present. The muscular coat consists of unstriped fibres
arranged in two layers, the outer longitudinal and the inner
circular (see fig. 2). In the large intestine the longitudinal
fibres, instead of being arranged evenly round the tube as
they are in the small, are gathered into three longitudinal
bands called taeniae (see fig. 1); by the contraction of
these the large intestine is thrown into a series of sacculi
or slight pouches. The taeniae in the caecum all lead to
the vermiform appendix, and form a useful guide to this
structure. In the rectum the three taeniae once more become
evenly arranged over the whole surface of the bowel, but
more thickly on the anterior and posterior parts. The
circular layer is always thicker than the longitudinal;
in the small intestine it decreases in thickness from the
duodenum to the ileum, but in the large it gradually increases
again, so that it is thickest in the duodenum and rectum.
The submucous coat is very strong and consists of
loose areolar tissue in which the vessels break up.
The mucous coat is thick and vascular (see fig. 2); it
consists of an epithelial layer most internally which
forms the intestinal glands (see EPITHELIAL, ENDOTHELIAL
AND GLANDULAR TISSUES.) External to this is the basement
membrane, outside which is a layer of retiform tissue, and
this is separated from the submucous coat by a very thin
layer of unstriped muscle called the muscularis mucosae. In
the duodenum and jejunum the mucous membrane is thrown into
a series of transverse pleats called valvulae conniventes
(see fig. 3); these begin about an inch from the pylorus and
gradually fade away as the ileum is reached. About 4 in.
from the pylorus the common bile and pancreatic ducts form a
papilla, above which one of the valvulae conniventes makes
a hood and below which a vertical fold, the frenulum, runs
downward. The surface of the mucous membrane of the whole
of the small intestine has a velvety appearance, due to
the presence of closely-set, minute, thread-like elevations
called vilii (see ffg. 2). Throughout the whole length of
the intestinal tract are minute masses of lymphoid tissue
called solitary glands (see fig. 2); these are especially
numerous in the Caecum and appendix, while in the ileum they
are collected into large oval patches, known as agminated
glands or Peyer's patches, the long axes of which,
from half an inch to 4 in. long, lie in the long axis of the
bowel. They are always found in that part of the intestine
which is furthest from the mesenteric attachment. In the
interior of the rectum three shelf-like folds, one above
the other, project into the cavity and correspond to the
lateral concavities or kinks of the tube. They are not
in the same line and the largest is usually on the right
side. They are known as the plicae recti or valves of
Houston. In the anal canal are four or five longitudinal
folds called the columns of Morgagni. (For further
details, see Quain's Anatomy, London, 1896; Gray's Anatomy,
London, 1905; Cunningham's Anatomy, Edinburgh, 1906.)
Embryology.--The greater part of the alimentary canal is
formed by the closing-in of the entoderm to make a longitudinal
tube, ventral and parallel to the notochord. This tube
is blind in front and behind (cephalad and caudad), but
the middle part of its ventral wall is for some distance
continuous with the wall of the yolk-sac, and this part of the
canal, which at first opens into the yolk-sac by a very wide
aperture, is called the mid gut. The part in front of it,
which lies dorsal to the heart, is the fore gut, while the
part behind the aperture of the yolk-sac is the hind gut.
The pharynx, oesophagus, stomach and part of the duodenum are
developed from the fore gut, a good deal of the colon and the
From A. Birmingham; Cunningham's Text-Book of Anatomy.
Fig. 3.--Valvulae Conniventes (natural size).
A, As seen in a bit of jejunum which
has been filled with alcohol and hardened.
B, A portion of fresh intestine spread out under water.
rectum from the hind gut, while the mid gut is responsible
for the rest. The cephalic part of the fore gut forms the
pharynx (q.v.), and about the fourth week the stomach
appears as a fusiform dilatation in the straight tube.
Between the two the oesophagus gradually forms as the embryo
elongates. The opening into the yolk-sac, which at first
is very wide, gradually narrows, as the ventral abdominal
walls close in, until in the adult the only indication of the
connexion between the gut and the yolk-sac is the very rare
presence (about 2%) of Meckel's diverticulum already referred
to. The stomach soon shows signs of the greater and lesser
curvatures, the latter being ventral, but maintains its straight
position. About the sixth week the caecum appears as a lateral
diverticulum, and, until the third month, is of uniform calibre;
after this period the terminal part ceases to grow at the
same rate as the proximal, and so the vermiform appendix is
formed. The mid gut forms a loop with its convexity toward
the diminishing vitelline duct, or remains of the yolk-sac,
and until the third month it protrudes into the umbilical
cord. The greater curvature of the stomach grows more
rapidly than the lesser, and the whole stomach turns over
and becomes bent at right angles, so that what was its left
surface becomes ventral. This turning over of the stomach
throws the succeeding part of the intestine into a duodenal
loop, which at first has a dorsal and ventral mesentery (see
COELOM AND SEROUS MEMBRANES.) The intestine now grows
very rapidly and is thrown into a series of coils; the caecum
ascends and passes to the right ventral to the duodenum, and
presses it against the dorsal wall of the abdomen; then it
descends toward its permanent position in the right iliac fossa.
From the ventral surface on the hinder (caudal) closed end
of the intestinal tube the allantois grows to form the
placenta and bladder (see URINARY SYSTEM, REPRODUCTIVE SYSTEM
and PLACENTA), and this region is the cloaca into which
the alimentary, urinary and generative canals or ducts all
open, but later two lateral folds appear which, by their
union, divide the cloaca into a ventral and a dorsal part,
the former being genito-urinary and the latter alimentary or
intestinal. In this way the rectum or dorsal compartment
is shut off from the genito-urinary. Later an ectodermal
invagination at the hind end of the embryo develops and
forms the anal canal; this is the proctodaeum, and for
some time it is separated from the hind (caudal) end of the
rectal part of the mesodaeum (or part of the intestinal
canal formed from the mesoderm) by a membrane called the
anal membrane. This is eventually absorbed and the
digestive tract now communicates with the surface by the anus.
F. Wood Jones (British Medical Journal, 17th of December
1904) has given a somewhat different description of the
development of the cloaca and anus, which better explains the
various abnormalities met with in this region but requires
further confirmation before it is generally accepted. For the
development of the mouth, pharynx, lungs, liver and pancreas from
the primitive alimentary canal, the reader is referred to the
special articles on those structures. (For further details, see W.
His, Anatomie menschlicher Embryonen (Leipzig, 1880-1885); C.
S. Minot's Embryology (New York, 1897); and J. P. M`Murrich,
Development of the Human Body (London, 1906). (F. G. P.)
Comparative Anatomy.--The primitive condition of the vertebrate
alimentary canal may be described as a straight, simple
tube, consisting of an anterior portion, the stomodaeum,
formed by an ectodermal invagination, the mesenteron,
a long median portion lined by endoderm, and a short
posterior portion, the proctodaeum, formed by ectodermal
invagination. In the lower vertebrates the primitive tube
subserved also the purpose of respiration, and traces of the
double function remain in the adult structure of all vertebrates
(see MOUTH, PHARYNX.) In fish, the pharynx, or branchial
region, suddenly becomes narrower, posterior to the gill-slits,
to form the oesophagus; in higher animals the oesophagus,
in the adult, is separated from the primitive pharyngeal
region and lies dorsal to it. Probably, in the primitive
vertebrata, the entire alimentary canal was lined with ciliated
cells. Traces of this ciliation persist in many living
forms. In the Ammocoete, the larval form of Petromyzon
(see CYCLOSTOMATA), the whole canal is ciliated except the
pharynx and the rectum; in the Dipnoi the epithelium of the
stomach and the intestines is ciliated; in Selachii that
of the posterior part of the gullet, and the spiral valve, is
ciliated; extensive ciliation may occur in almost any region
of the gut of the lower teleos. tomes, but in the higher
forms (Teleostei) it is generally absent. In the latter,
however, and in higher groups of vertebrates, a peculiar
striated border on the columnar cells lining the intestinal
tract has been held to be a final trace of ancestral ciliation.
The alimentary canal may be conveniently described in three
divisions, the oesophagus or gullet, the passage by which
food reaches the stomach, the stomach, typically an expanded
region in which the food remains for a considerable time
and is mechanically pulped, mixed with mucus and certain
digestive juices (see NUTRITION) and partly macerated,
the intestinal tract or gut, extending from the distal end
of the stomach to the cloaca or anus, in which the food is
subjected to further digestive action, but which is above all
the region in which absorption of the products of digestion
takes place, the refuse material together with quantities of
waste matter entering the gut from the blood and liver being
gradually passed towards the anus for discharge from the body.
The oesophagus is essentially merely a passage, as straight
as may be, from the pharynx to the stomach, varying in
length with the length of the neck and thoracic regions in
different animals, and in calibre with the nature of the
food. It is almost invariably lined with a many-layered
epithelium, forming a tough coating, readily repaired and
not easily damaged by hard food masses. It is occasionally
separated from the stomach by a slight constriction which may
be capable of contraction so as to prevent regurgitation.
There are few exceptions to this structural and functional
simplicity. In fishes (see ICHTHYOLOGY, Anatomy) the
swim-bladder is developed as a dorsal outgrowth of the oesophagus
and may remain in open connexion with it. In certain Teleosteis
(e.g. Liitodeira) it is longer than the length it has to
traverse and is thrown into convolutions. In many other
fish, particularly Selachiis, a set of processes of the
lining wall project into the cavity near the stomach and have
been supposed to aid in preventing food particles, or living
creatures swallowed without injury, escaping backwards into the
mouth. In some egg-eating snakes the sharp tips of the
ventral spines (hypapophyses) of the posterior cervical
vertebrae penetrate the wall of the oesophagus and are used
for breaking the shells of the eggs taken as food. In some
aquatic Chelonians, the food of which consists chiefly of
seaweeds, the lining membrane is produced into pointed processes
backwardly directed. In birds this region frequently presents
peculiarities. In Opisthocomus it forms an enormously
wide double loop, hanging down over the breast-bone, which
is peculiarly flattened and devoid of a keel in the anterior
portion. In many birds part of the oesophagus may be
