DIGESTION. L'39 DIGESTION. Food Starcb diastase Each piomiiieiil fooil is lUted upon by a par- ticular L'lizyuie adapted to transform it. The follow inj; table shows the principal sorts of jilant- foods. the enzymes by which they are digested, and the mo^t important products formed. I. Carbohydrates Enzyme Product ( maltose i dextrin IduUu inulaso leviilose Cellulose cytase a variety of sugar Saccharose invertase j B'"^™^^ Maltose gUicase glucose Pectins pectase pectic acid II. PttOTEIDS Albumins I tm.ain J peptones and Globulins; trypsin j fl„a|,y amides III. Other Foods Glywrides (fats) lipase { gjj;;'yj,".ij Glucosides emulsin | g'",';""'";.,'"! '"'" ( nous substances Some of the more jjromineut examjiles of di- gestion may be mentioned, ^■hcn a fungus lives upon or iu a dead or living body, it secretes enzjTiies which digest such of the substances it comes in contact with as it can attack; these may then be absorbed and used for its nourish- ment. During the day the leaves of green plants form carbohydrates faster than they can be car- ried away, and a portion is stored temporarily in the chloroplasts (q.v.) as starch-granules. Later, and especially during the night, when no food is formed, these starch-grains are digested and transferred to places of use or permanent storage. Toward spring potatoes lose their mealiness when cooked, and become gummy and sweetish on account of the digestion of starch into sugar by diastase. When the shoot starts to grow the process is hastened. In the germina- tion of spores and seeds, the stored foods, usually starch, oils, and proteids, are digested by appro- priate enz>niies. and the materials thus made available for the growing parts. DIGESTION, CoiCPAK.^TivE Pnysioi-ooY of. See Alimp^ntary SYSTEit, Evolution of. DIGESTION, Organs and Process of, in M.N. 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 be appro- priated by the various organs of the body. The general expression, 'function of digestion,' in- cludes several minor or subordinate processes. According to Jlilne Edwards, the acts of the di- ptstive function may be classed as follows : ( 1 ) The prehension of the food: (2) its mastication: (3) its insalivation: (4) its deglutition: (.5) its chymificatii)n or stomachal digestion; (6) its chylifieation or intestinal digestion: (7) defeca- tion; and (8) the absorption of the chyle. Be- fore examining these acts in succession, and the mechanism by which each is effected, we must have clear conceptions regarding the classifica- tion of food, the quantity of food, and other allied subjects, which are discussed in the article Food: and we must likewise have some knowl- edge of the eau.ses of those sensations which we call hiinr/rr and Ihirfit. which are, or ought to lie, our natural guides regarding the periods for taking food and the quantity to he taken. The immediate cause of ordinary hunger cannot 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 pneuinogastric nerves, from which the stomacli mainly derives its nervous fibres, if we correctly interpret the feelings of the animals on which the experiments were made. In extreme hunger, the sufferer coin- plains of a sense of sinking, which is referred to the region of the stomach, while general faint- ness 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. Tobacco and alcohol tend 1o limit the disintegration of the tissues, and thus to keep oft' or diminish hunger. When the sensa- tions of extreme hunger are not relieved by food, the body begins to feed upon its own tis- 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 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 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. ( 1 ) 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 tluids is effected in two ways; sometimes the liquid is poured into the tnouth, and is allowed to fall down the throat; in other cases, the tongue is used after the fa.shion of a piston, being drawn within the mouth so as to exhaust the anterior part of that cavity, and lltiids 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 muscles. It is by the varied move- ments of the lower teeth against the u|)per. through the action of these muscles, that the food is broken down or masticated. This operation is very important, since the more the food is broken down the more easily will it mix with the saliva and other fluids which participate in the diges- tive process. (.3) Insalivation is effected by the admixture of the secretions of the three pairs of salivary glands (the parotids, the submaxillaries, and the sublinguals) and of the Imecal mucus with the triturated food. (See Oland. ) The common saliva is a colorless, turbid, viscid, inodorous, and tasteless fluid, which, after standing for some time, deposits a layer or pavement epithelium (see KpiTllEl.ll'M) and mucus corpuscles. In the normal state, its reaction is alkaline, but the degree of alkalinity varies, and is greatest during and after meals. Saliva does not contain iilioFe
