Part 123

=Blood-vessels and Nerves.=—The stomach receives blood from all the branches of the cœliac artery. The gastric veins drain into the portal vein. The nerves are derived from the vagus and sympathetic nerves.

FIG. 262.—DIAGRAM OF ZONES OF MUCOUS MEMBRANE OF STOMACH OF HORSE. ]

THE SMALL INTESTINE

The =small intestine= (Intestinum tenue) is the tube which connects the stomach with the large intestine. It begins at the pylorus and terminates at the lesser curvature of the cæcum. Its average length is about seventy feet (ca. 22 meters). When distended its diameter varies from two to four inches (5 to 10 cm.). Its capacity is about twelve gallons (40 to 50 liters).

It is clearly divisible into a =fixed= and a =mesenteric= or floating portion. The fixed portion is termed the duodenum, while the mesenteric portion (Intestinum tenue mesenteriale) is arbitrarily divided into parts termed the jejunum and ileum.

The =duodenum= is about three to four feet (ca. 1 to 1.25 m.) long. Its shape is somewhat like a horseshoe, the convexity being directed toward the right. The =first part= is directed to the right and forms an ᔕ-shaped curve. The convexity of the first part of the curve is dorsal, of the second ventral. It lies on the middle and right lobes of the liver, and presents two dilatations (Ampullæ) with a constriction between them. The duodenal angle or head of the pancreas lies in the concavity of the second curve, and here, five to six inches (ca. 12 to 15 cm.) from the pylorus, the pancreatic duct and the bile-duct pierce the bowel wall. The =second part= passes upward and backward on the right lobe of the liver and, on reaching the right kidney and the base of the cæcum, it curves toward the median plane, opposite the last rib. The =third part= passes almost transversely from right to left behind the base of the cæcum, crosses the median plane under the third and fourth lumbar vertebræ, and turns forward to become continuous with the jejunum under the left kidney. The sacculations of the first part have a diameter of three to four inches (ca. 7.5 to 10 cm.).

It is attached by a short peritoneal fold termed the =mesoduodenum=. This fixes the first part of the duodenum closely to the liver and the right dorsal part of the colon; the remainder is somewhat less closely attached by it to the cæcum and right kidney, the sublumbar muscles, and (more closely) to the terminal part of the great colon and the first part of the small colon.

FIG. 263.—TOPOGRAPHY OF VISCERA OF HORSE, LEFT VIEW.

1.R., First thoracic vertebra; 1.L., first lumbar vertebra; 2.K., second sacral spine; S., scapula; A., humerus; St., sternum; B., ilium; O., femur; L., lung in complete expiration; l., dotted line indicating contour of lung in inspiration; H, pericardium; Z, costal part of diaphragm; Z′, tendinous center of diaphragm; l.N., left kidney; l.v.C., left ventral colon; v.Q., sternal flexure of colon; d.Q., diaphragmatic flexure of colon; D., small intestine; M., small colon; M′, rectum; a, coccygeus muscle; b, retractor ani; c, c, sphincter ani externus; d, constrictor vulvæ; e, sacro-coccygeus inferior; f, anterior gluteal artery; g, internal pudic artery; h, abdominal wall in section. (After Ellenberger, in Leisering’s Atlas, reduced.) ]

The =jejunum= and =ileum= together (Jejuno-ileum) constitute the mesenteric or floating portion of the small intestine. No distinct point exists at which to make the demarcation. With the exception of the last two or three feet, the mesenteric part of the intestine varies so much in position that only a general statement can be made. It lies in numerous coils (Ansæ) mingled with those of the small colon, chiefly in the dorsal part of the left half of the abdomen, from the visceral surface of the stomach to the pelvis. It may insinuate itself between the left portions of the colon and the abdominal wall; also between the ventral portions of the colon, reaching the floor of the abdomen. The terminal part of the intestine (ileum) ascends almost vertically, a little to the right of the median plane, to open into the cæcum at the lesser curvature of its base. The average diameter of the jejuno-ileum is about two and a half to three inches (ca. 6 to 7 cm.). In the cadaver one finds most of the tube presenting irregular constricted and dilated parts. The last three or four feet (ca. 1 meter) are usually tightly contracted, resembling somewhat the terminal part of the œsophagus. This part may be termed the ileum.

The mesenteric part is connected with the dorsal abdominal wall by the =great mesentery=. This is a wide fan-shaped fold, consisting of two layers of peritoneum, between which the vessels and nerves reach the bowel; it also contains the mesenteric lymph glands and some fat. The visceral border of the mesentery contains the intestine, while the parietal border or =root= (Radix mesenterii) is attached to a small area around the great mesenteric trunk under the first and second lumbar vertebræ. The root is thick, as it contains a large number of vessels and nerves placed close together. The mesentery is short at first, but soon reaches a length of one and a half to two feet (ca. 50 cm.)—sufficient to allow coils of the intestine to reach the abdominal floor, the pelvic cavity, or even to descend into the scrotum through the inguinal canal. Near its termination the intestine (ileum) leaves the border of the mesentery, so that the latter has a free edge which passes to the cæcum.

=Structure.=—The wall consists of four coats—serous, muscular, submucous, and mucous, enumerated from without inward.

The =serous coat= is complete except at the mesenteric edge, where the vessels and nerves reach the bowel.

The =muscular coat= consists of an outer longitudinal and an inner circular layer, the latter being the thicker. In the last few feet of the intestine the muscular coat is very thick, and being usually firmly contracted in the dead subject, gives the impression that this part of the bowel is of smaller caliber; such, however, is not the case during life.

FIG. 264.—DIAGRAM OF SECTION OF DIVERTICULUM DUODENI OF HORSE.

Solid line indicates mucous membrane. ]

The =submucous coat= is a layer of areolar tissue in which the vessels and nerves ramify. It contains also the duodenal glands and the bases of the solitary glands and Peyer’s patches.

The =mucous membrane= is soft and velvety. It has a grayish or yellowish-red color and is very vascular. About five or six inches from the pylorus it forms a pouch, the =diverticulum duodeni=, in which the pancreatic and hepatic ducts open. On a small papilla opposite this is the termination of the accessory pancreatic duct. At the ileo-cæcal opening the mucous membrane projects slightly into the cavity of the cæcum, forming the =ileo-cæcal valve=. The free surface is thickly beset with villi, small projections of the mucous membrane which can be seen well by placing a piece of the membrane in water. They are relatively short and thick and have a distinct neck in the horse. Each contains a central lymph vessel (lacteal), and around this a plexus of capillaries, lymphoid tissue, and unstriped muscle-fibers. They are important agents in absorption from the contents of the intestine. The epithelium is columnar, with many goblet cells. Underneath the basement membrane is a layer of unstriped muscle-fibers, the =muscularis mucosæ=.

The glands of the small intestine are of three kinds:

1. The =intestinal glands= (Glandulæ intestinales Lieberkuehni) are found throughout. They are simple tubular glands which open between the villi.

2. The =duodenal glands= (Glandulæ duodenales Brunneri) are found in the first twenty feet (ca. 6 meters) of the bowel. They are racemose glands, and are situated in the submucosa, so that their ducts perforate the muscularis mucosæ and the mucous membrane.

3. The =lymph follicles= (Noduli lymphatici) are found either scattered or in groups. In the former case they are termed =solitary glands= (Noduli lymphatici solitarii), in the latter =Peyer’s patches= (Noduli lymphatici aggregati). The solitary glands are about the size of a millet-seed or a small sago grain. Peyer’s patches are situated chiefly along the surface opposite to the mesenteric attachment and begin about three or four feet from the pylorus. They number one to two hundred, and are usually one to two inches (2 to 5 cm.) long and a quarter of an inch to one-half inch (ca. 2 to 14 mm.) wide. Larger ones occur in the terminal part, where one patch may have a length of seven to fifteen inches (ca. 17 to 38 cm.) and a width of half an inch to one inch (ca. 5 to 25 mm.) in young horses (Ellenberger). They undergo atrophy in old subjects.

FIG. 265.—PEYER’S PATCHES OF SMALL INTESTINE OF HORSE. ]

=Vessels and Nerves.=—The =arteries= of the small intestine come from the cœliac and anterior mesenteric arteries. The =veins= go to the portal vein. The =lymph vessels= are numerous and go to the mesenteric lymph glands. The =nerves= are derived from the vagus and sympathetic through the solar plexus.

THE LARGE INTESTINE

The large intestine (Intestinum crassum) extends from the termination of the ileum to the anus. It is about twenty-five feet (ca. 7.5 to 8 in.) in length. It differs from the small intestine in its greater size, in being sacculated, for the most part, possessing longitudinal bands, and having a more fixed position. It is divided into cæcum, great colon, small colon, and rectum.

THE CÆCUM

The =cæcum= (Intestinum cæcum) is a great cul-de-sac intercalated between the small intestine and the colon. It has a remarkable size, shape, and position in the horse. Its length is three to four feet (ca. 1 to 1.25 m.), and its capacity about seven to eight gallons (ca. 25 to 30 liters). It is conical in form, and is curved somewhat like a reversed comma. It is situated almost entirely to the right of the median plane, extending from the right iliac and sublumbar regions to the abdominal floor behind the xiphoid cartilage. Both extremities are blind, and the two orifices are placed close together on the concave curvature. It presents for description a base, a body, and an apex.