Part 55

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.

FIG. 37A.—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.