- THE OESOPHAGUS
Refer back for a moment to the diagram of the digestive tract in your last lecture. You will see the part labeled the oesophagus, which is a thick, muscular tube connecting the mouth to the stomach, and passing through the diaphragm. The tube is lined with mucous membrane, and the walls are made up of involuntary muscle. Once a ball, or bolus, of food has been forced into the oesophagus from the mouth by the process of swallowing, it is automatically pushed down the tube by an action known as peristalsis. This is shown in the diagram below:
Figure 1: An Oesophagus Showing the Peristalsis Action
The muscle in front of the bolus is relaxed, and the muscle behind is contracted, the whole process being a wave-like motion. Once a bolus is in the oesophagus, it must travel down to the stomach.
- 2. THE SIMPLE STOMACH
Animals that are non-ruminants are said to have a simple stomach, the most common farm animals in this category being the horse, the pig and the rabbit. The simple stomach is a bag, of which the walls are made of involuntary muscle, and the inside lined by a membrane of specialised cells. The oesophagus leads into the stomach, and once food has been processed, it passes from the stomach into the small intestine.
Figure 2: The Stomach of the Horse
Both the entrance to the stomach from the oesophagus, and the exit into the small intestine, are controlled by narrow rings of muscle called sphincters. These control the flow and passage of food into and out of the stomach. When the sphincter is contracted it prevents the passage of food, and when it is relaxed, food can pass through – see below.
Figure 3: Shows the Action of a Sphincter
The lining of the inside of the stomach consists of many folds, which increase the surface area and is in contact with the food inside the stomach. In addition, the stomach lining contains many gastric glands, or small pits, and these glands are lined with three types of cells. These are mucous cells, which produce a thick sticky fluid; parietal cells which produce hydrochloric acid and chief cells which produce enzymes. All these substances help in the process of food digestion. A gastric gland is shown by figure 6.
- 3. THE RUMINANT STOMACH
The ruminant stomach is found in animals, such as cattle, sheep and goats. It is designed to deal with large amounts of roughage in the form of grass and crop residues. It is much larger than the simple stomach. It is divided into four compartments; these are the rumen, the reticulum, the omasum and the abomasum – see figure 4.
Figure 4: A Ruminant Stomach
The oesophagus leads to the reticulum, a small compartment, the inner lining of which consists of intersecting ridges forming a ‘honey comb’ network. Food that is swallowed passes first of all into the reticulum.
Figure 5: Cells of the Reticulum
The reticulum leads into the rumen, which is by far the largest compartment of the ruminant stomach. The lining of the rumen consists of small, stalk-like projections called papillae, and the walls are made
of muscle so that the food in the rumen can be thoroughly churned and mixed. The rumen is never more than 2/3rds full of food and water, and a cow’s rumen can hold approximately a wheelbarrow full of food.
Figure 6: Gastric Gland of a Simple Stomach
The rumen leads into the omasum, which is a small compartment lined with long, muscular projections, rather like the pages of a book, as can be seen below.
Figure 7: An Omasum
It is useful to be able to recognise the parts of the ruminant stomach, and when you get the chance to examine the inside of a dead sheep or cow, look for the large rumen, the ‘honeycomb’ of the reticulum, and the long projections of the omasum.
The omasum leads to the abomasum, which is the ‘true’ stomach, and is very similar to the stomach of the non-ruminant. In general, the functions of the reticulum the rumen and the omasum are to break down the food material to the point where it can be dealt with by the true stomach, the abomasum. This is necessary because the ruminant animal lives on uncooked, vegetable matter, much of which is poor in food value so that vast quantities have to be eaten and digested to provide enough nutrients for the animal. Most simple-stomached animals, except the horse, eat much more concentrated foods, and even horses need some form of concentrate food to keep them in good condition.
- 4. THE SMALL INTESTINE
This is a long, muscular tube leading from the stomach or, in the case of the ruminant, the abomasum, to the large intestine. Food comes into the small intestine through the sphincter in the stomach and is pushed along by peristalsis, the same muscular action that pushes food down the oesophagus. This action is entirely involuntary; the animal has no control over it at all. In fact, if the animal gets excited, the action is speed up and food is passed along before it has been properly digested. The small intestine in cattle is about 35 metres long, and in sheep about 25 metres, but it is coiled up inside the animal.
The lining of the small intestine contains many glands which produce mucus and also enzymes that are required for the further digestion of food that has passed from the stomach. In addition, the lining contains many small stalk-like projections called villae, through which digested food passes from the small intestine into the bloodstream. Digested carbohydrates and proteins pass into the bloodstream, while digested fats pass through the villae into the lymphatic system. The small intestine is divided into three parts. The stomach leads into the duodenum, which leads to the jejunum and the last part which is called the ileum.
- 5. THE LARGE INTESTINE
The large intestine consists of the caecum and the colon. The ileum part of the small intestine leads into the caecum which is a large bag designed to hold food for further digestion and absorption. The horse has the largest caecum, because being a non-ruminant, but at the same time a grass-eating animal, much of the break-down of roughage takes place there. In other non-ruminants the caecum is small. In humans it is called the appendix and is not used at all; in fact, it can be removed altogether without any ill effects.
Food passes from the caecum into the colon where further absorption takes place, and where water is removed and absorbed back into the body. By now all that is left is waste material not required by the animal and passes into the rectum and out of the body through the sphincter of the anus. One of the effects of some infections and also laxative medicines is to prevent the absorption of water in the colon, so that a very watery liquid is passed out of the anus.
Figure 8: Cross Section through a Villus
