LARGE AND SMALL INTESTINE, ILEO-COLIC JUNCTION AND CAECUM.
In considering the anatomy of the human caecum and vermiform appendix
many structural conditions are encountered which can only be correctly
appreciated in the light of the physiology of the digestive tract. The
alimentary canal as a whole affords one of the most striking examples of
the adaptation of structure to function. The constant renewal of the
tissues of the body by the absorption of nutritive material, the
necessary concomitant egestion of undigestible remnants, the variety in
the quantity and character of the food habitually taken, all serve to
explain why the alimentary canal responds structurally in individual
forms so completely to the physiological demands made upon it. This will
become especially evident if we extend our observations to include, in
addition to man, a review of the corresponding structures in
representative types of the lower vertebrates. Moreover the human caecum
and appendix are in part rudimentary structures, representing a portion
of the alimentary tract which, in accordance with altered conditions of
food supply and nutrition, has lost its original functional significance
to the organism and which consequently exhibits the wide range of
structural variation which characterizes the majority of rudimentary and
vestigial organs.
The vermiform process of man and the higher primates is thus one of
several indications given in the structure of the alimentary canal (the
character of the dentition is another example) which suggests that at
one phylogenetic period the forms composing the order or their immediate
ancestors were largely or entirely herbivorous, and hence possessed a
more extensively developed caecal apparatus than their omnivorous
descendants of to-day. In approaching, therefore, the study of the
human caecum and appendix we will at once meet with conditions which call
for the simultaneous physiological and morphological consideration of
the adjacent small and large intestine.
Again many of the structural peculiarities which characterize the human
caecal apparatus can only be correctly valued by comparison with the
corresponding parts in the lower vertebrates. Our inquiry will,
therefore, most profitably include the following subdivisions of the
subject:
I. General review of the functional and structural characters of the
vertebrate large and small intestine.
II. Systematic consideration of the ileo-colic junction and the
connected structures in the vertebrate series.
III. Phylogeny of the types of vertebrate ileo-colic junction and caecum,
and their probable lines of evolution.
IV. Detailed morphology of the human caecum and vermiform appendix.
I. GENERAL REVIEW OF THE MORPHOLOGY AND PHYSIOLOGY OF THE VERTEBRATE
INTESTINE.
We have seen that the intestinal tube of all vertebrates is the product
of two of the embryonal blastodermic layers, the entoderm and mesoderm.
The former furnishes the characteristic and cardinal elements of the
digestive tract, viz., the secretory and absorbing epithelium of the
mucous membrane and of the accessory digestive glands, the liver and
pancreas.
From the mesoderm, on the other hand, are derived the muscular and
connective tissue coats which surround the epithelial tube and
contribute to the thickness of the intestinal wall, as well as the blood
vessels and lymphatics. The alimentary canal separates from the yolk-sac
of the embryo by the development of cranial, caudal and lateral folds,
and at an early period communicates with the neural canal by the
primitive postanal gut (cf. p. 23). This connection subsequently becomes
lost. The oral and anal openings, by means of which the alimentary canal
communicates with the exterior, are formed _secondarily_ by entodermal
invaginations which finally break through into the lumen of the canal
(cf. p. 24).
At an early embryonic stage the alimentary canal appears therefore as a
straight cylindrical tube running cephalo-caudad in the long axis of the
body-cavity, and suspended by the primitive mesentery from the ventral
aspect of the chorda dorsalis.
In Amphioxus, the cyclostomata, certain teleosts, dipnoeans and lower
amphibians the canal remains permanently in this condition (cf. Fig.
310).
In the remaining vertebrates the uniform non-differentiated tube of the
embryo develops further and appears more or less distinctly divided into
a proximal segment, the _foregut_, a central segment, the _midgut_, and
a distal segment, the _hindgut_, or _endgut_. This differentiation of
the tube into successive segments is closely connected with the
character and quantity of the food habitually taken and with the method
and rapidity of its elaboration in the process of digestion, absorption
and excretion. In general the _foregut_ is formed by the segment which
succeeds to the oral cavity, and includes the _pharynx_, _oesophagus_ and
_stomach_. The _midgut_ is composed of a longer or shorter narrower tube
of nearly uniform caliber, the _small intestine_, which follows the
gastric dilatation. Even in forms in which the stomach is not distinctly
differentiated (cf. p. 40) the connection of the biliary duct with the
intestinal canal serves to separate the fore- and midgut. The _hindgut_
or _large intestine_ is usually separated from the preceding segment by
an external circular constriction, with a corresponding annular valve or
fold of the mucous membrane in the interior.
The beginning of the large intestine is marked in many forms by the
development of an accessory pouch or diverticulum, the _caecum_. The
hindgut extends from its junction with the midgut to the cloacal or anal
opening.
1. Midgut or Small Intestine.
The small intestine is the segment of the alimentary canal in which
digestion of the non-nitrogenous food substances takes place, and which
affords the necessary area of mucous surface for the absorption of all
digested matters. Consequently the character and habitual quantity of
the food here elaborated exerts a very marked influence on the _length_
of the small intestine, _i. e._, on the extent of the digestive and
absorbing surface represented by its mucous membrane.
The relative length of the small intestine in any individual form will
vary with both the quantity and volume of the food and with the rapidity
of the metabolic processes. Animals, in which digestion is rapid and the
usual food small in bulk and concentrated in its nutrient qualities,
have a relatively short intestine, while the canal is longer in forms
subsisting on food which is bulky and which demands considerable time
for its elaboration. Hence we find the relatively shortest intestine in
carnivora, the longest in herbivora, while the canal in omnivora
occupies an intermediate position in regard to its relative length.
The rapidity of tissue-metabolism also exerts a marked influence on the
length and development of this portion of the alimentary canal.
In the warm-blooded animals (mammals and birds) the tissue-changes are
constant and rapid and call for a large amount of nutrition within a
given period, while the metabolic processes in the cold-blooded
vertebrates (reptiles, amphibia and fishes) are slow, these animals
being able to go without food for long periods. Consequently in the
former class the small intestine is relatively much longer than in the
latter. Thus in certain birds and herbivorous mammals the small
intestine exceeds the total length of the body many times. This
influence of the quantity and quality of the food on the length of the
intestinal canal is seen, for example, very well during the course of
development in the frog.
The increase in the length of the intestine, and the consequent varying
degrees of coiling and convolution, are therefore secondary
acquired characters, depending for their development upon the habitual
kind and volume of the food. Additional provisions for increasing the
efficiency of the digestive apparatus are encountered throughout the
whole of the intestinal canal. In many forms the digestive secretory and
absorbing area is augmented by the development of folds, valves,
diverticula, villi and papillae from the mucous surface of the intestine.
Certain valves and folds, moreover, both control the direction in which
the contents of the canal move and retain the same for a longer period
in the intestinal segment in which they develop. Such folds appear
especially well developed in the intestine of certain cyclostomes,
selachians and dipnoeans (cf. Figs. 203 and 204). In these forms the
alimentary canal is usually short and straight, and the fold which has a
typical spiral course and projects far into the lumen of the gut,
evidently makes up to a very large extent for the shortness of the
intestine, serving the threefold purpose of
(_a_) Increasing the digestive and absorbing surface;
(_b_) Prolonging the period of retention of the food-substances in the
intestine, and thus increasing the time available for elaboration and
absorption.
(_c_) Regulating the direction in which the intestinal contents move.
We will see presently that a similar spiral mucous fold is also
encountered in some of the higher vertebrates, especially in the large
intestine. Examples are found in the well-developed spiral valve in the
caeca of the ostrich (Fig. 341), the similar fold in the large intestine
of many rodents (Figs. 387 and 388) and in the crescentic plicae of the
primate large intestine (Figs. 471, 472 and 473).
To the same physiological category belong the _digestive diverticula_ of
the intestinal canal, such as the pyloric appendices of the midgut found
in many teleosts and ganoids (cf. p. 119) and the varieties of caeca or
blind diverticula of the hindgut encountered throughout the vertebrate
series. They all function as reservoirs which increase the available
digestive and absorbing surface and which in addition are especially
adapted to retain substances difficult of digestion until the processes
of elaboration have been completed.
=Divisions of the Small Intestine.=--In the higher forms the segment of
the small intestine which succeeds to the pylorus is distinguished as
the _duodenum_. Into it empty the ducts of the liver and pancreas. In
some animals a pear-shaped enlargement is found, corresponding to the
_duodenal antrum_ of the human intestine, as the dilated proximal
portion of the duodenum immediately beyond the pylorus is called.
Examples of this condition are furnished by the cetaceans, several
rodents, the llama and dromedary and the koala (Phascolarctos).
In the birds and in many mammals (_e. g._, dog, Fig. 200, and many
rodents, as the rabbit) the duodenum is drawn out into a long loop
surrounding the pancreas.
=Structure of the Small Intestine.= =1. Secretory Apparatus.=--The
glands whose ducts empty into the small intestine and which furnish the
digestive secretions, may be divided as follows:
(_a_) Glands situated in the substance of the intestinal walls.
Two kinds are distinguished:
1. Brunner's glands, small acinous glands confined to the first part of
the duodenum.
2. Glands of Lieberkuehn, small caecal pits distributed not only over the
entire small intestine, but also found in the mucous membrane of the
large intestine.
These structures furnish the intestinal juice, whose chief function is
the conversion of starches into sugar, while aiding in carnivorous
animals also the digestion of proteid substances. The glands are hence
best developed in herbivora, while in carnivora they are present in
diminished numbers since they assist in the digestion of proteid
substances.
The size and number of these glands also depends on the amount of food
digested within a given period. When a considerable quantity of
digestive fluid is required, in order to obtain the nutritive value of
the food for the organism rapidly, the glandular apparatus of the
intestine will be well developed. Hence mammalia, in whom these
conditions exist, possess both the glands of Brunner and of Lieberkuehn.
In birds the latter structures are still found, but the former are
absent, while amphibia and fishes are devoid of both kinds. In these
lower vertebrates the typical intestinal glandular apparatus of the
higher forms is to a certain extent replaced by small pits and
depressions of the mucous membrane bounded by reticular folds.
(_b_) _Glands situated outside the intestinal tube, into whose lumen
their ducts empty._
The liver and pancreas fall under this head. The liver functions in the
digestion of the fatty substances of the food, while the secretion of
the pancreas converts the starches into sugars, and aids in the
digestion of albumenoid substances and to a lesser degree in that of the
fats.
=2. Absorbing Apparatus of Small Intestine.=--The mucous membrane of the
intestine is provided with villi, containing lymphatics, by whose agency
the digested matters are absorbed. These structures are developed in
individual forms in direct proportion to the ease and rapidity with
which the food is habitually absorbed.
The more rapid and complete the digestion is the greater will be the
amount of digested nutritive material at any given time in the
intestine, and the greater will be the development of the absorbing
structures. Hence the villi of the small intestine are especially large
and prominent in the carnivora, while they are small and insignificant
in herbivora and omnivora. Intestinal villi are found in nearly all
mammals and in many birds. Fig. 300 shows the villi of the intestinal
mucous membrane in a carnivore mammal (_Ursus maritimus_, polar bear)
and Fig. 301 the same structures in the cassowary (_Casuarius
casuarius_) in which bird they are very well developed. The villi are
not confined to the two highest vertebrate classes, but are encountered
also in the mucous membrane of the midgut in certain reptiles, notably
the ophidia.
[Illustration: FIG. 299.--Schematic sagittal section of abdomen to
illustrate the intestinal branches of the abdominal aorta. The gastric
and hepatic arteries are shown for the sake of convenience as arising
together from the coeliac axis (_B_), hence the left and right
gastro-pancreatic folds carrying these vessels appear fused at their
beginning, separating the hepatic recess of the lesser peritoneal sac
(_A_) from the cavity of the omental bursa (_C_).]
[Illustration: FIG. 300.--Small intestine, of polar bear, _Ursus
maritimus_. Mucous surface. (Columbia University Museum, No. 782.)]
[Illustration: FIG. 301.--Duodenum, with entrance of pancreatic and
biliary ducts and well-developed diverticulum Vateri in the cassowary,
_Casuarius casuarius_ (Columbia University Museum, No. 1821.)]
[Illustration: FIG. 302.--Mucous membrane of midgut of _Boa
constrictor_. (Columbia University Museum, No. 1837.)]
Fig. 302 shows the intestine mucous membrane of the boa constrictor with
well-developed and prominent villous projections.
Some birds, such as the snipes, herons and crows, have in place of the
intestinal villi projecting folds of the mucosa, often arranged in a
reticular manner. This type is prevalent in amphibia and fish (Fig. 112,
distal segment of midgut). Collections of lymphoid tissue in the mucous
membrane of the small intestine, either aggregated to form Peyer's
patches (Fig. 309) or as solitary follicles, are only found in the two
highest vertebrate classes, birds and mammals. In the former they appear
scattered over the surface of the mucous membrane, in the latter they
may be arranged in aggregations or regular rows. They are not secreting
structures, but their exact function in absorption is not known. This
lymphoid or adenoid tissue in certain forms is especially well developed
at the ileo-colic junction, forming the _lymphatic sac_ of some rodents,
as lepus (cf. Fig. 386). It is not confined to the small intestine, but
is found in the large intestine as well. At times it appears especially
well developed in the terminal portion of the caecal pouch (appendix), as
in _Lepus_ (Fig. 388).
The _valvulae conniventes_ or _valves of Kerkring_ of the human small
intestine serve to very greatly increase the secreting and absorbing
mucous surface. They are not found in this complete development in any
other mammals, although a very few forms present a transverse
reduplication of the intestinal mucosa and the circular layer of
muscular fibers. An example of this is found in the intestinal mucous
membrane of a species of antelope, shown in Fig. 303.
[Illustration: FIG. 303.--Mucous surface of small intestine of a species
of African antelope, _Cervicapra arundinacea_. (Columbia University
Museum, No. 1843.)]
The complete development of the valvulae conniventes in man is possibly
also associated with a mechanical function in connection with the
upright posture. In some mammalia, as in certain rodents and the
porpoise (Fig. 304), the mucous membrane of the terminal part of the
small intestine is thrown into _longitudinal folds_.
[Illustration: FIG. 304.--Mucous surface of small intestine of _Phocaena
communis_, porpoise. (Columbia University Museum, No. 1057.)]
[Illustration: FIG. 305.--Mucous membrane of mid-gut of _Lophius
piscatorius_, the angler, 18 cm. caudad of pylorus. (Columbia University
Museum, No. 1838.)]
The mucosa of the midgut in the lower vertebrates may be smooth, or
thrown into longitudinal folds, or the longitudinal folds may become
connected by oblique and transverse secondary folds, resulting finally
in a more or less complicated reticulated pattern of crypts. A very good
example of the type-form from which the more complicated conditions are
derived is seen in Fig. 305, showing the mucous membrane of the midgut
in _Lophius piscatorius_, the angler. The specimen is taken 18 cm. from
the pylorus and shows a ground plan of longitudinal plicae connected by
short oblique cross folds.
[Illustration: FIGS. 306, 307.--Intestinal mucous membrane of
logger-head turtle, _Thalassochelys caretta_. (Columbia University
Museum, No. 1839.)]
[Illustration: FIG. 306.--Mid-gut.]
[Illustration: FIG. 307.--End-gut.]
Fig. 306, showing the midgut mucosa of the loggerhead turtle
(_Thalassochelys caretta_), exhibits the same arrangement further
developed, resulting in a fine reticulated pattern, while in the endgut
of the same animal the primitive longitudinal folding is resumed (Fig.
307).
The number and size of the human valvulae conniventes vary in different
parts of the small intestine (Fig. 309). They are not usually found in
the beginning of the duodenum (Fig. 308), but commence in the second or
descending portion.
[Illustration: FIG. 308.--Adult human subject. Mucous membrane of
pyloro-duodenal junction and of duodenum. (Columbia University Museum,
No. 1840.)]
[Illustration: FIG. 309.--Adult human subject. Mucous membrane of small
intestine, showing arrangement of valvulae conniventes in successive
portions of jejunum and ileum. (Columbia University Museum, No. 1841.)]
They become very large and closely packed immediately beyond the common
entrance of the biliary and pancreatic ducts and continue to be well
developed and numerous throughout the rest of the duodenum and upper
half of the jejunum (Figs. 308 and 309). From here on they become
smaller, more irregular and less closely packed, and finally in the
terminal two feet of the ileum disappear almost entirely (Fig. 309).
This varying development of the valvulae is the chief reason why a given
segment of the ileum weighs less than a corresponding length of the
jejunum. This reduction in the fold-formation of the intestinal mucosa
toward the terminal portion of the midgut is seen even in the lower
vertebrates. Thus in Fig. 112, showing the entire intestinal tract of
the conger eel, _Echelus conger_, in section, the plicae of the mucous
membrane in the proximal segment of the midgut, at and immediately
beyond the entrance of the biliary duct, are prominent and numerous.
This redundancy continues but slightly reduced in the descending limb of
the intestinal loop, while in the ascending limb and up to the
ileo-colic junction the folds are reduced to a fine reticulated
meshwork. Beyond the ileo-colic valve plate, in the short endgut, the
mucosa again presents numerous pointed reduplications.
II. ENDGUT OR LARGE INTESTINE.
In this segment of the intestinal canal the undigested remnants of the
food are collected and evacuated from time to time.
In addition, the mucous membrane of the large intestine absorbs all
_digested_ material which is passed from the small intestine. While
digestion of food-substances will not be _inaugurated_ in the large
intestine, material already in the process of digestion and mixed with
the intestinal juices of the preceding segment, will be further
elaborated in this portion of the canal and the nutritive products
absorbed. This is especially the case in herbivora and omnivora, whose
food is bulky, containing a large amount of refuse material, and is
hence only slowly digested. On the other hand the food of the carnivora
is easily and rapidly digested and absorbed. After passing through the
small intestine hardly any substances remain which are capable of
digestion and absorption. Hence the large intestine of herbivora and
omnivora is uniformly longer in proportion to the small intestine than
it is in carnivorous animals. In the former this segment of the canal
functions as an accessory digestive apparatus and hence, as we will see,
often develops accessory structural modifications, such as a large caecum
and spiral colon, while in the latter it acts almost solely as a canal
for the evacuation of the indigestible remnants.
Again, the large intestine is better developed in the higher animals, in
mammalia and to a lesser degree in birds, in whom the functional demands
for nutrition are active and require that a relatively large amount of
food should pass through the digestive tract in a given time. On the
other hand in the lower cold-blooded vertebrates the metabolism is less
active, less food is taken and it is not necessary to secure all the
nutrient material contained in the same for the organism. The great
differences observed in the vertebrate series in regard to length, width
and structure of the large intestine depend upon these physiological
conditions. The divisions of the human large intestine into caecum,
ascending, transverse and descending colon, sigmoid flexure and rectum
are found only in the primates, and here not uniformly.
In the lower vertebrate classes the endgut is very short, corresponding
only to the pelvic segment of the Mammalia (rectum), a colon proper
being absent in these forms (cf. Fig. 112, _Echelus conger_). The human
large intestine exhibits a very characteristic structure. Throughout the
greater part of the colon the longitudinal muscular layer is mainly
disposed in the form of three bands or taenia (ligamenta coli). The canal
itself is longer than these bands, thus producing a folding of the walls
in the form of three rows of pouches (cellulae coli), in the intervals
between the bands. The pouches of each row are separated from each other
externally by constrictions, internally by projecting crescentic folds
(plicae coli) (Figs. 471, 472 and 474).
This arrangement of the large intestine is also found in the monkeys
(Fig. 473) and in certain Rodents (Fig. 474).
In other mammals the large intestine is smooth and cylindrical and the
longitudinal layer of muscular fibers uniform (Fig. 475).
In general the vertebrate large intestine is _wider_ than the small,
usually in the proportion of 5:1 or 6:1.
In some ruminant Herbivora, however, the great length of the colon leads
to a reduction of the caliber in certain segments so that the large
intestine does not exceed the width of the small, or even falls below
the same.
The _length_ of the large intestine, as in man, is usually much less
than that of the small intestine. As already stated this disproportion
is more marked in Carnivora than in Herbivora.
The ratio in length of the large to the small intestine is very low in
the Seals (1:14), and in several Edentates, as _Myrmecophaga_,
_Tamandua_ and _Bradypus_ (1:9-11).
In the carnivorous mammals it ranges 1:5-7.
In some of the ruminant Herbivora, as the cow and sheep, it is 1:4,
while in the deer, horse, certain Rodents (as _Lepus_ and _Cricetus_) it
reaches as high as 1:2 or 1:3.
The large intestine is usually relatively short in birds, reptiles,
amphibia and fish.
In the Cassowary the length of the large to the small intestine is 1:6.
In some of the birds of prey (eagle) the proportion falls as low as 1:68
or 70.
Exceptions to the general rule are furnished by some of the herbivorous
Cetaceans and by the Dugong (_Halicore_) in whom the large intestine is
twice as long as the small. Again in the Ostrich the large intestine in
one example measured 40', while the length of the small intestine was
only 22'. This unusual development of the large intestine indicates the
necessity of retaining the food, which is bulky and difficult of
digestion, until the elaboration is completed. The same significance
belongs to the enormously developed caeca of these birds (cf. p. 204).
The separation of the small and large intestine is marked externally by
the _caecum_, when present, and internally by the _valve of the colon_.
The details of the vertebrate ileo-colic junction will be considered in
the following pages.
II. SERIAL REVIEW OF THE ILEO-COLIC JUNCTION AND CONNECTED STRUCTURES IN
VERTEBRATES.
I. FISHES.
In the Cyclostomata there is no differentiation between the mid- and
hindgut. Fig. 310 shows the entire alimentary canal of _Petromyzon
marinus_, the lamprey, caudad of the pericardium.
[Illustration: FIG. 310.--_Petromyzon marinus_, lamprey. Entire
alimentary canal below pericardium. (Columbia University Museum, No.
1575.)]
In some fishes the midgut is differentiated from the hindgut by an
external circular constriction, corresponding to an annular projecting
fold of the mucosa in the interior which resembles the pyloro-duodenal
valve. There is no caecum, and the short hindgut empties into the
cephalic and ventral aspect of the cloaca. Fig. 311 shows the entire
intestinal tract of a Teleost fish, _Echelus conger_, the conger eel.
The midgut, provided at the beginning with a short globular pyloric
appendix (cf. p. 119), constitutes the longest individual segment of the
canal. The hindgut, separated from the preceding by aconstriction, is
very short and of large caliber. Fig. 312 shows the broad annular valve
with central circular opening which separates mid- and hindgut in the
interior, and Fig. 313 the ileo-colic junction in section in the same
animal.
[Illustration: FIG. 311.--_Echelus conger_, Conger eel. Alimentary
canal, stomach, mid- and hindgut, liver, and spleen. (Columbia
University Museum, No. 1430.)]
[Illustration: FIG. 312.--_Echelus conger_, Conger eel. Ileo-colic
junction, opened. (Columbia University Museum, No. 1434.)]
[Illustration: FIG. 313.--_Echelus conger_, Conger eel. Section of
mid- and end-gut, with ileo-colic junction, hardened. (Columbia
University Museum, No. 1349.)]
A similar type of ileo-colic junction is seen in other Teleosts, as in
_Gadus callarias_, the cod (Fig. 314), _Pleuronectes maculatus_, the
flounder (Fig. 315), and in some Ganoids, as _Accipenser sturio_, the
sturgeon (Fig. 212). In some Selachians an appendicular diverticulum,
the so-called "rectal" or "digitiform gland," is found connected with
the terminal segment of the gut near the entrance of the same into the
cloaca (Fig. 316).
[Illustration: FIG. 314.--_Gadus callarias_, cod-fish. Ileo-colic
junction. Intestine on each side opened, with probe passed through
constricted opening of ileo-colic valve. (Columbia University Museum,
No. 1260.)]
[Illustration: FIG. 315.--_Pleuronectes maculatus_, flounder.
Ileo-colon, opened to show ileo-colic valve. (Columbia University
Museum, No. 1493.)]
[Illustration: FIG. 316.--_Galeus canis_, dog-shark, male.
Genito-urinary tract and cloaca _in situ_. (Columbia University Museum,
No. 1694.)]
[Illustration: FIG. 317.--_Accipenser sturio_, sturgeon. Alimentary
canal. (Columbia University Museum, Nos. 1826, 1827, and 1828.)]
II. AMPHIBIA.
The alimentary canal is simple and usually comparatively short. There is
no caecal pouch. Differentiation of mid- and endgut is usually marked
externally by a constriction and by the increased caliber of the
terminal intestinal segment.
[Illustration: FIG. 318.--_Rana catesbiana_, bull-frog. Alimentary canal
and appendages. (Columbia University Museum, No. 1454.)]
[Illustration: FIG. 319.--_Necturus maculatus_, mud-puppy. Alimentary
canal and appendages. (Columbia University Museum, No. 1582.)]
[Illustration: FIG. 320.--_Cryptobranchus alleghaniensis_, hellbender.
Ileo-colic junction. (Columbia University Museum, No. 1711.)]
Fig. 318 shows the alimentary canal of the bull-frog, _Rana catesbiana_,
Fig. 319 that of a Urodele Amphibian, _Necturus maculatus_, and Fig. 320
the ileo-colic junction isolated in _Cryptobranchus alleghaniensis_, the
hellbender.
III. REPTILIA.
In reptiles a well-marked differentiation of small and large intestine
is the rule.
Four types of ileo-colic junction are encountered in this class:
1. The transition from small to large intestine is marked by the greatly
increased caliber of the latter and by an annular valve in the interior.
An example of this type is furnished by _Alligator mississippiensis_
(Fig. 321) and a similar form is encountered in some lizards, as
_Heloderma suspectum_, the Gila monster (Fig. 322).
[Illustration: FIG. 321.--_Alligator mississippiensis_, alligator.
Ileo-colon; dried preparation. (Columbia University Museum, No. 179.)]
[Illustration: FIG. 322.--_Heloderma suspectum_, Gila monster. (Columbia
University Museum, No. 69/1536.)]
2. The large intestine immediately beyond the ileo-colic junction
protrudes along the convex border to form a rudimentary lateral caecum.
This type is found in many Chelonians, _e. g._, in _Pseudemys elegans_,
the pond turtle (Figs. 323 and 324) and _Chelydra serpentaria_, the
snapping turtle (Fig. 325).
[Illustration: FIG. 323.--_Pseudemys elegans_, pond-turtle. (Columbia
University Museum, No. 1069.)]
[Illustration: FIG. 324.--_Pseudemys elegans_, pond-turtle. Ileo-colic
junction, opened. (Columbia University Museum, No. 1524.)]
[Illustration: FIG. 325.--_Chelydra serpentaria_, snapping turtle.
Intestinal canal, pancreas, and spleen. (Columbia University Museum, No.
1369.)]
3. The ileo-colic junction is provided with a well-developed sacculated
caecal pouch derived from the proximal segment of the colon and divided
in the interior by folds into several secondary compartments.
[Illustration: FIG. 326.--_Iguana tuberculata_, iguana. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
243.)]
This type is found in some of the phytophagous lizards, as _Iguana
tuberculata_ (Figs. 326 and 327). The small intestine of this animal is
of considerable length and of uniform caliber from the pylorus to the
ileo-colic junction. The caecum is a large sacculated pouch developed
chiefly along the convex border of the large intestine opposite to the
mesenteric attachment.
[Illustration: FIG. 327.--_Iguana tuberculata_, iguana. Mid-gut,
ileo-colic junction, caecum, and end-gut; dried preparation. (Columbia
University Museum, No. 178.)]
[Illustration: FIG. 328.--_Iguana tuberculata_, iguana. Ileo-colic
junction and caecum in section. (Columbia University Museum, No. 1321.)]
[Illustration: FIG. 329.--Drawing taken from same preparation (No. 1321)
to elucidate more clearly internal structure of caecal pouch.]
[Illustration: FIG. 330.--_Cyclura teres_, smooth-backed cyclura.
Ileo-colic junction and caecum in section. (Columbia University Museum,
No. 1523.)]
The examination of the interior of this pouch reveals a complicated
structure (Figs. 328 and 329). Fig. 330 shows the same structures in a
closely allied form, _Cyclura teres_. The entrance of the small
intestine is guarded by an annular sphincter valve, whose central
circular opening leads into a proximal compartment of the caecum. This
compartment is in turn separated from the remainder of the caecal pouch
by a second circular valvular fold with central opening. Beyond this
valve the interior of the pouch carries a number of crescentic mucous
folds, corresponding to the external constrictions between the caecal
sacculations. The entire pouch gradually diminishes in caliber and
finally passes with a sharp angular bend into the terminal portion of
the endgut. At this point the lumen of the canal is slightly diminished
by a sphincter-like thickening of the muscularis, producing an annular
projection of the mucous membrane. The entire caecal pouch appears as a
specialized segment of the large intestine interposed between the
termination of the midgut and the terminal portion of the endgut, which
latter is characterized by uniform caliber and increased thickness of
the muscular walls.
The highly developed and complicated structure of the caecal apparatus in
_Iguana_ and allied forms exemplifies very clearly the influence of
_vegetable_ food on the development of this segment of the alimentary
tract when compared with the simple type of ileo-colic transition found
in _carnivorous_ lizards, as _Heloderma_ (Fig. 322). _Iguana_ subsists
on leaves, fruits and other vegetable matter and the caecal pouch is
invariably found filled with the firmer and less digestible portions of
this food. These are undoubtedly retained in the pouch by the series of
valves and folds until digestion and absorption of all available
nutritive material forwarded from the small intestine is completed. On
the other hand _Heloderma_ lives almost entirely on bird eggs, a
concentrated and easily digested food. Consequently the ileo-colic
junction in this lizard is exceedingly simple and rudimentary, marked
merely by a slight external constriction, with an annular valve in the
interior, and an increase in the caliber of the short hindgut,
resembling the form found in many teleost fishes.
4. Finally in some Ophidians a typical lateral caecal pouch of
considerable dimensions is found connected with the endgut immediately
beyond the ileo-colic junction.
An example of this reptilian type, closely resembling the corresponding
structure in many Mammalia, is presented by _Eunectes marinus_, the
anaconda, shown in Figs. 331 and 332.
[Illustration: FIG. 331.--_Eunectes marinus_, anaconda. Mid- and
end-gut, with ileo-colic junction and caecum. (Columbia University
Museum, No. 72/1535.)]
[Illustration: FIG. 332.--_Eunectes marinus_, anaconda. Mid- and
end-gut, with ileo-colic junction and caecum laid open. (Columbia
University Museum, No. 1709.)]
IV. ILEO-COLIC JUNCTION IN BIRDS.
In the birds the length of the intestine is subject to great variations.
The canal is short in species subsisting on fruits and insects, long in
those feeding on seeds, plants and fish. The large intestine,
immediately beyond the ileo-colic junction, is provided typically with
two symmetrical lateral caeca which extend in some forms for a
considerable distance cephalad on each side of the small intestine to
which they are bound by peritoneal connections.
[Illustration: FIG. 333.--_Buteo harloni_, black hawk. Ileo-colic
junction and caeca. (Columbia University Museum, No. 1502.)]
[Illustration: FIG. 334.--_Phalacrocorax dilophus_, double-crested
cormorant. Ileo-colic junction and caeca. (Columbia University Museum,
No. 67/1534.)]
[Illustration: FIG. 335.--_Gallus bankiva_, hen. Ileo-colic junction and
caeca. (Columbia University Museum, No. 1486.)]
[Illustration: FIG. 336.--_Chen hyperborea_, Canada snow-goose,
Ileo-colic junction and caeca. (Columbia University Museum, No.
47/1448.)]
As a rule carnivorous birds have short and rudimentary pouches (Figs.
333 and 334), whereas they are long in herbivorous forms (Figs. 335 and
336). Some carnivorous birds, as _Corvus_, _Strix_, etc., have fairly
long caeca (Fig. 337). In the passerine birds living on seeds and
insects, the caeca are of considerable length as they are also in some of
the piscivorous divers (Figs. 338 and 339). They are long in the Ratitae,
and in the Lamellirostra, who feed chiefly on plants (Fig. 340).
[Illustration: FIG. 337.--_Bubo virginianus_, great horned owl.
Ileo-colic junction and caeca. (Columbia University Museum, No. 672.)]
[Illustration: FIG. 338.--_Urinator lumme_, red-throated loon.
Ileo-colic junction and caeca. (Columbia University Museum, No. 1001.)]
[Illustration: FIG. 339.--_Merganser serrator_, red-breasted merganser.
Ileo-colic junction and caeca. (Columbia University Museum, No. 1798.)]
[Illustration: FIG. 340.--_Casuarius casuarius_, cassowary. (Columbia
University Museum, No. 1799.)]
[Illustration: FIG. 341.--_Struthio africanus_, African ostrich.
Ileo-colic junction and caeca. (Columbia University Museum, No.
48/1573.)]
The enormously elongated caeca of the African ostrich contain a spiral
fold of the mucous membrane in the interior (Fig. 341).
In place of the usual double avian caecum a single pouch is found in a
few forms, namely in the Herons (Fig. 342).
[Illustration: FIG. 342.--_Ardea virescens_, green heron. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1132 a.)]
[Illustration: FIG. 343.--_Urinator lumme_, red-throated loon. Small
intestine with caecal pouch; the remnant of the vitello-intestinal duct.
(Columbia University Museum, No. 997.)]
[Illustration: FIG. 344.--_Urinator imber_, great northern diver. Small
intestine with caecal pouch; the remnant of the vitello-intestinal duct.
(Columbia University Museum, No. 78/1573)]
In some birds the small intestine is also provided with a caecal pouch,
the remnant of the vitello-intestinal duct corresponding in its
significance to the occasional mammalian diverticulum of Meckel (Figs.
343 and 344). (cf. p. 35.)
V. ILEO-COLIC JUNCTION, CAECUM AND VERMIFORM APPENDIX IN THE MAMMALIA.
I. Subclass: Ornithodelphia.
I. Order: Monotremata.
In many particulars the anatomical structure of these animals reveals a
close relationship to the Sauropsida. They represent the mammalian class
in its lowest stage of evolution.
The ileo-colic junction in all the existing forms is direct, without
angular bend at the entrance of the small into the large intestine. The
caecum is a long narrow pouch, slightly dilated at the extremity, derived
from the beginning of the colon and extending backward along the free
margin of the small intestine. The caecum resembles in its general shape
and structure the pouches seen in many birds, except that it is
unilateral, while the birds normally have two symmetrical caeca. The
caecum of _Ornithorhynchus anatinus_, the platypus or duck bill, is shown
in Figs. 345 and 346, and that of _Echidna hystrix_, the spiny
ant-eater, in Fig. 347. These two animals represent the two genera into
which the order is divided.
[Illustration: FIG. 345.--_Ornithorhynchus anatinus_, duck mole.
Ileo-colon and caecum. (Columbia University Museum, No. 1499.)]
[Illustration: FIG. 346.--_Ornithorhynchus anatinus_, duck mole.
Ileo-colon and caecum. (Columbia University Museum, No. 1500.)]
[Illustration: FIG. 347.--_Echidna hystrix_, spiny ant-eater. Ileo-colon
and caecum. (Columbia University Museum, No. 1501.)]
II. Subclass: Didelphia.
II. Order: Marsupialia.
The Didelphia are represented by numerous species, which are united by
certain common anatomical characters of the reproductive organs and
dentition to form the order of the Marsupialia. The individual species
included within this order differ widely in abit, food, mode of
locomotion, etc., and consequently exhibit great diversity in the
structure of the skeletal and muscular systems and of the alimentary
canal. With the exception of the Opossums inhabiting the new world, the
families composing the order are confined to the Australian continent
and the adjacent islands. In respect to the alimentary tract in general
and the ileo-colic junction in particular, we are evidently dealing with
a group of animals which, while they retain the common characters above
indicated as uniting them in the marsupial order, yet have in the
structure of their digestive canal adapted themselves to widely
divergent conditions of food supply and environment. Consequently within
the confines of this single and largely isolated order, we encounter
nearly all the types of caecum and ileo-colic junction which are found
among the remaining mammalia. The group in its individual
representatives has passed, so to speak, through the different stages of
development and evolution which, on a very much larger scale, are
exhibited by the remaining mammalian orders.
We can, independently of the systematic zoological classification,
arrange the forms composing the order under the following types:
1. _Forms with large well-developed simple caeca, of uniform caliber,
with rounded globular termination._
This type is encountered among the herbivorous Marsupials, such as the
opossums, kangaroos and wallabys. Fig. 348 shows the structures in
_Didelphis virginiana_, the common opossum, Fig. 349 in a small species
of opossum from Trinidad, and Fig. 350 the same parts in _Halmaturus
derbyanus_, the rock wallaby.
[Illustration: FIG. 348.--_Didelphis virginiana_, opossum. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1533.)]
[Illustration: FIG. 349.--_Didelphis sp._? opossum. Ileo-colic junction
and caecum. (Columbia University, Study Collection.)]
[Illustration: FIG. 350.--_Halmaturus derbyanus_, rock kangaroo.
Ileo-colic junction and caecum. (Columbia University Museum, No. 727.)]
2. _Forms with enormously developed sacculated caeca, coiled spirally,
with or without additional convolutions of the proximal colon; the
terminal portion of the caecal pouch diminishes in caliber to form a
pointed appendage._
This type of caecum characterizes the _Phalangeridae_ or Phalangers and
the _Phasolarctidae_.
[Illustration: FIG. 351.--_Trichosurus vulpinus_, vulpine phalanger.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1800.)]
[Illustration: FIG. 352.--_Phascolarctos cinereus_, koala. Ileo-colic
junction and caecum. (Drawn from preparation.) (Columbia University
Museum, No. 1528.)]
Examples are shown in Figs. 351 and 352, representing the structures in
_Trichosurus vulpinus_, the vulpine phalanger, and _Phascolarctos
cinereus_, the koala.
3. _Forms with simple caeca of moderate size._
The _Peramelidae_ or bandicoots.
Fig. 353 shows the ileo-colic junction, caecum and proximal segment of
the colon in _Perameles nasuta_, the bandicoot.
[Illustration: FIG. 353.--_Perameles nasuta_, bandicoot. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1481.)]
4. _Forms with sacculated short caeca, whose terminal portion is reduced
to constitute a typical vermiform appendix._
The caecum of the _Phascolomyidae_ or wombats, resembles, in its general
structure and in the presence of a typical vermiform appendix, very
closely the corresponding parts of the alimentary canal in man and the
anthropoid apes. Fig. 354 shows these structures in _Phascolomys
wombat_, the common wombat.
[Illustration: FIG. 354.--_Phascolomys wombat_, wombat. Ileo-caecum and
appendix. (Columbia University Museum, No. 1508.)]
5. _Forms with simple direct ileo-colic junction without caecum._
In the purely carnivorous Marsupials, comprising the family of the
_Dasyuridae_, the reduction of the caecal apparatus, foreshadowed by the
appearance of the distal rudimentary segment as a vermiform appendix in
the wombats, has been carried to the complete elimination of the pouch.
The ileo-colic junction in these animals is simple, marked externally by
a circular constriction and internally by an annular valve. The colon
forms a very short terminal segment of the alimentary canal. The parts
are shown in Fig. 355 in a typical representative of the family,
_Dasyurus viverinus_, the Tasmanian devil.
[Illustration: FIG. 355.--_Dasyurus viverinus_, dasyurus, Tasmanian
devil. Intestinal canal. Ileo-colic junction. (Columbia University
Museum, No. 1463.)]
The structural modifications encountered in the digestive tract of these
carnivorous Marsupials can properly be regarded as the result of their
habitual diet, and we will meet with analogous and identical examples of
caecal reduction in the typical Carnivores among the placental mammals
(cf. p. 212).
III. Subclass: Monodelphia.
III. Order: Edentata.
In all probability the Sloths, Ant-eaters and Armadillos composing this
order represent a highly specialized remnant of an ancient group now
largely extinct. In respect to the ileo-colic junction the Edentates may
be arranged in two groups which offer, within the limited number of
existing species, a very complete transitional series.
I. SYMMETRICAL TYPE OF ILEO-COLIC JUNCTION.
1. _Differentiation in caliber, with direct funnel-like transition of
small into large intestine. No caecum._
Beyond the ileo-colic junction the caliber of the large intestine
increases gradually. The terminal ileum is thus implanted into the apex
of a funnel formed by the proximal segment of the colon.
Examples of this type are furnished by _Myrmecophaga jubata_, the great
ant-eater (Fig. 356), and by _Choloepus didactylus_, the two-toed sloth
(Fig. 357).
[Illustration: FIG. 356.--_Myrmecophaga jubata_, great ant-eater.
Ileo-colic junction. (Columbia University Museum, No. 1519.)]
[Illustration: FIG. 357.--_Choloepus didactylus_, two-toed sloth.
Ileo-colic junction. (Columbia University Museum, No. 714.)]
2. _Abrupt demarcation of small and large intestine, with median
transition of ileum._
The caliber of the intestine enlarges rapidly immediately beyond the
ileo-colic junction. This form is derived from the preceding by the
substitution of the abrupt ileo-colic transition for the gradual
funnel-shaped development of the large intestine.
The type is illustrated by _Tatusia novemcincta_, the nine-banded
armadillo (Fig. 358), and is also found in two other armadillos,
_Tolypeutes_ and _Xenurus_.
[Illustration: FIG. 358.--_Tatusia novemcincta_, nine-banded armadillo.
Ileo-colic junction. (Columbia University Museum, No. 176.)]
3. _The colon on each side of the ileo-colic junction is prolonged
backward along the small intestine, forming two symmetrical lateral
globular colic caeca._
This type, which is to be regarded as a further development of the
preceding form, is also found in the armadillos. Fig. 359 represents the
structures in _Dasypus sexcinctus_, the six-banded armadillo, and a
similar arrangement of the parts exists in _Chlamydophorus_, another
species of armadillo.
[Illustration: FIG. 359.--_Dasypus sexcinctus_, six-banded armadillo,
Ileo-colic junction and caeca. (Columbia University Museum, No. 1478.)]
4. _The caecal pouches are more completely differentiated, communicating
with the colon by a constricted neck._
This results in an arrangement which recalls the structure of many avian
caeca (cf. Fig. 337) and is seen in the double caecal pouches of
_Cyclothurus didactylus_, the little ant-eater (Fig. 360).
[Illustration: FIG. 360.--_Cyclothurus didactylus_, little ant-eater.
Ileo-colic junction and caeca. (Columbia University Museum, No. 1512.)]
II. ASYMMETRICAL TYPE OF ILEO-COLIC JUNCTION.
The second general group of the Edentates is characterized by the
gradual development of a single lateral asymmetrical caecum, in place of
the median symmetrical ileo-colic transition found in the forms just
considered. The species composing this group thus form a link leading up
to the right-angled accession of ileum to large intestine and the
lateral caecum characteristic of most other mammalia.
[Illustration: FIG. 361.--_Manis longicauda_, long-tailed pangolin.
Ileo-colic junction; dry preparation. (Columbia University Museum, No.
95.)]
[Illustration: FIG. 362.--_Manis longicauda_, long-tailed pangolin.
Ileo-colic junction. (Columbia University Museum, No. 328.)]
1. This type may be considered as being inaugurated by the form of
ileo-colic junction found in the _Manidae_ or _Pangolins_, as illustrated
by Figs. 361 and 362, taken from the long-tailed pangolin, _Manis
longicauda_. There is no caecum and only a slight differentiation in
caliber between the small and large intestine. The gut in all the forms
examined shows a very characteristic bend at the ileo-colic junction,
being twisted into a figure of 8 and held in place by mesenteric folds.
[Illustration: FIG. 363.--_Arctopithecus marmoratus_, three-toed sloth.
Ileo-colic junction. (Columbia University Museum, No. 1479.)]
2. The second stage, illustrated by _Arctopithecus (Bradypus)
marmoratus_, the three-toed sloth (Fig. 363), reveals a distinct
increase in the caliber and convexity of the large intestine opposite
the mesenteric border immediately beyond the ileo-colic junction.
[Illustration: FIG. 364.--_Tamandua bivittata_, Tamandua ant-eater.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1590.)]
[Illustration: FIG. 365.--_Tamandua bivittata_, Tamandua ant-eater.
Ventral view of abdominal viscera from the left side. (Study Collection,
Columbia University.)]
3. This leads in the third stage, represented by _Tamandua bivittata_,
the Tamandua ant-eater (Figs. 364 and 365), to the development of a
distinct lateral caecal pouch. I have had no opportunity of examining the
structures in _Orycteropus_, but from the published descriptions[8] the
large caecum of this animal would form the final link in this series.
[8] Flower and Lyddecker, "Mammals, Living and Extinct," p. 209.
IV. Order: Sirenia.
Of the two living representatives of this remarkable mammalian order the
dugong (_Halicore_) is described as possessing a single caecum, while the
caecal pouch of _Manatus americanus_, the manatee, is symmetrically bifid
at the extremity (Fig. 366).
[Illustration: FIG. 366.--_Manatus americanus_, American manatee.
Ileo-colic junction and bifid caecum. (Columbia University Museum, No.
673.)]
V. Order: Cetacea.
In the majority of the whales the ileo-colic junction is simple without
caecum, as in _Physeter_, _Delphinus_, _Monodon_ and _Phocaena_ (Fig.
367).
[Illustration: FIG. 367.--_Phocaena communis_, porpoise. Ileo-colic
junction. (Columbia University Museum, No. 1007.)]
A few forms have a small caecal pouch.
VI. Order: Ungulata.
The intestinal canal, in conformity with the herbivorous habit of the
group, is uniformly provided with a large caecum, and in many forms the
proximal segment of the colon immediately beyond the ileo-colic junction
is more or less extensively coiled in a spiral manner. This arrangement
is, without doubt, to be regarded as being functionally accessory to the
caecal apparatus, in the sense of increasing very much the area of the
secreting and absorbing surface and of prolonging the period during
which food-substances, which are slow and difficult of elaboration, are
retained in this segment of the alimentary canal.
1. SUBORDER: ARTIODACTYLA.
A. NON-RUMINANTIA.--In the _Suidae_ the caecum is large and the spiral
colon well developed (Fig. 368).
[Illustration: FIG. 368.--_Sus scrofa foet_, foetal pig. Ileo-caecum and
spiral colon _in situ_. (Columbia University Museum, No. 1111.)]
In the peccaries (_Dicotyles_) the terminal portion of the caecal pouch
is reduced, constituting a centrally implanted appendage.
[Illustration: FIG. 369.--_Dicotyles torquatus_, collared peccary.
Ileo-colic junction, caecum, and spiral colon. (Columbia University
Museum, No. 60/1462)]
[Illustration: FIG. 370.--_Dicotyles torquatus_, collared peccary.
Ileo-colic junction and caecum, isolated. (Columbia University Museum,
No. 1464.)]
Fig. 369 shows the ileo-colic junction and spiral colon in _Dicotyles
torquatus_, the collared peccary, and Fig. 370 the caecum and appendix of
the same animal detached from the spiral colon. In the hippopotamus, on
the other hand, the caecum is said to be absent. If this is the case the
animal forms an isolated exception among the Ungulates.
B. RUMINANTIA.--The caecum is very large and the spiral coil of the colon
extensive.
Fig. 371 shows the caecum of _Capra aegagrus_, the Bezoar goat, detached
from the adjacent intestine, and illustrates the type of the ruminant
pouch, of considerable length and caliber, without terminal reduction.
The same parts in a preparation of _Boselaphus tragocamelus_, the
nilghai, are shown in Fig. 372.
[Illustration: FIG. 371.--_Capra aegagrus_, Bezoar goat. Ileo-colic
junction and caecum, isolated; dried preparation. (Columbia University
Museum, No. 194.)]
[Illustration: FIG. 372.--_Boselaphus tragocamelus_, nilghai. Ileo-colic
junction and caecum, isolated. (Columbia University, Study Collection.)]
Fig. 373 shows the caecum and ileo-colic junction, together with the
spiral coil of the colon, in _Bos indicus_, the zebu, and Fig. 374 the
same structures with a typical example of the spiral colon from _Cervus
sika_, the Japanese deer; Fig. 375 is taken from a preparation of the
parts in a foetal sheep, while Fig. 376 shows the spiral colon isolated
in _Oryx leucoryx_, the oryx.
[Illustration: FIG. 373.--_Bos indicus_, zebu. Ileo-colic junction,
caecum, and spiral colon. (Columbia University Museum, No. 676.)]
[Illustration: FIG. 374.--_Cervus sika_, Japanese deer. Ileo-colic
junction, caecum, and spiral colon. (Columbia University, Study
Collection.)]
[Illustration: FIG. 375.--_Oris aries foet_, foetal sheep. Ileo-colic
junction, caecum, and spiral colon. (Columbia University Museum, No.
1379.)]
[Illustration: FIG. 376.--_Oryx leucoryx_, oryx. Spiral colon, isolated.
(Columbia University, Study Collection.)]
2. SUBORDER: PERISSODACTYLA.
In the horse and the rhinoceros the caecum is very large and of uniform
caliber.
[Illustration: FIG. 377.--_Tapirus americanus_, American tapir.
Ileo-colic junction, caecum, and colon. (Columbia University Museum, No.
624.)]
In the American tapir (Fig. 377) the large caecum tapers at its
extremity, to form a species of rudimentary appendix, resembling
somewhat the corresponding structure in _Dicotyles_ (cf. Figs. 369 and
370). The proximal segment of the colon is bent on itself in the form of
an extensive loop with closely adherent limbs, illustrating an early
stage in the development of the ruminant spiral colon (cf. p. 233).
3. SUBORDER: HYRACOIDEA.
This suborder is formed by the single family of the _Hyracidae_. In
addition to their other isolated and puzzling structural peculiarities
the members of this small group present a most unusual arrangement of
the intestinal canal, which is unique among living mammalia. In addition
to a large sacculated caecal pouch, situated in the usual position at the
beginning of the colon, the large intestine is provided further on with
two supplementary elongated pointed conical pouches (Fig. 378).
[Illustration: FIG. 378.--_Hyrax syriacus_, hyrax or coney. Intestinal
canal, with ileo-colic junction, proximal ileo-colic caecum, and distal
paired colic caeca. (Columbia University, Study Collection.)]
This unique arrangement, which is not found in any other known
vertebrate, may possibly be led back to a type-form encountered in
certain saurians (see p. 234).
4. SUBORDER: PROBOSCIDEA.
The caecum of the elephant is a very large sacculated pouch with rounded
termination, illustrated in Fig. 379, taken from the Asiatic elephant.
[Illustration: FIG. 379.--_Elephas indicus_, Asiatic elephant.
Ileo-colic junction and caecum. (Columbia University Museum, No. 995.)]
VII. Order: Rodentia.
With the exception of a single group, the dormice (_Myoxus_) (Fig. 380),
the rodents possess a well-developed caecal apparatus.
[Illustration: FIG. 380.--_Myoxus avellanarius_, common dormouse.
Alimentary canal. (Columbia University Museum, No. 1466.)]
In some forms the terminal portion of the pouch is reduced so as to
constitute an appendix. Many of these animals, in addition to the caecum
proper, have the proximal colon elongated and coiled in a spiral, and in
some this part of the large intestine is provided in the interior with a
spiral mucous fold. This latter structure functions again to increase
the extent of the mucous absorbing surface and to prolong the retention
of substances undergoing slow digestion and absorption.
Typical examples of the capacious sacculated rodent caecum, with a
terminal pointed reduced segment, are afforded by _Castor fiber_, the
beaver (Figs. 381 and 382) and by _Erethizon dorsatus_, the Canadian
porcupine (Figs. 383 and 384). Figs. 385 and 386 show the ileo-colic
junction, caecum and appendix in _Lepus cuniculus_, the rabbit. The
interior of the caecal pouch and of the proximal segment of the colon is
provided with a complete spiral valve (Fig. 387), while the appendix is
differentiated by the histological character of its mucous membrane
which is studded with closely packed adenoid follicles (Fig. 388). A
similar aggregation of lymphoid tissue is found in this animal at the
ileo-colic junction forming the s. c. _saccus lymphaticus_ (Fig. 387).
[Illustration: FIG. 381.--_Castor fiber_, beaver. Ileo-colic junction,
caecum, and proximal colon; ventral view. (Columbia University Museum,
No. 1607.)]
[Illustration: FIG. 382.--_Castor fiber_, beaver. Ileo-colic junction,
caecum, and proximal colon; dorsal view. (Columbia University Museum, No.
1607.)]
[Illustration: FIG. 383.--_Erethizon dorsatus_, Canadian porcupine.
Ileo-colic junction, caecum, and colon; ventral view. (Columbia
University, Study Collection.)]
[Illustration: FIG. 384.--_Erethizon dorsatus_, Canadian porcupine.
Ileo-colic junction, caecum, and colon; dorsal view. (Columbia
University, Study Collection.)]
[Illustration: FIG. 385.--_Lepus cuniculus_, rabbit. Ileo-colic junction
and caecum. (Columbia University Museum, No. 1568.)]
[Illustration: FIG. 386.--_Lepus cuniculus_, rabbit. Ileo-colic junction
with saccus lymphaticus, isolated, (Columbia University, Study
Collection.)]
[Illustration: FIG. 387.--_Lepus cuniculus_, rabbit. Ileo-colic junction
with saccus lymphaticus. Caecum and proximal segment of colon opened to
show spiral mucous fold in interior. (Columbia University Museum, No.
1587.)]
[Illustration: FIG. 388.--_Lepus cuniculus_, rabbit. Caecum and appendix
inverted to show spiral fold and structure of mucosa. (Columbia
University Museum, No. 1588.)]
The coils of the proximal colon encountered in many rodents are well
seen in _Dasyprocta agouti_, the agouti (Figs. 389 and 390), which
animal also illustrates a type of caecum found in several members of the
order. The pouch here is large, sacculated, uncinate, without reduction
of the terminal portion.
[Illustration: FIG. 389.--_Dasyprocta agouti_, agouti. Ileo-colic
junction, caecum, and colon. (Columbia University Museum, No. 24/1576.)]
[Illustration: FIG. 390.--_Dasyprocta agouti_, agouti. Ileo-colic
junction, caecum, and colon. (Drawing based on preparation shown in Fig.
388.)]
[Illustration: FIG. 391.--_Lagomys pusillus._ Ileo-colic junction,
caecum, and colon. (After Pallas, from Oppel, "Lehrbuch d. Vergl.
mikrosk. Anat. d. Wirbelthiere," II., Jena, 1897, p. 577, Fig. 314.)]
[Illustration: FIG. 392.--_Arvicola pennsylvanicus_, field mouse.
Alimentary canal. (Columbia University Museum, No. 815.)]
[Illustration: FIG. 393.--_Mus decumanus_, white rat. Ileo-colic
junction, caecum, and colon. (Columbia University Museum, No. 1574.)]
The relatively enormous size of the caecum in the _Muridae_ is shown in
Fig. 392, representing the entire visceral tract of _Arvicola
pennsylvanicus_, the meadow mouse. The pouch in these animals is large,
smooth and of uniform caliber (Fig. 393).
In some the colon beyond the entrance of the small intestine is provided
with a spiral mucous valve (Fig. 394).
[Illustration: FIG. 394.--_Arvicola riparius_, meadow mouse. Ileo-colic
junction, caecum, and colon. (Columbia University, Study Collection.)]
In the single instance of _Myoxus_ among the rodents, the ileo-colic
junction is simple, without any caecal pouch (Fig. 380).
VIII. Order: Carnivora.