ. We now pass uninterruptedly from
left to right across the ventral (original _left_) surface of the
stomach.
B. Changes in the Direction and Extent of the Dorsal Mesogastrium.
The effects of the altered position of the stomach on the disposition of
the abdominal space have just been considered in relation to the organ
itself, without reference to its natural connections with the parietes
and with adjacent viscera. Their true significance and their influence
on the adult anatomical arrangement of the abdomen is, however, only
appreciated when the changes in the arrangement of the peritoneal
membrane which they involve, are taken into account.
The dorsal mesogastrium changes more than any other portion of the
peritoneum in the course of development. It not only becomes displaced
and altered in direction by the rotation of the stomach, but in addition
it grows so extensively that it finally hangs down like an apron over
the entire mass of small intestines, forming the great omentum.
If we begin with the primitive disposition of the sagittal stomach and
dorsal mesogastrium shown in Fig. 171 it will be observed that both
structures together actually divide the dorsal portion of the abdominal
cavity into symmetrical right and left halves (Fig. 172).
[Illustration: FIG. 171.--Schematic representation of dorsal
mesogastrium before rotation of stomach.]
[Illustration: FIG. 172.--Semi-diagrammatic representation of
mesogastrium in human embryo of the sixth week. (Kollmann.)]
After rotation of the stomach (Fig. 173) the mesogastrium loses its
original sagittal direction. It follows the altered position of the
original dorsal border of the stomach, which has now become the caudal
margin or "greater curvature," by turning caudad and to the left, being
at the same time considerably elongated. This occurs during the second
month. Hence the dorsal mesogastrium, after leaving the vertebral
column, turns ventrad and to the left to reach its gastric attachment
along the greater curvature. This is the first indication of the
formation of the great omental or epiploic bursa.
[Illustration: FIGS. 173-175.--Schema of dorsal mesogastrium after
rotation of stomach.]
[Illustration: FIG. 173.--Early stage.]
[Illustration: FIG. 174.--Later stage, extension of mesogastrium beyond
stomach to left, with fundus of blind retrogastric pouch thus created at
X.]
[Illustration: FIG. 175.--After adhesion over area of dotted line
between dorsal mesogastrium and primitive parietal peritoneum. Secondary
line of transition from dorsal mesogastrium to parietal peritoneum at
X.]
The stomach is here considered as developing in situ and as influencing
by its growth and change of position the arrangement and direction of
the peritoneal layers with which it is connected. As a matter of fact it
is well to note that the stomach at first lies above the primitive
diaphragm or septum transversum, migrating, however, at an early period
into the subhepatic abdominal position. This migration produces a
corresponding increase in the length of the oesophagus (Fig. 34) and the
stomach, in consequence of this change in position, acquires its ventral
and dorsal mesogastrium. For the purpose of explaining the adult
peritoneal relations of the organ it is, however, more
convenient to regard the stomach as an abdominal organ from the
beginning and to deal with the subsequent changes in position from this
standpoint. The inaccuracy is slight and renders the comprehension of
the succeeding stages easier.
It will be noticed (Fig. 173) that the rudimentary retro-gastric space
or "lesser peritoneal sac" is bounded ventrally by the dorsal (the
primitive _right_) surface of the stomach, while its dorsal boundary is
furnished by the ventral (originally _right_) layer of the dorsal
mesogastrium.
In the primitive condition, therefore, dorsal mesogastrium and stomach
form together a straight line sagittal in direction and placed in the
median plane of the body. As the result of the developmental changes
above outlined this straight line becomes bent at the point where the
mesogastrium reaches the stomach (Fig. 173, x). The two component
elements of the line (stomach and mesogastrium) hinge on each other
here, and the angle which they form opens to the right.
The changes which are to be observed in the later stages depend
principally upon a peculiar feature characteristic of the development of
the dorsal mesogastrium. This feature consists in the extreme redundancy
of the membrane which grows out of proportion to the requirements of its
visceral connections, and to a certain extent becomes independent of the
direct mechanical purpose of carrying blood vessels to the viscera.
Hence in a transverse section at this period (Figs. 174 and 175) the
mesogastrium no longer passes in a direct line between its points of
attachment, viz. the greater curvature of the stomach and the vertebral
column, but extends beyond the stomach to the left. We will appreciate
the significance of this extensive growth of the mesogastrium especially
in considering the development of the spleen and pancreas. For the
present it will suffice to note (Figs. 174 and 175) that the growth has
carried the mesogastrium well to the left of the stomach, consequently
the retrogastric space is now bounded toward the left by the bend which
the original right leaf of the primitive sagittal mesogastrium takes in
order to reach its gastric attachment. The retrogastric space therefore
terminates toward the left in a blind pocket formed by this
reduplication of the mesogastrium.
One more factor is to be taken into consideration, namely the tendency,
already noted, of peritoneal surfaces to become adherent to each other.
Such adhesion involves the apposed surfaces of the mesogastrium and of
the primitive parietal peritoneum to the left of the vertebral column.
The dorsal (original _left_) layer of the mesogastrium adheres to the
parietal peritoneum covering the left side of the abdominal background
and the cephalic portion of the ventral surface of the left kidney up to
the end of the blind pouch which forms the extreme left limit of the
retrogastric space. Hence, after this process of adhesion is completed,
the dorsal wall of the retrogastric space is lined by secondary parietal
peritoneum covering the left kidney (original right leaf of primitive
mesogastrium) (Fig. 175). We obtain (Fig. 175 at x) an apparent
continuity of the parietal peritoneum with that portion of the
mesogastrium which, derived from the original left layer of the
membrane, appears now to extend, as the ventral one of two layers,
between the stomach and the abdominal parietes near the lateral border
of the left kidney. (Primitive gastro-splenic omentum.)
It should be remembered that the disposition of the peritoneum just
indicated is modified by the development of the pancreas and spleen,
both of which organs are intimately associated with the mesogastrium.
The foregoing statements and diagrams are therefore merely given for the
purpose of affording a general view of the extent, growth and changes of
the dorsal mesogastrium before proceeding to consider the development of
the pancreas and spleen in and from the membrane itself.
[Illustration: FIG. 176.--Schematic ventral view of stomach, duodenum,
and dorsal mesogastrium, after rotation of stomach and extension of
omental bursa caudad beyond greater curvature of stomach. The ventral
mesogastrium is detached along the lesser curvature.]
[Illustration: FIG. 177.--Semi-diagrammatic representation of peritoneal
membrane in human embryo. (After Kollmann.)]
In the view directly from in front the redundancy of the peritoneum
forming the mesogastrium is shown in Figs. 176 and 177. Just as the
membrane extends further to the left than required by its visceral
connection with the stomach, so the downward growth exceeds the demand
made by the rotation of the attached border (greater curvature) caudad
and to the left. The mesogastrium, forming, as it now does, the great
omentum, enlarges in descending toward the transverse colon (Fig. 177).
The bag thus formed can be distended with air in a foetus of from 8 to 9
cm. vertex-coccygeal measure, as shown in the figure. Consequently in
sagittal section the membrane is seen to extend caudad beyond the level
of the greater curvature, and must turn on itself and pass again
cephalad in order to reach the stomach (Fig. 178). By reason of this
excessive growth the limits of the primitive retrogastric space are
enlarged, not only toward the left, but more especially in the caudal
direction. The bend made by the mesogastrium in returning to the stomach
forms the blind extremity of a pouch which continues the retrogastric
space caudad beyond the stomach, and whose dorsal and ventral walls are
formed by the reduplicated mesogastrium. This pocket or pouch
constitutes the _omental_ or _epiploic bursa_ of the lesser peritoneal
cavity, for the great omentum is the direct product of this redundant
growth of the mesogastrium caudad. It will be observed that the great
omentum is made up of four peritoneal layers, the folding of the
double-layered mesogastrium naturally producing this result. The first
or ventral and the fourth or dorsal layer are derived from the original
left layer of the primitive sagittal mesogastrium; the intermediate
second and third layers, separated from each other at this stage by the
cavity of the omental bursa, are products of the primitive right leaf of
the mesogastrium. Since the entire retrogastric space with its
extensions becomes the "lesser cavity" of the human adult peritoneum, it
will be seen that its serous membrane is derived from the original right
leaf of the mesogastrium (second and third omental layers). After the
above-described adhesion of the mesogastrium to the parietal peritoneum
overlying the ventral surface of the left kidney, the membrane would be
traced in sagittal section (Fig. 179) from the dorsal surface of the
stomach caudad, lining the interior of the omental bursa (second layer)
to the turn or blind end of the pouch; thence cephalad as the third
omental layer, forming the dorsal wall of the epiploic bursa, to invest,
as secondary parietal peritoneum, the cephalic segment of the ventral
surface of the left kidney.
[Illustration: FIG. 178.--Schematic sagittal section through stomach and
dorsal mesogastrium, after rotation and formation of omental bursa.]
[Illustration: FIG. 179.--Schematic sagittal section through stomach and
dorsal mesogastrium after adhesion to prerenal parietal peritoneum.]
C. Development of Spleen and Pancreas in the Dorsal Mesogastrium and
Changes in the Disposition of the Great Omentum.
In order to obtain a correct conception of the adult human conditions it
is finally necessary to consider the development of the spleen and
pancreas in their connection with the dorsal mesogastrium and to note
the changes which are produced by adhesion of portions of the great
omentum to adjacent serous surfaces. It will be advisable to discuss
these subjects at first separately, and to subsequently combine all the
facts in an attempt to gain a correct impression of their share in
determining the disposition of the adult human peritoneum.
=1. Development of Spleen.=--The spleen develops from the mesoderm
between the layers of the dorsal mesogastrium, near its point of
accession to the greater curvature, in the region of the subsequent
fundus. It has therefore, like the stomach, originally free peritoneal
surfaces. After rotation of the stomach the organ lies between the two
layers of the membrane at the extreme left end of the retrogastric space
(Fig. 180).
[Illustration: FIG. 180.--Schematic transverse section of the abdomen,
showing early stage of development of spleen from extreme left end of
dorsal mesogastric pouch.]
=Vascular Connections.=--The splenic artery accedes to the mesal surface
of the spleen from the vessel which originally passed directly to the
dorsal border (subsequent greater curvature) of the stomach, between the
layers of the mesogastrium.
With the further growth of the spleen the segment of this vessel
situated between its origin from the coeliac axis and the hilum of the
spleen becomes relatively larger, forming the adult splenic artery,
while the continuation of the original vessel to the greater curvature
of the stomach appears now as a branch of the splenic artery, viz., the
arteria gastro-epiploica sinistra.
Through the development of the spleen the dorsal mesogastrium has been
subdivided into a proximal longer vertebro-splenic, and a distal shorter
gastro-splenic segment. The former, as we have seen, loses its identity
as a free membrane in the human adult, by fusing with the parietal
peritoneum investing the ventral surface of the left kidney. Hence,
after this adhesion has taken place, the splenic artery courses from the
coeliac axis to the spleen behind peritoneum which functions as part of
the general parietal membrane, but which is derived from the original
right leaf of the proximal vertebro-splenic segment of the primitive
mesogastrium (Fig. 181). On the other hand the distal segment of this
membrane, beyond the spleen, remains free, carrying, as the
gastro-splenic omentum, the left gastro-epiploic artery between its
layers from the splenic artery to the greater curvature of the stomach.
[Illustration: FIG. 181.--Schematic transverse section of the abdomen,
showing later stage of development of spleen and arrangement of
peritoneum after adhesion of dorsal layer of mesogastrium and primitive
prerenal parietal peritoneum.]
The lateral limit of the area of adhesion between mesogastrium and
parietal peritoneum is situated along the lateral border of the left
kidney. Hence, in the final condition of the parts, the main splenic
vessels at the hilum are situated between two peritoneal layers of which
the ventral (Fig. 181) appears as the parietal peritoneum forming the
dorsal wall of the retro-gastric space, while the dorsal layer (Fig.
181) forms a reflection from the mesal surface of the spleen, along the
dorsal margin of the hilum, to the adjacent lateral border of the left
kidney (lieno-renal ligament) and to the diaphragm. At this point of
adhesion subsequently firmer strands of connective tissue develop in the
serous reduplication forming the _ligamentum phrenico-lienale_ of
systematic anatomy. This process of adhesion takes place during the
second half of intra-uterine life. A connection with the colon, produced
by adhesion of the mesogastrium to the splenic flexure of the large
intestine, forms the adult _lig. colico-lienale_, while a similar
adhesion between great omentum, transverse mesocolon and phrenic
parietal peritoneum just caudad of the spleen, gives rise to the
_colico-phrenic_ or _costo-colic "supporting" ligament_ of the spleen.
On the other hand, the ventral one of the two layers constituting the
gastro-splenic omentum and including between them the left
gastro-epiploic artery, is formed by the distal part of the primitive
left layer of the mesogastrium, while the dorsal layer of the same fold
is the portion of the primitive right layer beyond the spleen, which
has not been converted into secondary parietal peritoneum, but forms now
part of the ventral wall of the lesser peritoneal sac between the spleen
and the stomach (Fig. 181) (lig. gastro-lienale). Since, therefore, the
gastro-splenic omentum is a specialized part of the fully-developed
dorsal mesogastrium, and since we have seen that the great omentum is
formed directly by the excessive growth of this membrane caudad, it is
not difficult to understand why in the adult human subject the ventral
layer of the gastro-splenic omentum is directly continuous with the
ventral layer of the great omentum along the greater curvature of the
stomach to which both are attached. The dorsal layer of the
gastro-splenic omentum would, in the same way, be continuous with the
second layer of the great omentum, lining the ventral wall of the
omental bursa, if it were not for the fact that in the adult adhesions
usually obliterate the cavity of the bursa.
Fig. 182 shows the stomach, left kidney, spleen and splenic flexure of
the colon hardened in situ and removed from the body of a two-year-old
child. The great omentum has been divided along the line of adherence to
the transverse colon.
[Illustration: FIG. 182.--Part of the abdominal viscera of child, two
years old, hardened _in situ_ and removed from body. The great omentum
has been detached along the line of the transverse colon. (Columbia
University, Study Collection.)]
[Illustration: FIG. 183.--The same preparation with the spleen removed,
showing lines of peritoneal reflection on mesial surface of the organ.]
In Fig. 183 the spleen has been removed from the preparation by division
of its peritoneal and vascular connections, and is shown in its mesal
aspect (gastric and renal surfaces, intermediate margin and hilum). It
will be seen that the peritoneal reflections are arranged in the form of
two concentric elliptical lines. The two ventral lines form the
gastro-splenic omentum and correspond to the reflection of the
peritoneum from spleen to left end of stomach carrying the gastric
branches derived from the splenic artery. The third line from before
backwards results from the division of the secondary parietal peritoneum
of the lesser sac, covering splenic artery, and ventral surface of
pancreas and derived from the dorsal mesogastrium; while the most dorsal
fourth line represents the divided reflection of the peritoneum from the
renal surface of spleen to lateral border of left kidney and diaphragm
(lig. lieno-renale).
Between the second and third lines of peritoneal reflection appears the
portion of the mesal surface of the spleen in contact with and invested
by the extreme left end of the lesser peritoneal sac.
Fig. 184, taken from an adult human subject with the viscera hardened in
situ, shows the left or splenic extension of the lesser peritoneal
cavity.
[Illustration: FIG. 184.--Upper abdominal viscera of adult human
subject, hardened _in situ_, with liver and colon removed and stomach
turned up. (Columbia University, Study Collection.)]
=2. Development of the Pancreas.=--The pancreatic gland is derived from
the hypoblast of the enteric tube. The secreting epithelium and that
lining the ducts of the adult gland is formed by budding and
proliferation of the intestinal epithelium. The gland develops primarily
from two outgrowths which are at first separate and distinct from each
other.
1. The proximal and dorsal bud grows directly from the hypoblast lining
the duodenum immediately beyond the pyloric junction.
In embryos of 8 mm. (four weeks) (Fig. 185) it appears as a small
spherical outgrowth connected by a slightly narrower stalk with the
epithelial intestinal tube.
[Illustration: FIG. 185.--Pancreatic and hepatic buds of human embryo of
four weeks. (Kollmann.)]
2. The distal and ventral outgrowth is separated from the preceding and
is from the beginning closely connected with the similar embryonic
outgrowth from the enteric tube which is to form the liver. This portion
of the pancreas is, strictly speaking, derived primarily from the
epithelium of the primitive hepatic duct and not directly from the
duodenum. This primary arrangement of the gland, being formed of two
main collections of budding hypoblastic cells, corresponds to the adult
system of the pancreatic excretory ducts. The proximal or dorsal
outgrowth furnishes that portion of the head of the gland whose
excretory system terminates in the _secondary pancreatic duct_ or _duct
of Santorini_, while the distal (ventral) outgrowth includes within its
area the termination of the principal pancreatic duct or _canal of
Wirsung_, which is closely connected with the end of the common
bile-duct at the intestinal opening common to both (Figs. 186-187). The
method of union of the two pancreatic outgrowths and their respective
share in building up the adult gland explains the usual adult
arrangement of the excretory system and its variations.
[Illustration: FIG. 186.--Pancreatic buds of human embryo of five weeks.
(Kollmann, after Hamburger.)]
[Illustration: FIG. 187.--Pancreatic buds of human embryo of six weeks.
(Kollmann, after Hamburger.)]
In the embryo of five weeks (Fig. 186) the two portions have grown in
length. The dorsal or proximal outgrowth, developing between the layers
of the mesoduodenum, is at this time the larger of the two, composed of
a number of glandular vesicles clustered around the stalk represented by
the parent duct.
The distal or ventral pancreatic growth, connected with the liver duct,
is as yet small and presents only a few vesicular appendages. The duct
of this portion empties in common with the hepatic duct into the
duodenum.
In embryos of the sixth to seventh week (Fig. 187), the two glandular
outgrowths have become connected with each other at a point which
corresponds exactly to the divergence of the duct of Santorini from the
main pancreatic duct (canal of Wirsung) in the adult gland (Fig. 188).
[Illustration: FIG. 188.--Human adult. Corrosion of pancreatic and
common bile-ducts: ventral view. (Columbia University Museum, No.
1712.)]
The secondary pancreatic duct (of Santorini) of the adult corresponds to
that section of the proximal or larger embryonic outgrowth situated
between the intestine and the point where the two glandular diverticula
fuse with each other. Hence the canal of Wirsung in the adult is a
compound product. It includes the duct system developed, in connection
with the bile duct, in the head of the gland, forming the intestinal
termination of the main duct. Its distal body portion on the other hand
is derived from the duct system of the originally larger proximal
outgrowth, including the entire peripheral portion which has become
secondarily added to the duct of the ventral outgrowth to form together
with it the canal of Wirsung. On the other hand the proximal portion of
the duct system of this originally larger part becomes secondarily
differentiated as the duct of Santorini.
Fig. 188 shows the normal adult arrangement of the pancreatic and
biliary ducts in a corrosion preparation of the canal.
The duct of Santorini in this case opened by a separate orifice into the
duodenum above the common opening of the biliary and pancreatic ducts
(cf. p. 113).
=Explanation of Adult Arrangement of Human Pancreatic Ducts and Their
Variations Dependent Upon the Embryonic Development.=--The smaller
distal embryonic outgrowth is, as we have seen, from its inception in
close connection with the duodenal end of the common bile-duct (Fig.
185).
The proximal outgrowth, situated nearer to pylorus and derived directly
from the duodenal epithelium, is the larger and forms the greater part
of the bulk of the adult pancreas (Figs. 186, 187).
If, notwithstanding this primitive arrangement, the distal duct (canal
of Wirsung) appears as the main pancreatic duct in the adult, while the
proximal (duct of Santorini) is secondary, this depends upon a union of
the products of the two outgrowths in such a manner that the greater
part of the duct system of the proximal and larger portion is
transferred to the distal duct to form the adult canal of Wirsung, while
the smaller segment of the proximal duct, between its opening into the
duodenum and the point of fusion of the two outgrowths, forms the adult
secondary duct of Santorini. This duct opens usually into the duodenum
upon a small papilla situated about 2.5 cm. above the common duodenal
termination of the bile-duct and canal of Wirsung (papilla Vateri) (Fig.
193). The duct of Santorini usually tapers toward the duodenal opening
from its point of departure from the main duct, its caliber gradually
diminishing in the direction indicated, so that it is smaller at the
duodenal opening than at the point of confluence with the main duct
(Fig. 189). Hence the secretion from the proximal head portion of the
pancreas, conveyed by this duct and its tributaries, passes usually into
the main pancreatic duct and not directly into the intestine through the
duodenal opening of the duct of Santorini. The latter is, however, thus
enabled to vicariously take upon itself the conduct of the pancreatic
secretion in cases of obstruction or obliteration of the main duct
(calculi, ulcers, cicatrices, etc.). In these cases of obstruction of
the main duct the duct of Santorini enlarges and performs its functions.
[Illustration: FIGS. 189-192.--Series of schemata showing normal and
variant adult types of biliary and pancreatic ducts.]
[Illustration: FIG. 189.--Usual human adult type.]
[Illustration: FIG. 190.--Persistence of early embryonal type.]
[Illustration: FIG. 191.--Duct of Santorini has no duodenal orifice.]
[Illustration: FIG. 192.--Duct of Santorini forms the only pancreatic
duct. Separate duodenal openings of biliary and pancreatic ducts,
resulting from failure of development of distal embryonal pancreatic
bud.]
[Illustration: FIG. 193.--Mucous surface of human duodenum, showing
entrance of biliary and pancreatic ducts and diverticulum Vateri.
(Columbia University Museum, No. 1842.)]
Occasionally, without obstruction of the main duct, the duodenal opening
of the duct of Santorini is large, and the flow of secretion evidently
the reverse of the usual, _i. e._, directly into the intestine.
In other cases, also without pathological conditions, the proximal duct
is the larger of the two and serves as the principal channel of
pancreatic secretion, the canal of Wirsung being small. This is
evidently a persistence and further development of the early embryonic
relative condition of the two outgrowths above described (Fig. 190). On
the other hand the duct of Santorini may not open at all into the
duodenum, terminating in small branches which drain the proximal part of
the head of the gland (Fig. 191).
Schirmer has examined the arrangement of the pancreatic ducts in 105
specimens. In 56 of these the duct of Santorini passed from the main
duct into the duodenum, opening upon a papilla situated 2.5 cm. above
the common opening of the bile duct and canal of Wirsung.
In 19 the duct of Santorini was well developed but did not open into the
duodenum.
In but 4 cases the duct of Santorini formed the only pancreatic duct,
the lower opening being occupied by the bile duct alone (Fig. 192). We
may assume in these cases failure of development of the distal outgrowth
connected with the primitive hepatic bud, leaving only the proximal
duodenal outgrowth to form the entire adult gland.
Figs. 188 and 189 show the normal arrangement of the duodenal openings
of the biliary and pancreatic ducts.
Figs. 190 to 192 show schematically the variations in the relative
development and the adult arrangement of the pancreatic ducts.
=Diverticulum and Papilla Vateri.=--From what has been said regarding
the embryonic union of the distal pancreatic outgrowth with the hepatic
bud it will be easy to recognize the corresponding features in the
arrangement of the adult duodenal termination of the common bile-duct
and canal of Wirsung. The dilated interior of the duodenal papilla
(diverticulum Vateri) corresponds to the embryonic segment between the
intestinal opening of the primitive liver duct and the point when this
duct gives off the distal larger pancreatic outbud (Figs. 186, 187, 188,
193 and 194).
[Illustration: FIG. 194.--Adult human subject. Mucous membrane of
pyloro-duodenal junction and of duodenum. (Columbia University Museum,
No. 1840.)]
The union of the pancreatic and biliary ducts to form the recess of the
diverticulum Vateri, which then opens by a single common orifice into
the duodenum, is better marked in some of the lower vertebrates than in
man.
[Illustration: FIG. 195.--Duodenum, with entrance of pancreatic and
biliary ducts and well-developed diverticulum Vateri in the cassowary,
_Casuarius casuarius_. (Columbia University Museum, No. 1821.)]
Fig. 195 shows the proximal portion of the duodenum of the cassowary
(_Casuarius casuarius_) with the biliary and pancreatic ducts and the
diverticulum at their confluence in section.
The development of these two main digestive glands as diverticula from
the intestinal canal also explains the direct continuity of the mucous
membrane of their ducts with that lining the duodenum, a fact which is
of considerable importance in the pathological extension of mucous
inflammations from the intestine to the duct system of the glands.
=Development of the Pancreas in Lower Vertebrates.=--In the embryo of
the _sheep_ two pancreatic buds are found, but the duct of the dorsal
(proximal) outgrowth (duct of Santorini) subsequently fuses entirely
with the main duct.
In the _cat_ there are likewise two pancreatic outgrowths.
In the _chick_ three pancreatic buds are visible about the fourth day.
_Amphibia_ likewise present three embryonic pancreas buds.
The ventral (distal) outgrowth is double, the two portions proceeding
symmetrically from each side of the hepatic duct. The single dorsal
outgrowth is derived directly from the duodenal epithelium. Later on all
these outgrowths fuse to form the single adult gland.
_Fish_ also possess several (up to four) embryonic pancreatic
outgrowths.
Recently in human embryos of 4.9 mm. cervico-coccygeal measure three
pancreatic outgrowths have been observed, all entirely distinct from
each other, one dorsal, budding from the epithelium of the primitive
duodenum and two ventral, proceeding from the grooved gutter which
represents the primitive ductus choledochus at this period. In embryos
of from 6 to 10 mm. the two ventral outgrowths have already fused, hence
only two buds, a single ventral and a dorsal, are now encountered.[4]
[4] Iankelowitz, Arch. f. Mikr. Anat., Bd. 46, 1895.
These observations place the development of the human pancreas in line
with the triple pancreatic outgrowths, two ventral and one dorsal
characteristic of the majority of the lower vertebrates, which have been
hitherto carefully examined. The ventral or distal bud is probably
double in the majority of vertebrates. The two segments fuse, however,
so early that the derivation of the pancreas from a double outgrowth, as
described above for the human embryo, practically obtains. In forms in
which the adult gland presents a number of separate openings into the
duodenum (cf. p. 118), the development would probably show multiple
embryonic outgrowths from the intestinal hypoblast.
In any case the dorsal pancreatic bud appears to have developed in the
vertebrate series before the ventral outgrowth and to be hence
phylogenetically the older structure.
COMPARATIVE ANATOMY OF THE PANCREAS.
With the exception of _Amphioxus_ and probably also of the
_Cyclostomata_, the gland appears to be present in all vertebrates,
varying, however, much in size, shape and relation to the intestinal
tube. Usually it appears as an elongated, flattened, more or less
distinctly lobulated organ, in close apposition to the duodenum between
the layers of the mesoduodenum. In all forms in which the gland is found
it is connected with the post-gastric intestine and marks the beginning
of the midgut. In structure the gland is usually acinous, resembling the
salivary glands. It is well developed in the selachians, forming a
triangular body connected with the beginning of the midgut (Fig. 202).
In some instances the gland elements do not extend beyond the intestine
itself, but remain imbedded in the wall of the midgut, as in
_Protopterus_. In certain adult teleosts the pancreas is surrounded by
the liver (Fig. 196), in others it does not appear as a compact gland
but is distributed in the form of finely scattered lobules throughout
the mesentery between the two layers of this membrane. On account of
this concealed position of the gland it was formerly believed that the
adult teleosts did not possess a pancreas. The pyloric caeca (cf. p. 119)
found in these forms were consequently considered to be homologous with
the pancreas of the higher vertebrates.
[Illustration: FIG. 196.--A portion of alimentary canal of _Pleuronectes
maculatus_, the flounder, with pancreas attached to biliary duct and
concealed in the substance of the liver, which has been removed.
(Columbia University Museum, No. 1491.)]
In _Myxinoids_ a peculiar lobulated glandular organ is found imbedded in
the peritoneal coat of the intestine near the entrance of the bile-duct,
into which its lobules open separately. This organ possibly corresponds
to the higher vertebrate pancreas.
An organ which may represent a dorsal pancreas is also developed in
_Ammocoetes_ (larva of _Petromyzon_), but its exact homology is still
doubtful. It is possible that a true pancreas has not yet developed in
the cyclostomata. In _Amphioxus_ no trace of a pancreas is found. In all
other vertebrates the gland is present. In certain amphibians, as the
frog, the single pancreatic duct opens into the common bile duct (Fig.
197).
[Illustration: FIG. 197.--Pancreas and biliary ducts of _Rana
esculenta_, frog. (Wiedersheim, after Parker; both from Ecker.)]
In lacertilians and in some chelonians a lateral offshoot of the
pancreas is directed transversely and is adherent to the spleen. Fig.
113 shows the gland in _Chelydra serpentaria_. While the gland usually
has a single duct, yet two ducts are found in a number of animals (many
mammals, birds, chelonians and crocodiles). At times three ducts are
encountered, as in the chicken and pigeon.
The arrangement of the pancreatic duct system among mammalia presents
the following variations:
1. Mammals with _one_ pancreatic duct, either connected with the
bile-duct or entering the intestine independently:
Monkeys, most rodents (except the beaver), marsupials, carnivora (except
dog and hyena), many ungulates (pig, peccary, hyrax, etc.), most
ruminating artiodactyla.
(_a_) The pancreatic duct joins the common bile-duct before entering the
duodenum in the monkeys, marsupials, carnivora, in the sheep, goat and
camel.
The point of entrance of the combined duct into the intestine varies. In
some forms it is near the pylorus, in others at some distance from the
same. The common opening is situated 11/2" to 2" beyond the pylorus in
carnivora, and one foot behind the same point in the goat and sheep.
(_b_) The pancreatic duct does not join the bile-duct, but empties
separately into the intestine, in most rodents and in the calf and pig.
In the calf the pancreatic duct opens into the duodenum 15' beyond the
bile-duct and 3' beyond the pylorus.
In the pig the pancreatic opening is 5"-7" beyond that of the bile-duct
and 6"-8" behind the pylorus.
2. Mammals with _two_ pancreatic ducts, of which one usually joins the
bile-duct: perissodactyla (except the ass according to Meckel),
elephant, beaver, several carnivora, dog, hyena, and according to
Bernard the cat. In the perissodactyla the proximal of the two
pancreatic ducts empties, either combined with the bile-duct, or
separate from it, but very close to it, 3"-4" behind the pylorus. The
second distal duct is smaller and opens several inches further down.
[Illustration: FIG. 198.--_Necturus maculatus_, mud puppy. Dissection of
intestinal canal, liver, pancreas, and spleen, with blood-vessels
injected. (Columbia University Museum, No. 1863.)]
[Illustration: FIG. 199.--Pancreas and pancreatic ducts of rabbit.
(Nuhn.)]
In most rodents the pancreatic entrance is placed at some distance from
the pylorus. Fig. 199 shows the arrangement of the parts in the rabbit,
in which animal the main distal pancreatic duct empties at a distance of
13"-14" from the pylorus into the end of the duodenum, which intestine
forms a very long loop, while the biliary duct, receiving the smaller
proximal pancreatic duct, opens near the pylorus.
In the _beaver_ the smaller proximal duct joins the bile-duct or even
enters the duodenum anterior to the bile-duct, nearer the pylorus,
while the distal larger pancreatic duct opens into the intestine
16"-18" behind the biliary duct. Of the two ducts found in the dog
(Fig. 200) the smaller proximal either joins the bile-duct or opens
into the intestine close to it, 1" to 11/2" beyond the pylorus. The
larger distal duct opens into the duodenum 1" to 11/2" behind the biliary
duct. Fig. 201 shows the dog's stomach and proximal portion of the
duodenum in section. The proximal smaller pancreatic duct here joins
the biliary duct, and opens with it by a single orifice into the
duodenum. The distal larger pancreatic duct opens independently into
the intestine further caudad.
[Illustration: FIG. 200.--Abdominal viscera of dog, showing arrangement
of pancreatic ducts. (Nuhn.)]
[Illustration: FIG. 201.--Section of dog's stomach, and proximal portion
of duodenum, with entrance of biliary and pancreatic ducts. (Columbia
University Museum, No. 1822.)]
The parts in _Hyaena_ present a similar arrangement.
Bernard always found _two_ pancreatic ducts in the _cat_, one large
principal duct and a second smaller accessory duct. Of these, the one
situated nearest to the pylorus always united with the bile-duct. The
pancreatic duct thus joining the bile-duct was sometimes the main duct,
sometimes the accessory smaller duct.
Since the main function of the pancreatic juice is the conversion of
starch into sugar, the gland appears better developed in general in
herbivora than in carnivora, without, however, disappearing in the
latter. In fact it is of considerable size in the carnivora, because the
secretion also acts on the albuminous food substances and, though to a
lesser degree, on the fats.
PYLORIC CAECA OR APPENDICES.
In the _Cyclostomata_ and _Selachians_ the intestinal canal is in the
main free from caecal appendages, while a large portion of the tube is
provided with a special fold of the mucous membrane which projects into
the lumen of the gut (spiral valve). Fig. 43 shows the straight
intestinal tract with the spiral valve of the longer distal segment in a
cyclostome, _Petromyzon marinus_ or lamprey. In Figs. 202 and 203 the
selachian (shark) intestine is represented in two examples, while the
similar spiral valve in a Dipnoean or lung fish, _Ceratodus_, is seen in
Fig. 204.
[Illustration: FIG. 202.--Alimentary tract with spleen and pancreas of
_Squalus acanthias_, the dog-fish. (Columbia University Museum, No.
1405.)]
[Illustration: FIG. 203.--Alimentary canal of _Galeus canis_, dog-shark,
in section, showing spiral intestinal valve. (Columbia University
Museum, No. 1429.)]
[Illustration: FIG. 204.--Alimentary canal with spiral valve of
_Ceratodus forsteri_, the Australian lung-fish (Barramunda). (Columbia
University Museum, No. 1645.)]
On the other hand in the Ganoids and in many Teleosts longer or shorter
finger-shaped diverticula of the midgut are found immediately beyond the
pylorus in the region of the bile-duct.
These pouches or diverticula of the intestine form the so-called pyloric
caeca or appendices of these fish. They vary very much in length,
diameter and number in different forms.
Thus but a single diverticulum appears in _Polypterus_ and _Ammodytes_
(Fig. 205). _Rhombus maximus_ and _Echelus conger_ (Figs. 112 and 206)
have two, and the same number appear in _Lophius piscatorius_ (Fig.
207). Perca has three and the _Pleuronectidae_ have three to five.
[Illustration: FIG. 205.--Alimentary canal of _Polypterus bichir_.
(Columbia University Museum, No. 1823.)]
[Illustration: FIG. 206.--Alimentary tract of _Echelus conger_, Conger
eel. Stomach, mid- and end-gut, liver, and spleen. (Columbia University
Museum, No. 1430.)]
[Illustration: FIG. 207.--Stomach, duodenum, and pyloric caeca of
_Lophius piscatorius_, angler. (Columbia University Museum, No. 1824.)]
Fig. 208 shows the stomach and the beginning of the midgut with four
pyloric caeca in _Pleuronectes maculatus_, and Fig. 209 the same parts of
this animal in section.
[Illustration: FIG. 208.--_Pleuronectes maculatus_, window-pane. Stomach
and mid-gut with pyloric caeca and hepatic duct. (Columbia University
Museum, No. 1432.)]
[Illustration: FIG. 209.--_Pleuronectes maculatus_, window-pane. Stomach
and mid-gut with pyloric caeca, in section. (Columbia University Museum,
No. 1433.)]
[Illustration: FIG. 210.--_Paralichthys dentatus_, summer flounder.
Stomach and mid-gut with pyloric caeca and liver. (Columbia University
Museum, No. 1431.)]
Fig. 210 shows the stomach and midgut of _Paralichthys dentatus_, the
summer flounder, with three well-developed conical pyloric caeca. On the
other hand in some forms the number of pyloric appendices is enormously
increased, while their caliber diminishes. Thus 191 caecal appendages are
found surrounding the beginning of the midgut in _Scomber scomber_. A
well-marked example of prolific development of the pyloric appendages is
furnished by the common cod, _Gadus callarias_ (Fig. 211). The
appendices are in the natural condition bound together by connective
tissue and blood vessels, so as to form a compact organ, resembling a
gland (Fig. 211, A), and a similar arrangement is found in _Thynnus
vulgaris_ and _alalonga_, _Pelamys_ and _Accipenser_ (Fig. 212).
[Illustration: FIG. 211.--Pyloric caeca of _Gadus callarias_, codfish.
(Columbia University Museum, No. 1825.) _A._ Bound together by
connective tissue and blood-vessels.
_B._ Dissected to show confluence of caeca to form a smaller number of
terminal tubes of larger calibre entering the intestine.]
[Illustration: FIG. 212.--Alimentary canal of _Accipenser sturio_,
sturgeon. Numerous pyloric caeca are bound together to form a gland-like
organ. (Columbia University Museum, Nos. 1826, 1827, and 1828.)
In the smaller upper figure on the left the stomach, mid-gut, and
pyloric caeca are seen in section, showing the lumen of the latter and
their openings into the mid-gut.
The lower left-hand figure shows the mid- and end-gut in section, the
latter provided with a spiral mucous valve.]
In some Teleosts (Siluroidea, Labroidea, Cyprinodontia, Plectognathi and
Leptobranchiates) the appendices are entirely wanting. If there are not
more than 8-10 appendices they usually surround the gut and empty into
the same in a circle. In other cases they are arranged in a single line,
or in a double row, opposite to each other (Fig. 213). Each appendix may
open into the intestine independently, this especially where the number
is limited and the individual pouches large (cf. Figs. 206-210), or
several may unite to form a common duct.
[Illustration: FIG. 213.--_Melanogrammus aeglifinus_, haddock. Stomach,
mid-gut, and pyloric caeca; spleen. (Columbia University Museum, No.
1598.)]
Fig. 211, _B_, shows the appendices in _Gadus callarias_, the cod, freed
by dissection from the investing connective and vascular tissue. It will
be noticed that a considerable number of the tubes unite to form ducts
of larger caliber which open into the intestine, as seen in the section
shown in Fig. 214.
[Illustration: FIG. 214.--Stomach and mid-gut of _Gadus callarias_,
codfish, in section, showing intestinal openings of pyloric caeca.
(Columbia University Museum, No. 1830.)]
The pyloric appendices apparently have the same _significance_ as the
spiral intestinal fold of the Selachians, Cyclostomes and
Dipnoeans, _i. e._, the production of an increase in the area of
the digestive and absorbing surfaces of the intestinal mucous membrane.
Hence, as stated, the appendices and the spiral fold are found to vary
in inverse ratio to each other. Thus, for example, _Polypterus_ (Fig.
205) still has a fairly well developed spiral fold and only a single
pyloric appendix, while _Lepidosteus_, with but slightly developed
spiral fold, has numerous appendices. It was formerly held that the
pyloric caeca and the pancreas were mutually incompatible structures, and
that where one is found the other will be wanting.
Hence the appendices were regarded as homologous with the pancreas of
the higher forms. Recent observations have shown that this view is not
strictly and entirely correct, while at the same time it merits
consideration in several respects.
It is true that the pancreas in certain teleosts is now known to be
present although concealed from observation in the liver or scattered in
the form of small lobules between the layers of the mesentery (cf. p.
117), and that in a number of fish, such as _Salmo salar_, _Clupea
harengus_, _Accipenser sturio_, both the appendices and the pancreas are
encountered. Consequently these structures are not identical or even
completely homologous, since they occur side by side in the same form.
On the other hand Krukenberg has demonstrated that the appendices
pyloricae may function physiologically as a pancreas by yielding a
secretion which corresponds to the pancreatic juice in its digestive
action. In the majority of forms, however, they apparently merely
increase the intestinal absorbing surface, secreting only mucus.
These structures are nevertheless very interesting and instructive since
they furnish a perfect gross morphological illustration of the embryonal
stages just considered in connection with the development of the
mammalian pancreas. In the adult ganoid or teleost these blind
diverticula or pouches, varying greatly in shape, number and size,
protrude from the intestine immediately beyond the pylorus, usually in
close connection with the duodenal entrance of the bile-duct. Two or
more of these pouches may unite to form a common duct or canal opening
into the intestine.
These forms, therefore, offer direct and valuable morphological
illustration of the manner in which the pancreas of the higher
vertebrates develops, _i. e._, as a set of hollow outgrowths or
diverticula from the hypoblast of the primitive enteric tube. We can
establish a consecutive series, beginning with forms in which only one
or two diverticula are found, and extending to types in which the number
of the little cylindrical pouches reaches nearly two hundred and in
which they are bound together by connective tissue and blood vessels so
as to closely resemble the structure of a glandular pancreas. This is
one of the most striking instances in which the minute embryological
stages of the higher types are directly illustrated by the permanent
adult conditions found in the lower vertebrates. [The same statement, as
we will see, holds good in reference to the development of the _liver_.]
RELATION OF THE PANCREAS TO THE PERITONEUM.
The gland becomes very intimately connected with the serous layers of
the primitive dorsal mesentery. In order to clearly comprehend the adult
serous relations it is necessary to make a distinction between two
divisions or portions of the gland, based upon the altered relations of
the primitive dorsal mesentery which result from the differentiation of
the primitive simple intestinal tube into stomach and duodenum.
1. The primary outgrowth of the pancreatic tubules from the duodenum,
_i. e._, the part which is to form the "head" of the adult gland, is
situated between the two layers of that division of the primitive dorsal
mesentery which forms, after differentiation of stomach and small
intestine, the _mesoduodenum_. Coincident with the rotation of the
stomach, as we have seen, the duodenum and mesoduodenum exchange their
original sagittal position in the median plane of the body for one to
the right of the median line, balancing, so to speak, the extension of
the stomach to the left (Fig. 218).
The original right layer of the mesoduodenum and the right surface of
the duodenum now look dorsad and rest in contact with the parietal
peritoneum investing the right abdominal background and the ventral
surface of the right kidney and inferior vena cava. We have already seen
that the descending portion of the duodenum in man becomes anchored in
this position by adhesion of these apposed peritoneal surfaces. This
fixation includes, of course, the structures situated between the layers
of the mesoduodenum, _i. e._, the head of the pancreas. Consequently,
after rotation and adhesion, this portion of the gland turns one surface
ventrad, invested by secondary parietal peritoneum, originally the left
leaf of the free mesoduodenum, while the original right surface of the
gland has become the dorsal and has lost its mesoduodenal investment by
adhesion to the primary parietal peritoneum.
2. In order to understand the way in which the body and tail of the
pancreas obtain their final peritoneal relations it is necessary to
consider the development of the dorsal mesogastrium to form the omental
bag. If we regard the primitive dorsal mesentery in the profile view
from the left side (Fig. 215) it will be seen that, as already stated,
the mesoduodenum is the first part of the membrane to be invaded by the
pancreatic outgrowth from the intestine. Cephalad of the mesoduodenum
the primitive dorsal mesogastrium (Fig. 215) is seen to protrude to the
left and caudad to form, as already explained, the cavity of the omental
bursa of the retrogastric space ("lesser peritoneal sac"). The further
growth of the pancreas carries the developing gland from the district of
the mesoduodenum into that portion of the dorsal mesogastrium which now
forms the dorsal wall of the omental bursa (Fig. 216).
[Illustration: FIG. 215.--Cephalic segment of primitive mesentery in
schematic profile view.]
[Illustration: FIG. 216.--Schematic profile view of primitive
mesenteries with formation of omental bursa and developing spleen and
pancreas.]
[Illustration: FIG. 217.--_Sos scrofa foet._, foetal pig. Portions of
thoracic and abdominal viscera hardened in situ. (Columbia University
Museum, No. 1449.)]
This double relation of the pancreas to the mesoduodenum and to the
mesogastrium forming the omental bursa is well seen in foetal pigs
between two and three inches in length (Fig. 217).
The head portion of the pancreas is seen developing between the layers
of the mesoduodenum, while the body and tail of the gland, extending to
the left, grows between the two dorsal layers of the omentum bursa
towards the spleen, which organ is found connected with the left and
dorsal extremity of the omental sac derived from the dorsal
mesogastrium.
Before the growth of the great omentum is pronounced the continuity of
the mesoduodenum and dorsal mesogastrium can be readily appreciated
(Fig. 218). But after the redundant growth of the membrane has carried
the great omentum further caudad, the stomach and the two omental layers
attached to the greater curvature lie in front of the structures
included between the two dorsal layers and conceal them from view (Fig.
177).
[Illustration: FIG. 218.--Schematic view of primitive mesentery after
intestinal rotation and incipient formation of omental bursa from dorsal
mesogastrium.]
In sagittal sections to the left of the median line (Figs. 221 and 222)
the pancreas now appears included between the layers of the great
omentum near their point of departure from the vertebral column. (This
point is of course identical with the prevertebral attachment of the
primitive dorsal mesogastrium from which the omentum is developed.)
[Illustration: FIGS. 221, 222.--Schematic sagittal sections through
stomach, pancreas, great omentum, and left kidney.]
[Illustration: FIG. 221.--Before adhesion between dorsal and
mesogastrium and parietal peritoneum.]
[Illustration: FIG. 222.--After adhesion.]
The foregoing considerations will, therefore, lead to the conclusion
that the pancreas presents, in regard to its peritoneal relations, two
distinct segments:
1. The portion adjacent to duodenum (head and neck of the gland) is
developed between the layers of the mesoduodenum.
2. The distal portion of the gland, comprising the body and tail,
develops between the layers of the great omentum (dorsal segment),
derived from the primitive dorsal mesogastrium.
The transections of the dorsal mesogastrium shown in Figs. 180 and 181
will now have to be amplified by the introduction of the body of the
pancreas between the two layers of the vertebro-splenic segment, in
addition to the splenic artery (Figs. 219 and 220).
[Illustration: FIGS. 219, 220.--Schematic transection of dorsal
mesogastrium, pancreas, spleen, and stomach.]
[Illustration: FIG. 219.--Before adhesion to primitive parietal
peritoneum (arrow indicates the direction in which the adhesion takes
place).]
[Illustration: FIG. 220.--After adhesion and formation of secondary line
of transition between mesogastrium and parietal peritoneum (lieno-renal
ligament).]
Hence the following facts will be understood:
1. In the adult the splenic artery supplies a series of small branches
to the pancreas as it courses along the cephalic border of the gland on
its way to the spleen.
2. After the above-described adhesion of the original left leaf of the
dorsal mesogastrium (vertebro-splenic segment) to the parietal
peritoneum (Fig. 220), the dorsal surface of the body of the pancreas
loses its peritoneal investment and becomes attached by connective
tissue to the ventral surface of the left kidney.
3. The ventral surface of the body of the pancreas is in the adult lined
by peritoneum of the "lesser sac"; in other words the organ has
practically assumed a "retro-peritoneal" position, its ventral
peritoneal covering appearing now as the dorsal parietal peritoneum of
the retro-gastric space.
4. When completely developed the extreme end (tail) of the pancreas
extends to the left, following the splenic artery, until it touches the
mesal aspect of the spleen at the hilus.
5. If we, therefore, leave out of consideration for the moment the
transverse colon and duodenum, which will be taken up presently, and
confine ourselves to the arrangement of the stomach, pancreas and great
omentum, a sagittal section to the left of the median line would result
as shown in Fig. 222, after the adult condition of adhesion has been
established.
The same process of fixation, which resulted in the anchoring of
duodenum and head of pancreas, extends to the body of the gland and the
investing omentum. The peritoneum lining the original left, now the
dorsal surface of the gland, fuses with the primitive parietal
peritoneum covering the diaphragm and the left kidney. The main body of
the pancreas in the adult appears prismatic, giving a triangular
sagittal section. The dorsal surface is adherent to the ventral surface
of the left kidney; the ventral surface is covered by the secondary
parietal peritoneum (original right layer of mesogastrium) which lines
the dorsal wall of the retrogastric space and omental bursa (lesser
peritoneal sac). The great omentum now appears to take its dorsal point
of departure along the sharp margin which separates this ventral surface
of the pancreas from a third narrower surface directed caudad. This
surface, under the conditions which we are at present examining, would
be lined by the peritoneum continued onto it from the dorsal layer of
the great omentum. This peritoneum merges along the dorsal margin of
this caudal surface of the pancreas with the general parietal peritoneum
covering the left lumbar region and the caudal part of ventral surface
of the left kidney. We have, therefore, along this line a secondary
transition from visceral to parietal peritoneum, obtained by the
obliteration of the original visceral peritoneum investing the dorsal
surface of the pancreas before adhesion to the parietal peritoneum.
The pancreas assumes, therefore, in the adult a secondary
retro-peritoneal position, covered on its ventral surface by peritoneum
of the "lesser sac," while the caudal surface is lined by part of the
general peritoneal membrane of the "greater sac." The dorsal surface,
denuded of serous covering by obliteration, is adherent to the crura of
the diaphragm, the aorta and the ventral surface of the left kidney.
It is now proper to compare the conclusions just derived from the study
of the development of the human dorsal mesogastrium and connected
structures (spleen and pancreas) with the conditions presented by the
corresponding parts in one of the lower mammalia, which illustrate some
of the human embryonal stages. Here again the abdominal cavity of the
cat forms an instructive object of study.
The purpose of the following comparison should be twofold:
I. The mesogastrium, spleen and pancreas in the cat will clearly
illustrate the process of human development above outlined.
II. The abdominal viscera of the cat, if properly arranged, will enable
us to complete the consideration of this region by including the very
important relations which the transverse colon and third portion of the
duodenum bear in man to the great omentum and pancreas.
I. SPLEEN, PANCREAS AND GREAT OMENTUM OF CAT.
After opening the abdominal cavity it will be seen that the great
omentum can be lifted up, exposing the subjacent coils of the small and
large intestine, to which it adheres at no point. In other words the
entire dorsal surface of that part of the original mesogastrium which
forms the great omentum is free. It will be remembered that this is not
the case in the adult human subject, because here the dorsal surface of
the great omentum adheres to the transverse colon. Consequently in man
only that portion of the dorsal surface of the omentum can be seen which
extends between the transverse colon and the caudal free edge of the
membrane.
It will be noted that on the left side the spleen is connected by its
mesal surface to the omentum and through it with the stomach
(gastro-splenic omentum). In other words the cat illustrates the human
embryonal stage in which the spleen has appeared between the layers of
the dorsal mesogastrium at the extreme left or blind end of the
retrogastric pouch formed by the rotation of the stomach and elongation
of the mesogastric membrane, but _before_ the adhesion has taken place
between the original _left_ (now _dorsal_) layer of the vertebro-splenic
segment of the mesogastrium and the primitive parietal peritoneum
apposed to it (Fig. 219). Consequently the dorsal wall of the "lesser"
sac in the cat is still composed of the two layers of the free
vertebro-splenic segment of the mesogastrium, the primitive right (now
ventral) layer not having been converted, as is the case in man, into
secondary parietal peritoneum by adhesion of the original left (now
dorsal) layer to the primitive prerenal parietal peritoneum.
If we now examine the relation of the pancreas to the peritoneum we can
establish the following facts:
1. The portion of the gland adjacent to the duodenum, corresponding to
the "head" of the human organ, is included between the two layers of
the mesoduodenum. This membrane is free, so that the dorsal surface of
this portion of the pancreas is seen to be invested by the dorsal layer
of the mesoduodenum (Fig. 223). The duodenum and the mesoduodenum, the
latter containing the head of the pancreas between its layers, can be
turned toward the median line, so as to expose the entire ventral
surface of the post-cava and right kidney. To illustrate the arrangement
which is found in the adult human subject the descending duodenum and
pancreas should be allowed to fall over to the right so as to cover the
vena cava and the mesal part of the ventral surface of right kidney. The
adult human condition will now be produced if we assume that the
structures are fixed in this position by the obliteration of the apposed
serous surfaces, viz., the parietal peritoneum over kidney and vena cava
on the one hand and the right layer of the mesoduodenum and the dorsal
visceral peritoneum of the duodenum on the other.
[Illustration: FIG. 223.--Abdominal viscera of cat, hardened and removed
from body, showing relation of pancreas to mesoduodenum and dorsal
mesogastrium, respectively. (Columbia University Museum, No. 728.)]
2. In following out the pancreas of the cat in its entire extent,
proceeding to the left of the pylorus, it will be seen that the body of
the gland has extended between the two dorsal layers of the great
omentum (primitive dorsal mesogastrium) over to the spleen (Fig. 223).
Consequently the arrangement in the cat corresponds to the stage in the
human development shown in Fig. 219 and Fig. 221 in which adhesion of
the dorsal surface of the pancreas to the parietal peritoneum has not
yet taken place.
It will be quite easy to reconstruct from the facts as demonstrated by
the arrangement of the parts in the cat, the stage in the development of
the lesser peritoneal sac in which the dorsal wall of the space is still
formed by the proximal portion of the free dorsal mesogastrium (great
omentum) and the structures included between its two layers.
It must then become apparent that the entire serous surface which in the
adult human subject we regard as "parietal peritoneum of the lesser sac"
lining the dorsal wall of the retrogastric space is derived from what
originally was the right layer of the primitive sagittal dorsal
mesogastrium.
II. RELATION OF GREAT OMENTUM TO TRANSVERSE COLON, TRANSVERSE MESOCOLON
AND THIRD PART OF DUODENUM.
The second purpose to be accomplished by the study of the cat's
abdominal cavity at this stage is the correct appreciation of the adult
human conditions which are produced by areas of adhesion between the
transverse colon, transverse mesocolon and third part of the duodenum on
the one hand, and the dorsal mesogastrium, as great omentum, with the
structures contained between its layers, on the other.
Perform the manipulations of the large and small intestine in the cat
(see p. 67) which are required in order that the tract may be arranged
so that it will correspond in general to the topographical conditions
presented by the adult human subject. Locate the transverse colon and
mesocolon and the third portion of the duodenum produced by these
manipulations in imitation of the corresponding human structures. Then
proceed to plot the different parts out successively as they would
appear in a sagittal section (Fig. 224).
[Illustration: FIG. 224.--Schematic sagittal section of abdominal
viscera of cat, after the intestines have been rotated to correspond to
the adult human disposition, to show lines of peritoneal reflection
before adhesion.]
The following facts are to be noted and indicated on the plan of the
section:
1. The great omentum is free, hanging down from the greater curvature of
the stomach over the coils of intestine. Turning the omentum up it will
be observed that the body of the pancreas is included between the two
dorsal layers of the membrane.
2. The omentum, containing the pancreas, can be lifted up, exposing the
next succeeding structure, viz., the transverse colon and mesocolon. In
the cat the large intestine has been brought over, by the manipulations
above indicated, into a transverse position so as to represent the human
transverse colon and its mesocolon. It is therefore necessary to
remember that in this mammal the fixation of the transverse mesocolon in
the position indicated, by adhesion of ascending and descending mesocola
to the parietal peritoneum of the abdominal background, has not yet
occurred. Consequently the membrane must be held in the transverse
position in order to represent the human arrangement.
It will of course be observed that both surfaces of the transverse
mesocolon established in this way are free, not adherent to either
omentum or pancreas on the one hand, nor to the transverse duodenum on
the other.
3. The third or transverse portion of the duodenum is seen to be
attached by the distal part of the mesoduodenum, both of the serous
surfaces of the membrane being free. The duodenum having been brought
from right to left transversely across vertebral column and aorta,
underneath the superior mesenteric artery, the mesoduodenum, in the
segment corresponding to the transverse duodenum, exchanges its original
sagittal position for one in a horizontal plane, with cephalic
(primitive left) and caudal (primitive right) surfaces.
Now compare the above arrangement of the intestines and peritoneum in
the cat at once with the conditions presented in the adult human
subject, reserving certain intermediate stages, as exhibited by some of
the lower monkeys, for subsequent study.
[Illustration: FIG. 225.--The same figure indicating the areas of
adhesion and peritoneal obliteration (shaded) which produce the
arrangement of the adult human peritoneum.
1. Area of adhesion between opposed surfaces of great omentum and
transverse mesocolon and colon.
2. Area of adhesion between parietal peritoneum, duodenum, and caudal
layer of transverse mesocolon.
3. Adhesion of opposed walls of omental bursa leading to obliteration of
distal portion of pouch and producing "gastro-colic" ligament of adult
human subject.]
The examination of a similar sagittal section representing schematically
the adult human arrangement of the parts (Fig. 225) will reveal the
following points of difference as compared with the cat:
1. The peritoneum covering the dorsal surface of the pancreas, derived
from the primitive dorsal mesogastrium, has become adherent to the
parietal peritoneum, as previously described.
2. The cephalic surfaces of the transverse colon and mesocolon fuse with
the corresponding area of the dorsal (4th) layer of the great omentum
(dorsal mesogastrium).
In the human foetus in the 4th month the connection is still so slight
that the omentum can readily be separated from the transverse colon and
mesocolon.
Further dorsad the cephalic layer of the transverse mesocolon adheres to
the serous investment of the caudal surface of the pancreas, derived, as
we have seen, from the same dorsal layer of the great omentum.
3. The duodenum and mesoduodenum are fixed by adhesion on the one hand
to the parietal peritoneum, on the other to the caudal layer of the
transverse mesocolon near the root of that membrane.
4. The cavity of the omental bursa is usually obliterated in the adult
caudad of the level of the transverse colon, by adhesion of the apposed
surfaces of the two intermediate omental layers.
We have therefore three general areas of secondary peritoneal adhesion
to deal with (Fig. 225), viz.:
1. Dorsal layer of primitive } { Parietal peritoneum, cephalic
mesogastrium (great } { layer of transverse
omentum) including the } { mesocolon and cephalic surface
serous investment of the } to { of transverse colon.
dorsal and caudal surfaces } {
of the pancreas (Fig. 225, } {
1). } {
2. Transverse duodenum } { Parietal peritoneum and
and mesoduodenum (Fig. } to { caudal layer of transverse
225, 2). } { mesocolon.
3. Between the apposed serous surfaces of the intermediate omental
layers (Fig. 225, 3).
[Illustration: FIG. 226.--Schematic sagittal section of adult human
peritoneum].
These areas of adhesion result naturally in the production of secondary
lines of peritoneal transition as follows:
1. Figs. 225, 1; 226, 1, from the omentum, dorsal layer, to the caudal
surface of transverse colon, caudal layer of transverse mesocolon and
caudal surface of the pancreas.
2. Figs. 225, 2; 226, 2, from the caudal layer of the transverse
mesocolon across the transverse portion of the duodenum to the parietal
peritoneum and mesentery of the jejuno-ileum.
3. Figs. 225, 3; 226, 3, between the intermediate omental layers,
forming the secondary caudal limit of the lesser sac.
These changes consequently result in the rearrangement of the adult
human peritoneum in accordance with the following schema (Fig. 226):
We trace the peritoneum as the ventral or superficial layer of the great
omentum from the greater curvature of the stomach caudad around the
distal free edge of the omentum and cephalad, as the dorsal layer, to
the ventral border of the transverse colon. Here apparently this layer
is continued across the caudal surface of the large intestine and beyond
as the caudal layer of the transverse mesocolon. While this condition
obtains practically in the adult it is to be remembered that the
adhesion (at 1 in Fig. 225) prevents us from lifting the omentum away
from the colon, and that consequently the apparent continuity of the
dorsal layer of the great omentum with the caudal layer of the
transverse mesocolon is the result of this peritoneal fusion.
Near the dorsal attachment or "root" of the transverse mesocolon the
caudal layer of the membrane becomes continuous with the parietal
peritoneum investing the transverse portion of the duodenum on its
ventral aspect, which peritoneum in turn passes into the free mesentery
of the jejuno-ileum (Fig. 225, 2). Comparison with the previous figures
will show that we are dealing here with another area of secondary
peritoneal fusion.
If we now open the "lesser peritoneal cavity" by dividing the two layers
of the omentum attached to the greater curvature of the stomach (Figs.
225 and 226 in direction of arrow) we will apparently reach the upper or
cephalic surface of the transverse mesocolon. This layer can be followed
dorsad to the sharp border which separates the ventral and caudal
surfaces of the pancreatic body and the membrane can be traced thence
over the ventral surface of the gland to the diaphragm. (The connections
with the liver and stomach shown schematically in the diagram (Fig. 225)
are to be considered in detail subsequently.)
In the adult the peritoneal surface just described appears as the
cephalic layer of the transverse mesocolon and its continuation dorsad.
From the facts previously considered it will be at once apparent that we
are really dealing here with a part of the third layer of the primitive
omentum. We do not see the original cephalic layer of the transverse
mesocolon. This membrane has become fused with the fourth omental layer,
and its free serous surface obliterated in the stretch between the
vertebral column and the transverse colon. Hence the human adult
transverse mesocolon is apparently composed of _two_ layers; the
cephalic of these layers appears as peritoneum of the "lesser sac," in
conformity with its derivation from the original third omental layer
lining the interior of the omental bursa. The caudal layer, on the other
hand, is a part of the general or "greater" peritoneal membrane. The
entire adult transverse mesocolon, hence, comprises _four_ peritoneal
layers, of which only two remain as permanently free serous surfaces.
These differ in their derivation, the cephalic layer being a part of the
primitive dorsal mesogastrium (third omental layer), while the caudal
layer is part of the primitive mesocolon. Between these two layers of
the adult transverse mesocolon are included the two obliterated
embryonic membranes, _viz._, the fourth omental layer and the original
dorsal layer of the transverse mesocolon.
Caudad the two layers of the adult transverse mesocolon surround the
transverse colon and are continuous along the ventral margin of the
intestine with the layers of the great omentum. Toward the vertebral
column these layers again diverge. The cephalic layer, lining the
"lesser peritoneal cavity" invests the ventral surface of the pancreas.
The caudal layer continues over the caudal surface of the body of the
gland and transverse portion of the duodenum into the parietal
peritoneum and the free mesentery of the jejuno-ileum. Consequently the
returning layers of the great omentum are said to surround the
transverse colon and unite along the dorsal border of the intestine to
form the transverse mesocolon, which membrane is continued dorsad toward
the vertebral column as two layers. At the "root" of the transverse
mesocolon these layers are then described as diverging, the cephalic
passing up to line the ventral surface of the pancreas, while the caudal
continues over the caudal surface of the pancreas and third portion of
the duodenum into the parietal peritoneum and mesentery.
Wherever in this discussion of the transverse mesocolon the transition
between the caudal layer of the membrane and the "parietal" peritoneum
is referred to it is necessary to remember that this "parietal"
peritoneum is the _secondary_ investment of the abdominal background,
formed by the surface of the ascending and descending mesocolon which
remains free after the opposite surface and the vertical segments of the
large intestine have been anchored by adhesion to the _primary_ parietal
peritoneum (cf. p. 81, Fig. 158).
A summary at this point of the course of the dorsal mesogastrium, in
forming the great omentum and its subsequent connections, would show us
that the membrane first enlarges and descends towards the transverse
colon (Fig. 177). The omental bag is formed by the descending or
superficial segment (starting from the greater curvature of the
stomach), turned toward the observer in the figure, and by the ascending
or deep layer which is attached above to the dorsal abdominal wall, in
front of the vertebral column and aorta along the original line of
origin of the dorsal mesogastrium. Gradually growing and descending
further, the deep segment becomes attached to the transverse colon. It
also becomes connected, especially on the left side, with the
diaphragmatic peritoneum (phrenicocolic lig.), so that its original
starting point is no longer distinct. Finally the development of the
spleen and pancreas between the layers of the dorsal segment and their
subsequent connections obscure the original conditions.
Fig. 297 shows the primitive condition at a time when the connection
with the transverse colon and mesocolon has not yet taken place.
The omental bag or bursa epiploica develops in the region of the dorsal
mesogastrium and the viscera included between its layers, by changes in
the position and extent of the membrane which finally result in placing
a part of the right half of the primitive coelom cavity behind the
stomach. Up to the sixth week the line of origin of the dorsal
mesogastrium is from the mid-dorsal line of the abdomen. It deviates
from this origin to the left because the great curvature of the stomach
to which it is attached turns in this direction. On this account, and
because of the rapid growth of this portion of the mesogastrium, a bag
or space is formed behind the stomach. The entrance into this space is
situated to the right of the lesser curvature, behind the peritoneal
layers connecting the same with the liver (lesser or gastro-hepatic
omentum and hepato-duodenal ligament). The ventral wall of this space is
formed by the dorsal surface of the stomach itself, the dorsal wall by
the mesogastrium, turning to the left and presenting its original right
surface, now directed ventrad. The caudal limit of the retro-gastric
space is given by the turn of the mesogastrium to reach its attachment
along the greater curvature of the stomach (rudiment of great omentum).
The stomach, in contributing to produce these changes, passes from the
vertical to the oblique and finally into the transverse position. The
pylorus, formerly directed caudad, passes up and to the right. The
fundus develops and the original left side of the stomach becomes the
ventral, the right side the dorsal. The original dorsal border, now the
greater curvature, moving caudad, carries the attached dorsal
mesogastrium with it into its new position. The mesogastrium now pouches
to form the great omentum and rapidly enlarges. At first hardly
projecting beyond the greater curvature, it increases in length until it
forms a four-layered apron which hangs down as a loose sac over the
transverse colon and the coils of the small intestine (Fig. 177). In the
foetus of six months the cavity of the omental bag extends caudad as far
as the lower edge of the omentum. Later adhesions between the peritoneal
surfaces lining the interior of the bursa limit this extension.
The omental bursa is therefore formed by a ventral lamella, consisting
of two peritoneal layers, which hangs down from the greater curvature of
the stomach and passes around the caudal free edge of the omentum into
the double-layered dorsal lamella, which ascends, over the transverse
colon, to the original starting point of the dorsal mesogastrium along
the front of the vertebral column and aorta. Hence the "great omentum"
is originally composed of four layers of peritoneum.
The dorsal double lamella becomes adherent over a considerable area to
the parietal peritoneum of the dorsal abdominal wall. In this way the
organs developed between the two layers of the lamella obtain their
final fixed position. The pancreas becomes anchored and appears in the
adult as a "retro-peritoneal" structure, while the spleen is attached by
the "phrenico-lienal ligament" to the diaphragm.
In addition the dorsal omental lamella adheres in the fourth month to
the cephalic layer of the transverse mesocolon and to the transverse
colon.
Important illustrations of some of the intermediate stages in the human
development of this portion of the peritoneal tract are afforded by the
permanent adult conditions found in the abdominal cavity of some of the
lower primates, notably certain of the cynomorphous monkeys.
[Illustration: FIG. 227.--Abdominal cavity of _Macacus rhesus_, Rhesus
monkey, with the small intestine removed. (Columbia University Museum,
No. 63/1831.)]
Fig. 227 shows the abdominal cavity and disposition of the peritoneum in
a macaque monkey (_Macacus rhesus_, male) in the ventral view, with the
coils of small intestines removed and the omentum lifted up and
reflected upon the ventral body wall. The following important points of
difference from the arrangement in the _cat_ on the one hand, and in
_man_ on the other, are to be noted:
1. The large intestine presents the typical primate course, with an
ascending, transverse and descending colon. The ileo-caecal junction is
situated in the right iliac fossa.
2. The ascending and descending mesocola are still _free_, not having
become adherent to the parietal peritoneum along the dorsal abdominal
wall. Hence the caudal portions of the ventral surfaces of the two
kidneys are still covered by the _primitive parietal peritoneum_.
3. The great omentum is not yet adherent to the transverse colon and
mesocolon except for a short distance on the extreme right. At this
point the dorsal layer of the omentum has begun to contract adhesions to
the hepatic flexure of the colon and ascending colon, but the rest of
the transverse colon is free. Differing from the human arrangement is a
line of adhesion, uniformly present in these monkeys, between the dorsal
surface of the omentum along its right edge and the ventral
surface and right border of the _caecum_ and _ascending colon_, parts
which normally are not adherent to the omentum in man.
4. Hence in tracing the omentum to the left of the limited adhesion to
the hepatic flexure and ascending colon, _i. e._, nearly throughout the
entire extent of the transverse colon, we find the membrane passing
freely without adhesion over the cephalic surface of the transverse
mesocolon, which preserves its original free condition, independent of
the omentum. This arrangement is shown in the schematic sagittal section
in Fig. 230.
5. Tracing the omentum dorsad beyond the transverse colon and mesocolon
the pancreas is reached. Here we encounter the first extensive area of
omental or mesogastric adhesion. The omental peritoneum continues over
the ventral and caudal surfaces of the gland, investing the same, but
the dorsal surface has lost its serous covering and is anchored to the
ventral surface of the left kidney. Hence a sagittal section would show
the arrangement of the monkey's omentum as indicated in the schematic
Figs. 229 and 230. Making now a general comparison of the peritoneal
membrane of this animal with that of man, and of both with the preceding
common embryonal condition, we can draw the following conclusions,
indicated schematically in the five figures 228-232.
[Illustration: FIGS. 228-232.--Schematic sagittal sections of dorsal
mesogastrium and omental bursa, in man, monkey, and cat.]
[Illustration: FIG. 228.--Common embryonal condition, as illustrated by
cat, after rotation and formation of omental bursa.]
[Illustration: FIG. 229.--Area of adhesion between dorsal mesogastrium
and primitive parietal peritoneum in _Macacus_, producing condition
shown in Fig. 230.]
[Illustration: FIG. 230.--Arrangement of great omentum as found in
_Macacus rhesus_, shown without reference to areas of peritoneal
obliteration.]
[Illustration: FIG. 231.--Corresponding section of human adult
peritoneum showing, along dotted lines, area of peritoneal adhesion.]
[Illustration: FIG. 232.--Section showing human adult peritoneum without
reference to area of adhesion.]
1. The dorsal layer of the monkey's omentum in its proximal segment
behaves in the same way as in man, _i. e._, it becomes adherent to the
primitive parietal peritoneum down as far as the caudal margin of the
dorsal surface of the pancreas included between the primitive
mesogastric layers forming by their further growth the omental apron.
Therefore we find, as in the human subject,
(_a_) The pancreas adherent to the ventral surface of the left kidney.
(_b_) A portion of the ventral surface of the kidney, cephalad of the
pancreas, and the dorsal wall of the retrogastric (lesser peritoneal)
space lined by secondary parietal peritoneum derived from the third
layer of the omentum (original right layer of dorsal mesogastrium).
2. The monkey differs from adult man in the behavior of the dorsal
omental layer in relation to the cephalic surface of the transverse
mesocolon. The adhesion, which in the human subject fuses this layer
with the transverse colon and mesocolon, does not occur in the monkey.
Hence we have in this animal the following conditions:
(_a_) The omentum is non-adherent to the transverse colon and transverse
mesocolon.
(_b_) The caudal surface of the pancreas is lined by its original
mesogastric peritoneum.
(_c_) The transverse mesocolon is formed by the original two layers of
the primitive dorsal mesentery; hence its cephalic layer is not
"peritoneum of the lesser sac" as is the case in man.
(_d_) The caudal part of the ventral surface of the left kidney below
the pancreas, is covered by the original parietal peritoneum.
(_e_) Only one point or line of _secondary peritoneal transition_
exists, where the dorsal layer of the omentum in the adult becomes
continuous with the parietal peritoneum covering the caudal surface of
the pancreas and the ventral surface of the left kidney.
_Note_: In the schematic sections shown in Figs. 228 to 232 the
transverse colon is represented as far removed from the ventral surface
of the left kidney, in order to make the peritoneal lines of the
mesocolon more clear. Actually a sagittal section which would divide the
kidney would cut the transverse colon at its extreme left end, where it
turns close to the ventral surface of the left kidney and then follows
its lateral border to form the splenic flexure (Fig. 235). The caudal
part of the ventral surface of the left kidney in the adult human
subject is covered by the peritoneum which, as secondary parietal
peritoneum, is derived from the upper part of the right leaf (later
ventral leaf) of the descending mesocolon. Hence it should be remembered
that these diagrams present _combinations_ of sections. A section which
will show the full development of the transverse mesocolon is mesad of
the kidney; while a section through the kidney would be too far laterad
to show the transverse mesocolon.
[Illustration: FIGS. 233-235.--Series of schematic sagittal sections
through left kidney and adrenal, pancreas, and transverse colon, to show
development of adult peritoneal relations.]
[Illustration: FIG. 233.--Embryonic condition, as illustrated by cat,
after rotation of intestine. Pancreas free between dorsal layers of
great omentum. Transverse colon and mesocolon free. Kidney behind
primitive parietal peritoneum.]
[Illustration: FIG. 234.--Area of adhesion between: 1. Primitive
parietal peritoneum. 2. Mesogastrium forming great omentum. 3. Colon and
mesocolon.]
[Illustration: FIG. 235.--Adult human arrangement, shown without
reference to obliterated areas.]
Figs. 233, 234 and 235 show sagittal sections through the left kidney
with the adult arrangement of the peritoneum and colon and the embryonic
and adhesion stages leading to the same.
It will be observed that in all the schematic sections of the early
embryonic stages the two layers of the transverse mesocolon are shown
without dorsal attachment, as turning with the formation of a fold (Fig.
228 at x) into two layers descending ventrad of the parietal peritoneum.
This is because the dorsal attachment of the mesocolon is at this stage
still in the median line and would hence not be encountered by a
sagittal section through the kidney, and because the two layers of the
transverse mesocolon, immediately after rotation of the large intestine,
are still directly continuous with the two layers of the descending
mesocolon. That is to say, the cephalic layer of the transverse
mesocolon is continuous with the dorsal (originally the left) layer of
the descending mesocolon, and the caudal layer of the transverse
mesocolon with the ventral (originally the right) layer of the
descending mesocolon, which is, in the human subject, to assume
subsequently the character of parietal peritoneum after the dorsal layer
and the primitive parietal peritoneum have become obliterated by
adhesion (Fig. 235).
Fig. 236 shows this continuity of the descending and transverse
mesocolon as a permanent adult condition in the macaque. The fold of
transition between the two is seen at x in Fig. 228. It will be noticed
that the ventral surface of the left kidney, caudad of the adherent
pancreas, is covered by the primitive parietal peritoneum, corresponding
to section in Fig. 230.
RELATIONS OF SPLEEN AND OMENTUM IN _MACACUS RHESUS_.
The spleen in this animal has not contracted any extensive adhesions to
the parietal peritoneum (the phrenico-lienal lig. of anthropotomy is not
developed). It can be turned mesad so as to expose the lateral border
and an adjacent segment of the ventral surface of the left kidney, as
well as the dorsal surface of the tail of the pancreas at its tip, still
covered by mesogastric peritoneum. Hence in the monkey the adhesion of
the original vertebro-splenic segment of the mesogastrium, including the
pancreas, to the primitive parietal peritoneum is less complete than in
man.
[Illustration: FIG. 236.--Abdominal viscera of _Macacus cynomolgus_, Kra
monkey. (Columbia University Museum, No. 1801.)]
MEDIAN ATTACHMENT OF DESCENDING MESOCOLON AND ITS RELATION TO THE
MESOCOLON OF THE SIGMOID FLEXURE IN THE _MACAQUE_.
Fig. 236 shows the abdominal viscera, hardened in situ, of _Macacus
cynomolgus_, the Kra monkey, in the ventral view and from the left side.
The great omentum is lifted up, the pancreas is adherent to the ventral
surface of the left kidney, the caudal portion of which is covered by
the primary parietal peritoneum, which can be exposed by turning the
still free descending mesocolon mesad. The mesocolon retains its
primitive attachment to the median line ventrad of the large
prevertebral blood vessels. It is readily seen that adhesion between the
left leaf of this free descending mesocolon and the parietal peritoneum
down to the level of the iliac crest would produce the conditions found
in the human adult, with an attached descending colon and a free sigmoid
flexure; also that limited adhesion of the mesocolon of the sigmoid
flexure to the parietal peritoneum would produce, as previously
explained (cf. p. 97), the intersigmoid peritoneal fossa.
=2. Ventral Mesogastrium and Liver.=--The peritoneal reflections from
the stomach to the liver, and the arrangement of the membrane in
connection with the latter organ, remain for consideration.
Certain complicated adult conditions, encountered in this part of the
abdominal cavity, make it desirable to arrange the subject for purposes
of study under the following subdivisions:
I. The development of the liver and of its vascular system, and the
significance of the adult circulation of the liver and of the foetal
remnants connected with the organ.
II. The anatomy of the ventral mesogastrium and the changes produced in
the arrangement of the membrane by the development of the liver.
=I. A. Development of the Liver.=--The liver, like the pancreas, is
developed from the duodenum as an outgrowth from the hypoblast lining
the enteric tube. As we have previously noted, the first outgrowth of
the hepatic diverticulum is closely associated with the distal
pancreatic outbud; in fact the latter arises as a derivative from the
hepatic duct rather than as a distinct outbud from the intestinal tube.
(This close association of the hepatic duct with the pancreas is well
seen in the arrangement of the concealed pancreas of some teleosts (cf.
p. 117, Fig. 196).)
In point of time the liver is the first accessory structure to develop
by budding from the primitive alimentary canal, the pancreas and lung
following.
[Illustration: FIG. 237.--Longitudinal section of an embryo of
_Petromyzon planeri_, four days old. (Minot, after Kupffer.)]
In the primitive type of development, as seen in _Petromyzon_ and in the
Amphibia, the liver appears very early, as a diverticulum of the
embryonic intestinal tube, near its cephalic extremity, projecting on
the ventral aspect down into the mass of yolk-cells (Fig. 237). The
short stretch of the primitive alimentary canal cephalad of the hepatic
diverticulum corresponds to the foregut. With the development of the
heart the primitive foregut becomes divided into pharynx and
post-pharyngeal segment (oesophagus and stomach). The hepatic
diverticulum then lies immediately dorsad of the caudal or venous
extremity of the heart. Hence it is probable that the liver is an older
organ in the ancestral history of the vertebrates than the pharynx or
even the heart. The liver diverticulum lies in close connection with the
omphalo-mesenteric veins which return the blood from the yolk-sac to the
heart. In the course of further development, as will be seen below, the
liver comes into very intimate relations with the venous circulation.
In human embryos of 3.2 mm. the primitive hepatic duct appears as a wide
hollow pouch composed of hypoblast cells, growing between the two layers
of the ventral mesogastrium, which membrane, extending between the
ventral border of the primitive stomach and the ventral abdominal wall,
will be subsequently considered in detail. The liver, in developing
between the layers of the ventral mesogastrium, approaches very early
the _septum transversum_ or rudimentary diaphragm and becomes connected
with the same. A mass of mesodermal cells, derived from the mesogastrium
and from the primitive mesodermal intestinal wall surrounding the
hypoblastic lining of the tube, covers the caecal termination of the
primitive hepatic duct, forming the so-called embryonic _hepatic ridge_.
This mesodermal tissue accompanies the duct in its further growth and
branching, forming the connective tissue envelope, known in the adult as
the capsule of Glison. The primitive hepatic duct is directed cephalad
in the mesogastrium between the vitelline duct and the stomach (Fig.
101).
In embryos measuring 4.25 mm. the duct is 0.24 mm. long. Later (in
embryos of 8 mm.) the primitive single duct divides into two secondary
branches, indicating, even at an early stage, the adult arrangement of
the duct, as formed by the union of the right and left hepatic ducts
(Fig. 185).
The gall-bladder in embryos of this size (8 mm.) is a well-defined caecal
diverticulum, branching caudad from the main hepatic duct.
The vesicular mucous surface is thus derived from the enteric hypoblast
in the same way as the epithelial lining of the bile-ducts and
capillaries. The external muscular and fibrous coats of the gall-bladder
are developed from the mesoderm of the mesogastrium.
It is to be noted that at an early stage the gall-bladder is derived
from the main duct close to the intestine, the latter duct being very
short. Later on the common duct grows in length, making the liver more
and more a gross anatomical organ distinct from the intestine. The
cystic duct develops as the result of a similar increase in length of
the cystic diverticulum. The two principal secondary branches of the
hepatic duct give origin to sprouts or buds. These are derivatives of
the hypoblastic cells of the larger ducts and may from the beginning be
hollow, possessing a lumen continuous with that of the parent duct
(Selachians, Amphibians). In warm-blooded animals these sprouts are at
first solid, forming the s. c. _hepatic cylinders_, and only
subsequently become hollowed out with the further development of the
biliary duct system of the liver. The rapid growth of the organ leads to
a great increase in the number of the hepatic cylinders. They spread out
on all sides, finally coalescing with adjacent buds so as to form an
interlacing network whose meshes are filled by blood vessels. After the
hepatic cylinders have become canalized they preserve the same
arrangement, hence the resulting biliary capillaries of the adult form
an anastomosing network. Amphioxus and the amphibians have a single
hepatic outgrowth (Fig. 49).
In the Selachians the liver arises as a ventral outgrowth at the hinder
end of the foregut immediately in front of the vitelline duct, thus
bringing the liver from the beginning into close proximity with the
vitelline veins entering the heart. Almost as soon as formed the
outgrowth develops two lateral diverticula, opening into a median canal.
The two diverticula are the rudimentary lobes of the liver and the
median canal uniting them is the rudiment of the common bile-duct and
gall-bladder.
In the Teleosts the liver arises quite late (in the trout about the 25th
day) as a solid outgrowth from the intestinal canal close to the heart.
In the Amniota the liver arises in the same position as in the Anamnia,
but, at least in birds and mammals, shows its bifurcation almost, if not
quite, from the start. The two forks embrace between them the
omphalo-mesenteric or vitelline veins just before they empty into the
sinus venosus of the heart.
In the chick the liver appears between the 56th and 60th hour, the right
fork being always of greater length but less diameter than the left. The
hepatic outbud in the rabbit appears during the 10th day, and during the
11th day begins to send out branches.
In man, as above stated, the bud appears well marked in embryos of 3
mm.
[Certain adult variations make it appear possible that there are two
human embryonic hepatic buds, a cranial and a caudal, as is the case in
birds.]
=I. B. Comparative Anatomy of the Liver.=--The liver, phylogenetically a
very old organ, occurs in all vertebrates, for the caecal diverticulum of
the intestine of amphioxus (Fig. 49) has probably the significance of a
hepatic outbud.