Caducibranchiata the anterior end of the enlarged rectum
lies very close to the distal extremity of the stomach, and
the gut, between these two regions, is greatly lengthened,
forming a loop with many minor loops borne at the periphery
of an expanse of mesentery, recalling the Meckelian tract of
birds and mammals. In the tadpole this region is spirally
coiled and is still longer relatively to the length of the
whole tract. In Hyla and Pipa there is a small caecum
comparable with the colic caecum of birds and mammals.
In Reptilia the configuration of the intestinal tract does
not differ much from that in Batrachia, the length and
complexity of the minor coils apparently varying with the general
configuration of the body, that is to say, in reptiles with a
long, narrow, and snake-like body the minor loops of the gut
are relatively short and unimportant, whilst in those with
a more spacious cavity, such as chelonians, many lizards and
crocodiles, the gut may be relatively long and disposed in many
minor coils. There is comparatively little differentiation
between the mid-gut and the gut in cases where the whole
gut is long; in the others the hind-gut is generally marked
by an increase of calibre. A short caecal diverticulum,
comparable with the colic caecum of birds and mammals, is
present in many snakes and lizards and in some chelonians.
In fishes, batrachians and reptiles the intestinal tract
is swung from the dorsal wall of the abdominal cavity by a
mesentery which is incomplete on account of secondary absorption
in places, and which grows out with the minor loops of the
gut. There are also traces, more abundant in the lower
forms, of the still more primitive ventral mesentery.
Intestinal Tract in Birds and Mammals.--There is no doubt but
that the similarity of the modes of disposition of the alimentary
tract in birds and mammals points to the probability of the
chief morphological features of this region in these animals
having been laid down in some common ancestor, although we
FIG. 4.--Intestinal Tract of Chauna chavaria.
c.c. Colic caeca. p.v. Cut root of portal vein.
d. Duodenum. r.v. Rectal vein.
g. Glandular patch. s. Proventriculus.
l.l. Meckel's tract. y. Meckel's diverticulum, or
l.i. Hind-gut. Yolk-sac vestige.
have not yet sufficient exact knowledge of the gut in Pisces,
Batrachia and Reptilia to find amongst these with any
certainty the most probable survival from the ancestral
condition. The primitive gut must be supposed to have run
backwards from the stomach to the cloaca suspended from the
dorsal wall of the body-cavity by a dorsal mesentery. This
tract, in the course of phylogeny of the common ancestors
of birds and mammals, became longer than the straight length
between its extreme points and, consequendy, was thrown into
a series of folds. The mesentery grew out with these folds,
but the presence of adjacent organs, the disturbance due to
the outgrowth of the liver, and the secondary relations brought
about between different portions of the gut, as the out-growing
loops invaded each other's localities, disturbed the primitive
simplicity. Three definite regions of outgrowth, however,
became conspicuous and are to be recognized in the actual
disposition of the gut in existing birds and mammals. The
first of these is the duodenum. In the vast majority of
birds, and in some of the simpler mammals, the portion of
the gut immediately distal of the stomach grows out into a
long and narrow loop (fig. 4, d), the proximal and distal
ends of which are close together, whilst the loop itself may
remain long and narrow, or may develop minor loops on its
course. In mammals generally, however, the duodenum is complex
and is not so sharply marked off from the distal portion
of the gut as in birds. The second portion is Meckel's
tract. It consists of the part generally known as the small
intestines, the jejunum and ileum of human anatomy, and
Fig. 5,--Intestinal Tract of Canis vulpes. S, cut
end of duodenum; C, caecum; R, cut end of rectum.
stretches from the distal end of the duodenum to the caecum or
caeca. It is the chief absorbing portion of the gut, and in nearly
all birds and mammals is the longest portion. It represents,
however, only a very small part of the primitive straight
gut, corresponding to not more than two or three somites of the
embryo. This narrow portion grows out to form the greater part
of what is called the pendent loop in mammalian embryology.
Its anterior or proximal end lies close to the approximated
Fig. 6.--Intestinal Tract of Macropus bennetti. S,
cut end of duodenum; R, cut end of rectum; C, caecum;
C2, accessory caecum; C.L, colic loop of hind-gut.
proximal and distal ends of the duodenal loop, whilst its
distal end passes into the hind-gut at the colic caecum or
caeca. In the embryos of all birds and mammals, the median
point of Meckel's tract, the part of the loop which has
grown out farthest from the dorsal edge of the mesentery, is
marked by the diverticulum caecum vitelli, the primitive
connexion of the cavity of the gut with the narrowing stalk
of the yolk-sac (fig. 4, y.) Naturally, in birds where the
yolk-sac is of great functional importance this diverticulum
is large, and in a majority of the families of birds persists
throughout life, forming a convenient point of orientation. In
mammals, no doubt in association with the functional reduction
of the yolk-sac, this diverticulum, which is known as Meckel's
diverticulum, has less importance, and whilst it has been
observed in a small percentage of adult human subjects has not
been recognized in the adult condition of any lower Mammalia.
In birds, Meckel's tract falls into minor folds or loops,
the disposition of which forms a series of patterns
remarkably different in appearance and characteristic of
different groups. In fig. 4 an extremely primitive type is
represented. In mammals Meckel's tract remains much more
uniform; it may be short, or increase enormously in length,
but in either case it falls into a fairly symmetrical shape,
suspended at the circumference of a nearly circular expanse
of mesentery. Where it is short it is thrown into very
simple minor loops (figs. 5, 6 and 7); where it is long,
these minor loops form a convoluted mass (figs. 8 abd 9).
FIG. 7.--Intestinal Tract of Tapir. S, cut end of
duodenum; R, cut end of rectum; C, caecum; CL, colon.
The third portion of the gut should be termed the hind-gut and
lies between the caecum or caeca and the anus, corresponding to
the large intestines, colon and rectum of human anatomy. It is
formed from a much larger portion of the primitive straight gut
than the duodenum and Meckel's tract together, and its proximal
portion, in consequence, lies very close to the origin of the
duodenum. In the vast majority of birds, the hind-gut in the
adult is relatively extremely short, often being only from
Fig. 8.--Intestinal Tract of Giraffe. S, cut end of duodenum;
R, cut end of rectum; C, caecum; P.C.L, post-caecal
loop; S.P, spiral loop; SF, third loop of hind-gut.
one-eighth to one-thirtieth of the whole length of the
gut. A certain number of primitive birds, however, have
retained a relatively long condition of the hind-gut (fig.
4), the greatest relative length occurring in struthious
birds, and particularly in the ostrich, where the hind-gut
exceeds in length the duodenum and Meckel's tract together.
Mammals may be contrasted with birds as a group in which
the hind-gut is always relatively long, sometimes extremely
long, and in which, moreover, there is a strong tendency to
differentiation of the hind-gut into regions the characters
of which are of systematic importance. The first region is
the colon, which forms a very simple expansion in mammals
such as Carnivora (fig. 5), where the whole hind-gut is
relatively short, or a series of simple loops in mammals
in which the whole gut has a primitive disposition (e.g.
Marsupialia, fig. 6). In the odd-toed Ungulata, the colon
(fig. 7) forms an enormously long loop, the two limbs of which
are closely approximated and the calibre of which is very
large. In Ruminantia (fig. 8) the colon is still more
highly differentiated, displaying first a simple wide loop,
then a complicated watchspring-like coil, and finally a very
long, irregular portion. In the higher Primates (fig. 9)
it forms one enormous very wide loop, corresponding to the
ascending, transverse and descending colons of human anatomy,
and a shorter distal loop, the omega loop of human anatomy.
Other striking patterns are displayed in other mammalian groups.
The second region of the hind-gut is usually known as the rectum.
and although it is sometimes lengthened it is typically little longer
than the portion of the primitive straight gut that it represents.
FIG. 9.--Intestinal Tract of Gorilla. S, cut end of
duodenum; R, cut end of rectum; C, vermiform appendix of
caecum; X, X2, X3, cut ends of factors of the portal vein.
Adaptations of the Intestinal Tract to Function.--The chief
business of the gut is to provide a vascular surface to which
the prepared food is applied so that the nutritive material
may be absorbed into the system. Overlying and sometimes
obscuring the morphological patterns of the gut, are many
modifications correlated with the nature of the food and
producing homoplastic resemblances independent of genetic
affinity. Thus in birds and mammals alike there is a direct
association of herbivorous habit with great relative length of
gut. The explanation of this, no doubt, is simply that
the vegetable matter which such creatures devour is in a
form which requires not only prolonged digestive action,
but, from the intimate admixture of indigestible material,
a very large absorbing surface. In piscivorous birds and
mammals, the gut is very long, with a thick wall and a
relatively small calibre, whilst there is a general tendency
for the regions of the gut to be slightly or not at all
defined. Fish, as it is eaten by wild animals, contains a
large bulk of indigestible matter, and so requires an extended
absorbing surface; the thick wall and relatively small
calibre are protections against wounding by fish bones. In
frugivorous birds the gut is strikingly short, wide and simple,
whilst a similar change has not taken place in frugivorous
mammals. Carnivorous birds and mammals have a relatively short
gut. In birds, generally, the relation of the length and
calibre of the gut to the size of the whole creature is
striking. If two birds of similar habit and of the same
group be compared, it will be found that the gut of the larger
bird is relatively longer rather than relatively wider.
The same general rule applies to Meckel's tract in mammals,
whereas in the case of the hind-gut increase of capacity is
given by increase of calibre rather than by increased length.
The Colic Caeca.--These organs lie at the junction of
the hind-gut with Meckel's tract and are homologous in
birds and mammals although it happens that their apparent
position differs in the majority of cases in the two
groups. In most birds, the hind-gut is relatively very
short, and the caecal position, accordingly, is at a very
short distance from the posterior end of the body. whereas
in most mammals the hind-gut is very long and the position of
the caecum or caeca is relatively very much farther from the
anus. Next, in most birds, the caeca when present are
paired, whereas in most mammals there is only a single
caecum. On the other hand, in certain birds (herons) as a
