DIGESTION, ORGANS AND PROCESS OP. The function or process of digestion is one of the chief of those organic functions which are directly concerned in maintaining the life of the individual, inasmuch as it is that through which the animal is enabled to receive aliment, and to prepare or modify it for being assimilated to, and appropriated by, the various organs of the body, or, in other words, for being converted into blood.
The general expression, " function of digestion," includes several minor or subordi nate processes. According to Milne Edwards, the acts of the digestive function may be classed as follows: 1. There is the prehension of the food; 2. Its mastication; 3. Its insalivation; 4. Its deglutition; 5. Its chymification or stornachal digestion; 6. Its chylitieation or intestinal digestion; 7. Defecation; and 8. The absorption of the chyle.
Before examining these acts in succession, and the mechanism by -which each is effected, we must have clear conceptions regarding the classification of food, the quantity of food, and other allied subjects, which are discussed in the article DIET; and we should likewise have some knowledge of the causes of those sensations which we call hunger and thirst, which are, or ought to be, our natural guides regarding the periods for taking food, and the quantity to be taken. The immediate cause of ordinary hunger cannot be explained; but that it is due to some peculiar condition of the gastric mucous mem brane, seems probable from the fact, that the sensation continues after division of the pneumogastric nerves, from which the stomach mainly derives its nervous fibers, if we correctly interpret the feelings of the animals on which the experiments were made. In extreme hunger, the sufferer complains of a sense of sinking, which is referred to the region of the stomach, while general faintness and sometimes considerable pain are present. Hunger, or the want of food which occasions it, may be diminished by rest, sleep, or any cause that retards the general change which is perpetually going on in all the tissues of the body. We have shown in the article DIET, that tobacco and alcohol have
a power of limiting the disintegration of the tissues, and thus of keeping off or diminish ing hunger. When the sensations of extreme hunger are not relieved by food, the body begins to feed upon its own tissues, and the symptoms of starvation (q.v.) begin to manifest themselves. The period at which death occurs from abstinence, varies greatly in different animals—young animals always dying sooner than older ones. In man, total privation of food usually causes death in about a week; but if a little drink be allowed, life is considerably prolonged.
Thirst is dependent upon a peculiar condition (probably undue dryness) of thef mucous membrane of the upper part of the digestive tube. The thirst in febrile afrec tioDs is, however, probably due to the morbid state of the blood.
We now proceed to the consideration of the different acts of which the digestive function is made up.
1. In the act of prehension, man and many of the lower animals (monkeys,squirrels,etc.) employ the hands or anterior extremities and mouth; the lips and anterior teeth, and, to a certain extent, the tongue, being also employed in this function. In the lower animals, however, the modes of prehension are various. Some (like the giraffe) twist the tongue around the leaves, and young 'branches of trees; others (the ant-et: Wen) have a remark ably long tongue, covered with a viscid secretion, and by thrusting this organ into ant hills, etc., secure their prey; and in the chameleon among reptiles, and the woodpecker among birds, the tongue seems specially developed for prehensile purposes. In the elephant, this act is accomplished by the prolongation of the nostrils into the organ popularly known as the trunk, In other mammals (the ruminants and solipeds), the large pendulous lips are the organs employed. In birds, the bill (which is a modifica tion of the lips) is always the prehensile organ of that class.