DIGESTION, ORGANS AND PROCESS OF, IN MAN. The process of digestion is one of the chief of those organic functions 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 it to he appro priated by the various organs of the body. The general expression. 'function of digestion.' in eludes several minor or subordinate processes. According to Milne Edwards, the acts of the di gestive function may be classed as follows: ( ) The prehension of the food: (2) its mastication: (3) its insalivation: (4) its deglutition: (5) its ehrmification or stomachal digestion; ( 6) its ehylifieation or intestinal digestion; (7) defeca tion: and (S) the absorption of the chyle. lte fore examining these acts in succession. and the mechanism by which each is effected, we must have clear conceptions regarding the tion of food, the quantity of food, and other allied subjects, which are diseussed in the article Form: and we must likewise have some knowl edge of the causes of those sensation, which we call hunger and thirst, which are. Or ought to he, our natural guides regarding the periods for taking food and the quantity to he taken. The immediate cause of ordinary hunger eannot be explained: but that it is due to some peculiar condition of the gastric, mucous membrane seems probable from the fact that the sensation continues after division of the pneumogastrie nerves, front which the stomach mainly derives its 111'1'1'011S tibres, if we correctly interpret the feelings of the animals on which the experiments were made. In oxtreme hunger. the ,ffifi.rer emn a of sinking. which is referred to the region of the stomach, while general fa int uess and sometimes considerable pain are present. Ilunger, or the want of food which occasions it, may lie diminished by rest, sleep. or any cause that retards the general change which is perpetually going on in all the of the body. Tobacco and alcohol tend to limit the disintegration of the tissues, and thus to keep ()II' or diminish hunger. When the sensa
tions of extreme hunger are not relieved by food, the body begins to feed upon its OW II sues, 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 anima Is always dying sooner than older ones. In Man. total privation of food usually causes death in about a but if a little drink be allowed, life is considerably prolonged. Thirst is depen dent upon a peculiar condition ( probably undue dryness) of the mucous membrane of the upper part of the digestive tube. The thirst in febrile affections 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.
(11 In the act of prehension, man and the other vertebrates employ the hands or anterior ex tremities and mouth, the lips and anterior teeth. and, to a certain extent. the tongue. The prehen sion of fluids is effected in two ways: the liquid is poured into the mouth. and is allowed to fall down the throat : in other cases, the tongue is used after the fashion of a piston. being drawn within the month so as to exhaust the anterior part of that cavity. and fluids are thus forced to enter by atmospheric pressure.
(2) Mastication is effected in the cavity of the mouth by means of the teeth (q.v.), or, in their absence in some animals, by hardened surfaces that perform their functions. The upper jaw is immovable; but the lower jaw, with its teeth, is capable of moving in all directions by means of powerful museles. It is by the vitriol 1110 Ve /Bents of the lower teeth against the upper. through the action of these muscles. that the food is broken down or masticated. This operation is very important, since the more the fond is broken down the more easily will it mix with the saliva and other fluids which participate in the diges tive process.