temporarily dilated, forming a ``crop,'' as for instance in
birds of prey and humming birds. In the flamingo, many ducks,
storks, and the cormorant the crop is a permanent although
not a highly specialized enlargement. Finally, in the vast
majority of seed- eating birds, in gallinaceous birds, pigeons,
sandgrouse, parrots and many Passeres, particularly the
finches, the crop is a permanent globular dilatation, in which
the food is retained for a considerable time, mixed with a
slight mucous secretion, and softened and partly macerated by
the heat of the body. Many birds feed their young from the
soft contents of the crop, and in pigeons, at the breeding
season, the cells lining the crop proliferate rapidly and
are discharged as a soft cheesy mass into the cavity, forming
the substance known as pigeon's milk. Amongst Mammalia, in
Rodentia, Carnivora, elephants and ruminants, the wall of
the oesophagus contains a layer of voluntary muscle, by the
contraction of which these animals induce anti- peristaltic
movements and can so regurgitate food into the mouth.
Stomach.--Where the oesophagus passes into the stomach, the
lining wall of the alimentary tract changes from a many-layered
epithelium to a mucous epithelium, consisting of a single layer
of endodermal cells, frequently thrown into pits or projecting
as processes; from being chiefly protective, it has become
secretory and absorbing, and maintains this character to the
distal extremity where it passes into the epiblast of the
proctodaeum. In most cases the course of the alimentary
canal from the distal end of the oesophagus to the cloaca or
anus is longer than the corresponding region of the body, and
the canal is therefore thrown into folds. The fundamental
form of the stomach is a sac-like enlargement of the canal,
the proximal portion of which is continuous with the line
of the oesophagus, but the distal portion of which is bent
in the proximal portion, the whole forming an enlarged bent
tube. At the distal end of the tube the intestinal tract
proper begins, and the two regions are separated by a muscular
constriction. In fishes the stomach is generally in one of two
forms; it may be a simple bent tube, the proximal limb of which
is almost invariably much wider than the distal, anteriorly
directed limb; or the oesophagus may pass directly into an
expanded, globular or elongated sac, from the anterior lateral
wall of which, not far from the oesophageal opening, the duodenum
arises. In Batrachia and Reptilia the stomach is in most
cases a simple sac, marked off from the oesophagus only by
increased calibre. In the Crocodilia, however, the anterior
portion of the stomach is much enlarged and very highly
muscular, the muscles radiating from a central tendinous
area on each of the flattened sides. The cavity is lined by
a hardened secretion and contains a quantity of pebbles and
gravel which are used in the mechanical trituration of the
food, so that the resemblance to the gizzard of birds is well
marked. This muscular chamber leads by a small aperture into
a distal, smaller and more glandular chamber. In birds the
stomach exhibits two regions, an anterior glandular region, the
proventriculus, the walls of which are relatively soft and
contain enlarged digestive glands aggregated in patches (e.g.
some Steganopodes), in rows (e.g. most birds of prey) or
in a more or less regular band. The distal region is larger
and is lined in most cases by a more or less permanent lining
which is thick and tough in birds with a muscular gizzard,
very slight in the others. In many birds, specially those
feeding on fish, the two regions of the stomach are of equal
width, and are indistinguishable until, on opening the cavity,
the difference in the character of the lining membrane becomes
visible. In other birds the proventriculus is separated
by a well marked constriction from the posterior and larger
region. In graminiferous forms the latter becomes a
thick-walled muscular gizzard, the muscles radiating from
tendinous areas and the cavity containing pebbles or gravel.
In mammals, the primitive form of the stomach consists of a
more or less globular or elongated expansion of the oesophageal
region, forming the cardiac portion, and a forwardly curved,
narrower pyloric portion, from which the duodenum arises.
The whole wall is muscular, and the lining membrane is richly
glandular. In the Insectivora, Carnivora, Perissodactyla, and
in most Edentata, Chiroptera, Rodentia and Primates, this
primitive disposition is retained, the difference consisting
chiefly in the degrees of elongation of the stomach and the
sharpness of the distal curvature. In other cases the cardiac
portion may be prolonged into a caecal sac, a condition most
highly differentiated in the blood-sucking bat, Desmodeus,
where it is longer than the entire length of the body. There
are two cardiac extensions in the hippopotamus and in the
peccary. In many other mammals one, two or three protrusions
of the cardiac region occur, whilst in the manatee and in
some rodents the cardiac region is constricted off from the
pyloric portion. In the Artiodactyla the stomach is always
complex, the complexity reaching a maximum in ruminating
forms. In the Suidae a cardiac diverticulum is partly
constricted from the general cavity, forming an incipient
condition of the rumen of true ruminants; the general cavity
of the stomach shows an approach to the ruminant condition
by the different characters of the lining wall in different
areas. In the chevrotains, which in many other respects show
conditions intermediate between nonruminant artiodactyles
and true ruminants, the oesophagus opens into a wide cardiac
portion, incompletely divided into four chambers. Three of
these, towards the cardiac extremity, are lined with villi
and correspond to the rumen or paunch; the fourth, which
lies between the opening of the oesophagus and the pyloric
portion of the stomach, is the ruminant reticulum and its
wall is lined with very shallow ``cells.'' A groove runs
along its dorsal wall from the oesophageal aperture to a
very small cavity lined with low, longitudinally disposed
folds, and forming a narrow passage between the cardiac and
pyloric divisions; this is an early stage in the development
of the omasum, psalterium or manyplies of the ruminant
stomach. The fourth or true pyloric chamber is an elongated
sac with smooth glandular walls and is the abomasum, or rennet
sack. In the camel the rumen forms an enormous globular
paunch with villous walls and internally showing a trace of
division into two regions. It is well marked off from the
reticulum, the ``cells'' of which are extremely deep, forming
the well-known water-chambers. The psalterium is sharply
constricted off from the reticulum and is an elongated chamber
showing little trace of the longitudinal ridges characteristic
of this region; it opens directly into the relatively small
abomasum. In the true ruminants, the rumen forms a capacious,
villous reservoir, nearly always partly sacculated, into which
the food is passed rapidly as the animal grazes. The food
is subjected to a rotary movement in the paunch, and is thus
repeatedly subjected to moistening with the fluids secreted
by the reticulum, as it is passed over the aperture of that
cavity, and is formed into a rounded bolus. Most ruminants
swallow masses of hairs, and these, by the rotary action of the
paunch, are aggregated into peculiar dense, rounded balls which
are occasionally discharged from the mouth and are known as
``hair-balls'' or ``bezoars.'' The food bolus, when the animal
is lying down after grazing, is passed into the oesophagus
and reaches the mouth by antiperistaltic contractions of the
oesophagus. After prolonged mastication and mixing with
saliva, it is again swallowed, but is now passed into the
psalterium, which, in true ruminants, is a small chamber with
conspicuous longitudinal folds. Finally it reaches the large
abomasum where the last stages of gastric digestion occur.
In the Cetacea the stomach is different from that found in
any other group of mammals. The oesophagus opens directly
into a very large cardiac sac the distal extremity of which
forms a long caecal pouch. At nearly the first third of
its length this communicates by a narrow aperture into the
elongated, relatively narrow pyloric portion. The latter
is convoluted and constricted into a series of chambers that
differ in different groups of Cetacea. In the Sirenia
the stomach is divided by a constriction into a cardiac and
a pyloric portion, and the latter has a pair of caeca. In
most of the Marsupialia the stomach is relatively simple,
forming a globular sac with the oesophageal and pyloric
apertures closely approximated; in the kangaroos, on the
other hand, the stomach is divided into a relatively small,
caecal cardiac portion and an enormously long sacculated
and convoluted pyloric region, the general arrangement of
which closely recalls the large caecum of many mammals.
Intestinal Tract.--It is not yet possible to discuss the
general morphology of this region in vertebrates as a group,
as, whilst the modifications displayed in birds and mammals
have been compared and studied in detail, those in the
lower groups have not yet been systematically co-ordinated.
Fishes.--In the Cyclostomata, Holocephali and a few
Teleostei the course of the gut is practically straight from
the pyloric end of the stomach to the exterior, and there is
no marked differentiation into regions. In the Dipnoi, a
contracted sigmoid curve between the stomach and the dilated
intestine is a simple beginning of the complexity found in other
groups. In very many of the more specialized teleosteans, the
gut is much convoluted, exhibiting a series of watchspring-like
coils. In a number of different groups, increased surface for
absorption is given, not by increase in length of the whole gut,
but by the development of an internal fold known as the spiral
valve. This was probably originally a longitudinal fold
similar to the typhlosole of chaetopods. It forms a simple
fold in the larval Ammocoete, and in its anterior region
remains straight in some adult fish, e.g. Polypterus, but
in the majority of cases it forms a complex spiral, wound
round the inner wall of the expanded large intestine, the
internal edge of the fold sometimes meeting to form a central
column. It occurs in Cyclostomata, Selachii, Holocephali,
Chondrostei, Crossopterygii, Amiidae, Lepidosteidae and
Dipnoi. A set of organs peculiar to fish and known as the
pyloric caeca are absent in Cyclostomata and Dipnoi,
in most Selachii and in Amia, but present, in numbers
ranging from one to nearly two hundred, in the vast majority of
fish. These are outgrowths of the intestinal tract near
the pyloric extremity of the stomach, and their function is
partly glandular, partly absorbing. In a few Teleostei
there is a single caecal diverticulum at the beginning of
the ``rectum,'' and in the same region a solid rectal gland
occurs in most elasmobranchs, whilst, again, in the Dipnoi
a similar structure opens into the cloaca. These caeca have
been compared with the colic caeca of higher vertebrates,
but there is yet no exact evidence for the homology.
In the Batrachia the course of the intestinal tract is
nearly straight from the pyloric end of the stomach to the
cloaca, in the case of the perennibranchiates there being no
more than a few simple loops between the expanded ``rectum''
and the straight portion that leaves the stomach. In the
