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Comparison Between the Digestive Tracts of a Carnivore, a Herbivore and Man
Part 3: The gut of a herbivore — The sheep
The sheep is a herbivore. While all carnivores' digestive tracts are similar, herbivores' digestive systems vary widely. There are two basic types of herbivore:
The latter type are called ruminants because they 'ruminate' or chew a cud as part of their digestive process. A ruminant's stomach is complex, having four chambers. They also have a large caecum. Herbivores with simpler stomachs have a relatively larger caecum to help with digestion.
As we all know, a sheep is a herbivore that eats grass. A woolly ball on legs on the outside, a sheep's inside is unbelievably complicated. (2) The total length of the sheep's digestive tract is about twenty-seven times as long as the animal's body length. This dimension is common to all herbivores.
The first major difference between the herbivore and the carnivore is the sheer amount of food the herbivore is forced to eat. While a carnivore can usually manage with one small meal a day, the herbivore must eat so much that it is continually eating and its stomach is never empty.
The mouth. A sheep has no incisors or canine teeth in its upper jaw. It doesn't bite grass off; it tears it off. The sheep's molars are flat and its jaw movements are rotary, designed for grinding rather than for crushing or tearing. The sheep's salivary glands are very important. They produce the prodigious amounts of saliva necessary to fully permeate the food during rumination. While chewing is of little importance to the dog, it is vital to the sheep.
The rumen. As a sheep grazes, the grass passes straight into the first chamber of the stomach, the rumen . This has a capacity of some four gallons. When the rumen is full, and the sheep has an opportunity, it regurgitates small parcels or 'cuds' of food back to its mouth for chewing and further mixing with saliva. The saliva of a sheep does not contain amylase necessary for digesting starch, so this 'chewing the cud' must merely be to aerate, macerate and mix the saliva more thoroughly to aid digestion of the grass.
The rumen does not contains any digestive juices but it does contain billions of bacteria and protozoa which begin the process of breaking down the cellulose cell walls into cellobiose to begin the process of releasing the nutrients inside. This is a process entirely missing from the digestive system of a carnivore. Some carbohydrates are converted to fatty acids and others are absorbed by bacteria and other micro-organisms to be converted into other substances. About seventy percent of the cellulose is absorbed directly into the bloodstream from the rumen.
The reticulum. The next chamber after the rumen is the much smaller reticulum , with a capacity of about four pints. It is here that small parcels of food are compacted into cuds for regurgitation to the mouth for rumination. These then return to the rumen for more bacterial breakdown.
The omasum. In time, the contents of the rumen and reticulum pass to the third chamber, the omasum . This holds about a gallon of material. Again, the food is subjected to attack and breakdown by bacteria and other micro-organisms. Note that although we are three-quarters through the stomachs of the sheep, we have yet to encounter any digestive enzymes. All these chambers are solely concerned with the breakdown and liquefaction of the food into such a form that it can be digested when it is eventually subjected to such enzymes.
The obomasum. The fourth and last chamber of the sheep's stomach, the obomasum , which holds about two gallons, is the sheep's true stomach. The obomasum has glands which secrete hydrochloric acid, pepsin and a weak fat-splitting enzyme called lipase. All of these enzymes are much weaker in concentration that in the dog's digestive system. These enzymes break down the plant proteins and fats and, much more importantly, they kill and absorb the billions of bacteria and other micro-organisms that have done all the work so far. In this way plant protein is transformed into animal protein within the herbivorous digestive tract, making it possible for herbivores to survive without even traces of animal protein in their diet.
Intestine. From here on digestion takes place much as it does in the dog. The difference is the bacterial breakdown of the plant cell walls by the first three chambers of the sheep's stomach, which has no parallel in the carnivorous dog.
The sheep's digestive system is very wasteful, unlike that of the dog, over fifty percent of the food eaten is excreted.
Now that we have considered both a carnivore and a herbivore, in Part 4 we will look at the gut of a human to see where it fits.
Reference
2. MacGregor R. Structure of the Meat Animals . The Technical Press Ltd. London, 1952.
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