Notes on Veterinary Anatomy

by Charles James Korinek

The oesophagus, or the continuation of the gullet, is a tube extending from the gullet to the stomach, and is used to convey the food to that organ. It is made up of two coats, the muscular and the mucous. The former contains fibers which, when once the food enters the tube, contract behind it, forcing it down to the stomach. Its lining is a continuation of the mucous membrane of the mouth and the gullet. In tracing the oesophagus or tube down the neck from the gullet, note that it passes down the left side of the neck, entering the thoracic, or chest cavity, between the lungs over the heart through the large muscular curtain known as the diaphragm, then enters the stomach an inch or two after passing the diaphragm.

The Stomach.—This organ is very small in the horse in comparison with that of the ox. It holds only about four gallons, and is situated just back of the curtain which separates it from the lungs. It lies mostly to the left side. The walls of the stomach being composed of three coats. That on the outside is called the serous membrane, a name applied to membranes which line closed cavities, such as the abdominal cavity. The inside lining is a continuation of the mucous membrane lining the organs before mentioned. The lining in the left part of the stomach or the part where the food is prepared for digestion is the same color as that of the mouth. The lining of the right part of the stomach, which is the true digestive part, is of a deep red color resembling velvet, and when placed under a microscope has the appearance of a honey-comb. When the stomach is empty this membrane is thrown into loose folds. Several small openings may be noticed through which the gastric juice and pepsin from the glands, situated in the walls of the stomach, enters. These are very important fluids as they assist greatly in digesting the food. The third coat is known as the muscular coat already mentioned. Its action is to give the stomach a churning motion, rolling the food around and mixing it with the juices. The opening to the stomach is guarded by a valve which prevents the food from passing back through the gullet. There is also a valve at the opening of the bowels, preventing any coarse, undigested food from entering them. The stomach is held in its place by five large ligaments, and is well supplied by blood-vessels and nerves. Digestion of the food takes place very quickly in the horse in comparison to other animals. Frequently a change of food or working too soon after eating will interfere with the digestion, thus setting up what is known as indigestion. This is a very painful disease in the horse. After the food is acted upon by the juices in the stomach it changes into what is known as chyme, which passes into the bowels.

The Bowels.—They are divided into two parts—the large and small.

The small bowels are 72 feet in length, about one inch in diameter, and are made up of three coats, same as the stomach. The serous coat on the outside contains small glands which secrete an oily material to lubricate the outside of the bowels, which comes in contact with the inner wall of the abdominal cavity. The muscular coat, made up of muscular fibers, is situated between the other two coats, the same as in the stomach; its action is to contract the bowels, giving them motion to convey the food along through them. The mucous coat is a continuation of the mucous coat of the stomach. Along this coat are found small glands known as villi lacteal; these absorb the nourishment from the food as it passes along through the bowels and pours it into the blood. The small intestines or bowels are attached on the upper side to what is known as the mesentery, which is attached above to the roof of the abdominal cavity. It can be seen in any of the smaller animals upon examination. About six inches from the stomach, in the bowels, are found two openings. One of these receive the hepatic duct, a tube for the purpose of carrying the bile from the liver to be poured in on the food as it passes through the bowels. The other opening is for the duct of the gland known as the pancreas. It secretes a clear fluid known as the pancreatic juice. These juices act on the food in the first part of the small intestines, changing it into chyle. After this, the action of the rest of the intestines is to absorb the nourishment out of the food as it is passing back. The small intestines and stomach, when in a healthy condition, should be found empty one hour after food has been eaten. The small bowels or intestines are situated mostly on the left side just behind the stomach.

The large bowels have three coats, the same as the small ones. The first part of the large bowels is known as the blind bowel or caecum, and is about three feet in length; this is generally the first thing to protrude when opening a horse’s abdominal cavity. Its use is to act as a reservoir to hold the water and fluid of the food; from this organ the water and fluid parts are mostly taken up into the system. The next part of the large bowels is known as the large colon; it lays along the floor of the abdominal cavity, is about nine feet in length and is doubled on itself three times. In this bowel the solid part of the food is found. Here digestion is brought about by the contraction and expansion of the muscles of the bowel and the nourishment taken from it, after which it is worked back out of this bowel and enters what is known as the floating colon. This is about ten feet in length and about two inches in diameter, or double the size of the small bowel. It is thrown into folds or pleats, and as that portion of the food containing no nourishment passes through it is worked into balls which pass back and are emptied into the rectum or back bowel. This is situated at the back part of the abdominal and pelvic cavity back of the small bowels or intestines and like them, is suspended by a fold paritoneum.

The rectum or back bowel is sometimes known as the straight bowel. It is about 18 inches long and forms the last part of the bowels or intestines. Its coats are a continuation of those of the large bowel, but each is thicker and heavier. Above this bowel are the bones of the sacrum, below it, in the horse the bladder and other small glands. Below the rectum of the mare are situated the womb and the vagina, the latter being the passage into the womb from the outside. The bones which help to form the pelvic cavity are situated at the sides, and at the back immediately under the tail is what is known as the annus. The use of the rectum is to hold the balls as they pass back from the floating colon. When the rectum becomes so full that there is pressure on the sides of the wall thus stimulating the nerves the muscular coat contracts and forces the contents back towards the annus. At the same time the muscles of the annus dilate, causing the faeces to pass out.

Accessory Organs of Digestion.—In the abdominal region these organs are the Liver, the Pancreas, and the Spleen.

The Liver is the largest gland in the body and is situated between the stomach and the diaphragm. The liver of the horse weighs from ten to twelve pounds. It is of a dark brown color, well supplied with blood and nerves, and is held in place by several strong ligaments. A bitter, greenish colored fluid called the bile is secreted from the liver and emptied into the digestive system, where it plays an important part in its action on the food. There is no gall bladder in the horse, but simply a tube passing from the liver to the small bowel into which it empties the bile about six inches back of the stomach. It is important to note that it is in this tube that gall stones sometimes collect.

The Pancreas is another very important gland. It is of a grayish, fatty color, and may be found near the roof of the abdominal cavity in front of the kidneys. The Pancreas secretes a clear, colorless fluid called the pancreatic juice. This fluid, like the bile from the liver, also plays an important part in the digestion of food. It is carried down from the Pancreas by a duct or tube emptying into the small bowel just back of that of the liver.

The Spleen is a long, flat gland about fifteen inches in length, situated along the left side of the stomach and to which it is closely attached. It is of a grayish red color and feels quite soft. It is ductless, there being no secretion passing from it. The function of this gland is not clearly understood, but by many it is supposed to regulate the temperature of the stomach during the process of digestion and to act as a reservoir for the blood. Some speak of it as the burying ground of the red corpuscles. It is well supplied with blood vessels and nerves and weighs about two pounds in an average sized horse.