secrete a mucus to keep the surface moist. The surface of the mucous
membrane is covered by stratified squamous epithelium.
The submucosa connects the mucous membrane to the muscular coat.
The wall of the crop is provided with strong muscles. The fibrous coat,
or tunic, connects it with the surrounding structures.
_Function._—The crop is a storehouse for the food during the hours of
feeding, the food when needed by the stomach being gradually discharged
from the crop by the contraction of its muscular walls. The subcutaneous
cervical muscles which cover this reservoir aid in this discharge.
In the _act of deglutition_, the food, after being subjected to the
fluid supplied in the mouth by the adjacent glands, is poised upon the
tongue and swallowed partly by a sudden jerk of the head, and partly by
means of the pressure of the tongue against the hard palate, the food
then passes down the esophagus and lodges first in the crop, till needed
by the stomach, when it is passed through the second portion of the
esophagus to the first portion of the stomach, the proventriculus. The
time during which food remains in the crop depends upon the nature of
the food. Animal food will, in part, be retained about eight hours and
vegetable foods may not all be passed on for from sixteen to eighteen
hours.
=The Stomach= (Fig. 31, No. 7; Fig. 32, _B_).—The stomach, or
ventriculus, of fowls is made up of two portions, namely, the pars
glandularis, or proventriculus, and the pars muscularis, or gizzard.
=The Proventriculus.= _Location._—The proventriculus lies in the
superior part of the groove formed by the two lobes of the liver, is
inferior to the aorta, and is directed slightly to the left,
communicating anteriorly with the second portion of the esophagus and
posteriorly emptying into the pars muscularis, the gizzard.
_Shape._—The proventriculus is round transversely and elongated, in
fact, nearly fusiform. In the hen of average size it measures about 1.62
inches long and 0.8 inch in diameter.
_Structure._—The wall of the proventriculus has four coats, the mucous,
submucous, muscular, and serous.
The inner mucous coat which is raised in folds, is lined with columnar
epithelial cells. The mucous membrane contains lymphoid tissue. The
mucous coat throughout contains simple tubular glands which secrete a
highly acid fluid which finds its way to the surface through small
cylindrical ducts lying at right angles to the inner surface of the
mucous membrane (Fig. 32, No. _B_, 1).
[Illustration:
FIG. 32.—Gizzard and stomach of the fowl.
_A._ 1, Alveolar glands. 2, Mucous membrane of the inner surface of
the proventriculus showing the tubular glands. 3, Connective tissue
between the alveolar glands. 4, An artery. 5, Muscularis mucosa.
_B._ Photograph of the proventriculus and gizzard. 1, Proventriculus
showing stomata of glands. 2, End of duodenum. 3, Gizzard showing
hornified epithelium formed into grooves. 4, The heavy muscular
walls.
]
The submucosa, connects the mucous and muscular coats and the muscularis
layer throws the mucous membrane into folds.
Outside the muscularis mucosa there is another layer (Fig. 32, _A_), of
simple tubular glands, grouped in lobules, and lined by cuboidal cells
and separated from each other by clefts. These tubular glands converge
toward the center and open into the same cavity.
The middle muscular coat can be divided into three layers: two thin,
longitudinal layers, and a thick, circular layer interposed between the
other two.
_Function._—The function of the proventriculus appears to be to soak the
food with a secretion. The secretion from the proventriculus is similar
to that produced by the fundus glands of the stomach of mammals. It
contains acid and a ferment-pepsin.
=The Gizzard= (Fig. 31, No. 18). _Location._—The gizzard, or muscular
stomach, occupies a portion of the central part of the abdominal cavity
(Fig. 33, No. 4). It lies slightly to the left and just behind the
liver, the proventriculus, and the spleen, and rests upon a mass of
intestines. The gizzard communicates at its anterior portion with the
proventriculus and with the duodenum. These openings are close together.
The gizzards of a large number of hens of average size averaged in
weight as follows: full, 0.215 pound; empty, 0.126 pound. The gizzard
stands perpendicularly and somewhat obliquely in the abdominal cavity.
_Shape._—In shape the gizzard is roundish, flattened laterally.
_Structure._—The walls of the gizzard are very thick and are made up of
three coats: mucous, muscular, and serous.
The cavity of the gizzard is covered by a thick skin-like structure
possessing a heavy stratified squamous epithelial layer which is thrown
somewhat into ridges (Fig. 32, _B_). This membrane becomes thinnest near
the edges.
The mucous membrane, being cornified and readily detachable, is by some
anatomists considered as a special membrane (Fig. 37, _B_).
At the pyloric opening there is a valve formed by a fold of the mucous
membrane, which prevents grit and large particles of food from passing
out of the gizzard.
[Illustration:
FIG. 33.—View of viscera of the left side of a hen. 1, The base of the
heart. 2, Proventriculus. 3, Left lobe of the liver. 4, Gizzard. 5,
End of the duodenal loop. 6, Pancreas. 7, Free portion of the small
intestine. 8, Lungs. 9, Diaphragm.
]
[Illustration:
FIG. 34.—View of the viscera of the right side of a hen. 1, Base of
the heart. 2, Lungs. 3, Right lobe of the liver. 4, Duodenal loop.
5, Pancreas. 6, Cæcum. 7, Large intestine or rectum. 8, Free portion
of the floating small intestine. 9, Gall-bladder. 10, Right kidney.
11, The ribs. Note the lungs pushing up between them.
]
On each side it has a powerful fleshy muscle, the _muscularis
lateralis_. These musculares laterales are hemispherical, consist of
very closely packed fibers extending transversely, and are attached to
strong anterior and posterior ligamentous tendons. They are joined at
the edge of the organ by a strong aponeurosis. The muscular fibers are
red but do not possess cross striations; they are of the smooth type.
The _muscularis intermedii_, thinner and less developed than the
musculares laterales, occurs on each of the anterior and the posterior
parts of the gizzard.
The serous coat covers over the greater part of the external surface of
the gizzard and closely adheres to the muscular coat.
_Function._—The hard callous pads of the gizzard, operated by the
powerful muscles above described, together with grit, act like mill
stones and make reduction to fineness very complete.
At the posterior part there is a sacculated portion containing glands of
the long tubular type which secrete a fluid ferment similar to that
secreted by the glands of the pyloric portion of the stomach of mammals.
These glands also exist in a small band near the entrance of the
gizzard. Here the food is mixed with strongly acid secretion containing
pepsin which makes gastric digestion perfect.
[Illustration:
FIG. 35.—A median antero-posterior section through the body of a
1-pound pullet. 1, First portion of the esophagus. 2, Crop. 3,
Second portion of the intestine. 4, Proventriculus. 5, Gizzard. 6,
Spleen. 7, Liver. 8, Heart. 9, Point where duodenum was severed from
gizzard. 10, Point where duodenum was severed. 11, Duodenum. 12,
Pancreas. 13, Cæcum. 14, Floating small intestine. 15, Ovary. 16,
Oil sac.
]
=The Small Intestine= (Fig. 31, No. 8 and 9).—The small intestine,
intestinum tenue, is the tube which connects the gizzard with the large
intestine. It is divided into two parts, the duodenum and the free
portion. Of the three parts as considered in the mammalian intestine,
only the first, the _duodenum_, can be distinguished. There is no
demarcation between the jejunum and the ileum. The jejunum and the ileum
or that part which represents these sections, are in coils suspended
from the free border of the mesentery the other border of which is, in
turn, attached to the dorsal wall (Fig. 64). The length of the small
intestine in a hen of average size is about 61.7 inches.
[Illustration:
FIG. 36.
_A._ Section of the duodenum of the fowl. 1, Villus. 2, Gland. 3, Mass
of lymphoid tissue. 4, Muscularis mucosa. 5, Longitudinal layer. 6,
Circular muscular layer. 7, Serous layer.
_B._ A section from A at _a_. 1, The striated free border of the
cells. 2, Goblet cell. 3, Columnar cell. 4, Nucleus of cell.
]
The wall of the small intestine is provided with four coats, as follows:
a mucous, submucous, muscular coat made up of two layers—the outer,
longitudinal and the inner circular layer, and an external serous.
[Illustration:
FIG. 37.—Histological studies of various anatomical parts.
_A._ A transverse section of the first portion of the esophagus of a
fowl. 1, The outer longitudinal muscular layer. 2, The circular
muscular layer. 3, The submucosa. 4, The muscularis mucosa. 5,
Stroma. 6, Epithelial layer. 7, The lenticular glands. 8, The lumen.
_B._ A transverse section of the proventriculus. 1, The outer
longitudinal muscular layer. 2, The middle muscular layer. 3, The
inner longitudinal muscular layer. 4, Stroma. 5, The muscularis
mucosa. 6, The submucosa. 7, Stroma. 8, Tubular glands. 9,
Tubulo-alveolar glands. 10, A tubulo-alveolar gland with the tubular
glands cut transversely, 11, The lumen of the proventriculus.
_C._ A section of the inner wall of the gizzard. 1, The hyaline mucous
membrane. 2, Branched tubular glands. 3, Submucosa. 4, Muscle. 5, A
connective tissue septum.
_D._ A transverse section of the small intestines. 1, The outer
longitudinal muscular layer. 2, The inner circular muscular layer.
3, The muscularis mucosa. 4, The stroma. 5, Brunner’s glands. 6, A
villus. 7, Mass of lymphoid tissue.
_E._ A transverse section through the cecum. 1, The outer longitudinal
muscular layer. 2, The inner circular muscular layer. 3, The
submucosa. 4, The muscularis mucosa. 5, The stroma. 6, Tubular
glands. 7, The lumen.
_F._ The cerebellum showing the arbo-vitæ. 1, White, fiber portion. 2,
The granular layer. 3, The layer of Purkinje cells. 4, The molecular
layer.
]
The mucous membrane which lines the intestine is thick, soft, and highly
vascular. It has a velvety appearance, due to numerous long, thin
projecting _villi_. The villi (Fig. 36, _A_) are concerned in the
absorption of the digested food, absorbing principally the emulsified
fats. Each villus is covered with a single layer of high columnar
epithelial cells. Some of these, the so-called goblet cells, provide
mucin which lubricates the mucous surface. These cells are found in all
mucous surfaces and prevent the surface from becoming dry. There are a
few goblet cells near the summits of the villi. Openings of simple
intestinal tubular glands the duodenal glands, or the glands formerly
known as _Brunner’s glands_, are located between the villi. These glands
secrete the succus entericus, or intestinal juice. These openings, or
stomata, are lined with granular cells. The reaction of the contents of
the small intestine is strongly acid, but gradually less so in
proportion to the distance down the intestinal tract until the cæca are
reached, where the reaction is found to be faintly acid, neutral, or
slightly alkaline.
_Function._—The function of the small intestine is that of digestion and
absorption.
=The Duodenum.= _Location and Shape._—The duodenum, a small tube,
originates from the gizzard about ½ inch to the right of the entrance of
the proventriculus. Extending from left to right, it passes under and
behind the gizzard along the inner side of the right abdominal wall
backward to the posterior portion of the wall, and a trifle more than
half way toward the left side, where the loop rests. This loop, the
duodenal loop, is about 5 inches in length (Fig. 31, No. 8). The two
branches of the loop the first and the second portions or the descending
and ascending limbs, are loosely held by connective tissue, and have the
pancreas lodged between them.
=The Free Portion of the Small Intestine.= _Location._—That section of
the small intestine following the duodenum is called the free portion of
the small intestine and occupies the space between the abdominal air-sac
and the median line of the abdominal cavity. It is disposed in coils and
is suspended from the dorsal wall of the abdomen by a thin membrane, the
mesentery.
The bile ducts enter the small intestine about 14 inches from its point
of origin. The pancreas also pours its contents into the small
intestine.
=The Large Intestine= (Fig. 31, No. 12). _Location._—The large intestine
extends in a straight line along the inferior border of the vertebral
column communicating anteriorly with the small intestine, and the cæca
and posteriorly with the cloaca.
_Shape._—The diameter of the large intestine is approximately twice that
of the small intestine. In fowls of average size its mean length is 4.61
inches. The large intestine has sometimes been spoken of as the rectum,
or straight gut.
_Structure._—Like the small intestine the large intestine has four
coats: an inner mucous, a submucous, a middle muscular and an outer
serous. The folds of mucous membrane of the large intestine have tubular
glands lined with columnar cells (Fig. 37, _A_, No. 1).
_Function._—The large intestine is similar in function to that of the
small intestine, in that digestion and absorption may take place within
it.
=The Cæca= (Fig. 31, No. 11). _Location._—The fowl has two cæca
extending forward from their point of origin at the juncture of the
small and large intestine.
_Shape._—The cæca average 7.61 inches in length. They are large in
caliber toward the blind extremity and are constricted near their
origin.
_Structure._—The parietal coats are continued from the small intestine.
Ebarth has described an elevated body in the cæcum, which is located
about 4 millimeters from the opening and is composed entirely of
lymphoid tissue.
[Illustration:
FIG. 37_A_.—Photomicrograph of a transverse section of large intestine
and oviduct just anterior to the cloaca, showing the tubular glands
of the large intestines, mucosa of oviduct with intervening
structures. 1, The tubular glands of large intestine, or rectum. 2,
Muscular wall of intestine. 3, Wall of the oviduct. 4, Mucosa of the
oviduct.
]
The cæca are usually partly filled with a soft pultaceous material of a
pasty consistency.
=The Cloaca= (Fig. 31, No. 13). _Location._—The rectum opens by a
circular, valvular orifice into the dilated portion just in front of the
anus, which dilatation is called the cloaca.
_Shape._—The cloaca is saccular in shape.
_Structure._—The cloaca is divided into two portions, the coprodeumal
and the urodeumal. That portion of the cloaca into which the intestine
empties is called the _coprodeum_; and the ureter and oviduct empty into
the _urodeum_. The seminiferous tubules, carrying the semen from the
testes in the male empty in teatlike projections on the cloacal mucous
membrane into the urodeumal portion. The cloacal walls are similar in
structure to the large intestines.
On the dorsal wall of the cloaca between it and the spine, is a small
sac, called the _bursa of Fabricius_, which has a duct communicating
with the cloaca. The mucous membrane of this sac is thrown into folds
and is studded with glands. The bursa of Fabricius is larger in the
young than in the adult bird. It apparently atrophies as the bird
becomes older. When the bird is four months old this bursa is best
developed, and at this age it may be as large as 2 or 3 centimeters in
diameter.
_Function._—The function of the cloaca is to give passage way to the
feces, the urine, and the egg and to act as an organ of copulation.
=Course of the Food.=—The food first enters the mouth, after being
picked up by aid of the beak. From here it passes through the pharynx
and first portion of the esophagus to the crop, without mastication, as
the bird is not provided with teeth. The food is passed from the crop by
aid of its muscular walls as needed; thence through the second portion
of the esophagus to the proventriculus, an expansion in the digestive
tube just before it terminates in the gizzard. The glands of the
proventriculus produce a secretion in which the food is soaked before
passing into the gizzard. The gizzard is provided with strong muscular
walls which, by aid of grit, thoroughly reduce the food to fineness.
From the gizzard the food passes through the first portion of the small
intestine, where it is subjected to the action of the bile from the
liver, the pancreatic juice from the pancreas, and of the succus
entericus from the glands of the intestinal wall. The food is then
passed into the cæca. The indigestible portion of the food passes from
the cæca through the large intestine, or rectum, to the cloaca and
thence to the external world.
The digestive functions of the bird are very potent and rapid. This
compensates for the waste caused by their extensive, frequent, and
energetic motions, and is in accordance with the rapidity of their
circulation and their high state of irritability.
THE ACCESSORY ORGANS OF DIGESTION
The accessory organs of digestion are the liver, pancreas, and some
anatomists include also the spleen. The first two manufacture fluids
containing ferments which aid in splitting or digesting the food.
=The Liver= (Figs. 31, 33 and 34). _Location._—The liver, hepar, lies
ventrally and posteriorly to the heart. It is related anteriorly with
the diaphragm, inferiorly with the sternum, posteriorly with the gizzard
and intestine, and superiorly with the ovary, oviduct and proventriculus
and laterally with the abdominal wall.
_Shape._—The liver is a voluminous deep livid brown gland, soft and
friable in texture. It is divided into two principal lobes, a right and
a left.
The right lobe is larger than the left. In the hen of average size the
liver weighs 35 grams. The parietal surface is convex and smooth. The
surface which lies against the viscera is irregularly concave. The
visceral surface furnishes exit for the bile duct and passage for the
nerves and blood-vessels. This part is called the porta.
The left lobe may be cleft from below so deeply as to form two lobes on
that side.
_Structure._—Each lobe is covered by a double serous membrane, one
closely adherent, the other surrounding the structure loosely. These
tunics, which are reflections of the peritoneum, are continued from the
base of the liver, over both the anterior, and the posterior surface.
The loose layer is formed by the air cells surrounding the lobes. The
thin border of the liver is usually free.
The two lobes of the liver are connected by a narrow isthmus of liver
tissue. Occasionally there is a bird in which there occurs a _lobus
Spigelii_ located at the posterior of the liver between the two
principal lobes.
The apex of the heart sacculates the diaphragm backward, so that part of
this apex lies between the right and the left lobes (Fig. 43, No. 6 and
7). A ligament, the _falciform_, extends from the apex of the
pericardial membrane, and attaching it rather firmly to the central
connective tissue, or interlobar ligament. This ligament also has
attachments to the inner surface of the sternum. The _broad ligament_ of
the liver is formed posteriorly by a fold of the peritoneum.
The _interlobar_, or _principal, ligament of the liver_ is formed by a
large and strong duplicature of the peritoneum, which makes a
longitudinal division in the abdominal cavity similar to the lateral
division made by the thoracic mediastinum in mammals. It is reflected
upon the pericardium from the linea alba and the middle line of the
sternum, and passes deeply into the interspace of the lobes of the
liver. It is attached to these lobes throughout their whole length and
connects them below to one side of the gizzard. The lateral and
posterior part of the liver attach to the adjacent air cells, and the
whole viscus is thus kept fixed in its position during rapid and violent
movements of the bird.
The remains of the umbilical veins are traceable within the duplicature
of the membranes forming the septum. These remains thus represent the
round ligament of mammals.
[Illustration:
FIG. 38.—Cellular structure of liver, pancreas, and trachea.
_A._ Liver. 1, Liver cells. 2, Sinusoid. 4, Nucleus of cells.
_B._ Pancreas. 1, Island of Langerhans. 2, Alveolar cells. 3, Duct.
_C._ Trachea. 1, Ciliated epithelia. 2, Glands. 3, Hyaline cartilage.
_D._ Section of wall of ovum at Fig. 57 letter _d_. 1, Yolk. 2,
Granular membrane. 3, Theca. 4, Blood-vessel.
]
A microscopic study of the liver of the fowl shows a compact mass of
liver cells polyhedral in shape, with large nuclei (Fig. 38, _A_). The
liver tissue differs from that of mammals in that there is no clearly
outlined lobular arrangement; neither is the outline of the individual
cell so well marked. The parenchymatous portion is made up of columns of
liver tissue. These columns anastomose and show narrow channels between.
They are best seen in the young chick.
_Function._—One of the functions of the liver is to secrete bile. A
_gall-bladder_, which receives part of the bile secreted by the right
lobe of the liver, is located on the posterior face of this lobe.
Extending from this gall-bladder there is a duct, the _cystic duct_,
which empties into the small intestine toward the extremity of the
second branch of the duodenal loop. Another duct called the _hepatic
duct_, proceeds directly from the two lobes of the liver and empties
into the intestine just in front of the cystic duct.
=The Pancreas= (Fig. 31, No. 17). _Location and Shape._—The pancreas, an
organ lying in the duodenal loop is a yellowish-white, lobulated gland,
elongated in shape. Its average length in fowls of average size is 4.96
inches, and the average weight about 0.008 pound. The pancreas is
divided into lobes, which in turn are divided into lobules.
_Structure._—The pancreas has a supporting connective tissue. The
lobules are made up of small alveolar glands, which are lined with
columnar epithelial cells. The ducts leading from the alveoli are small;
these unite to form larger ducts in which the epithelium is taller.
Between the alveoli throughout the pancreas there are clusters of
polyhedral cells which form the _islands of Langerhans_. These islands
are said to produce an internal secretion, or hormone, which is absorbed
by the blood or lymph capillaries and thus enters the circulation (Fig.
38, _B_).
_Function._—The function of the pancreas is to secrete a fluid
containing digestive ferments.
=The Spleen= (Fig. 31, No. 21). _Location._—The spleen, or lien, lies in
a triangle formed by the proventriculus, the liver, and the gizzard.
_Shape._—It is a reddish-brown body shaped like a buckeye, is small in
size, weighing only about 0.005 pound.
_Structure._—The outer surface of the spleen is covered by a reflection
of the peritoneum. After this covering is removed, there is observed a
firm, white, fibrous layer, the _cortical portion_. This covering sends
into the interior small and large trabeculæ, forming a framework and
dividing it into acini, or compartments. The spaces are filled with a
dark red parenchymatous material called _splenic pulp_. The framework,
in both deep and surface portions, are found elastic fibers and smooth
muscular fibers.
The spleen is essentially a lymphatic organ, its peculiar structure
depending largely upon the arrangement of the blood-vessels. Compact
lymphatic tissue occurs in the spleen in spherical, oval, or cylindrical
collections of closely packed lymphoid cells. These masses are known as
the _Malpighian bodies_, or _splenic corpuscles_. They are distributed
throughout the splenic pulp. Each splenic corpuscle contains one or more
small arteries. These extend near the periphery of the corpuscle and
more rarely in the center.