Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/729

NUTRITION Vomiting may, within certain limits, be inhibited by taking a series of deep and rapid inspirations at a time when the sense of nausea is becoming unbearable. True vomiting seems to be impossible without the aid of the abdominal muscles, as, for example, when the abdomen is laid open. In such conditions emetics cause active opening of the car diac orifice and movements of the stomach-walls, but not the full and free expulsion of the contents. Vomiting is usually a reflex act, the centre for which lies in the medulla oblongata near the centre for respiration, though it is reasonable to suppose that it may occur not only as a reflex act but as a result of direct stimula tion of the centre or centres associated with it. Secretions of the Alimentary Canal. Production of the Alimentary Juices. We have described the movements of the alimentary tube by which the food is triturated and agitated, and finally propelled from mouth to anus. One object of these movements is to mix together the food and certain solvent juices which are poured upon the food at various points. We have now to give an account of these juices and their properties. They are produced in, or by the agency of, the epithelium cells lining the interior of glands which are either situated in the walls of the alimentary canal or which empty their secretion into it. Though these cells derive the materials necessary to their metabolic activity from the blood, the substances which they elaborate and which are characteristic of the secre tions that they help to form are not found in the blood, but are products of the activity of the protoplasm of the cells themselves. The act of secretion is a function of the living cell, and not a mere act of filtration or diffusion through the vascular walls to the cell -substance. It often involves the elaboration of entirely new substances. The act of secretion is, or may be, under the control of the nervous system ; it may be started, inhibited, and the products of the secretion may be variously modified, by the stimulation of distant nerves. In some glands cer tainly, and possibly in all, the elaboration of the specific secretion of the gland takes place in two well-defined stages, and the two stages are indicated by differences in the anatomical appearances of the gland-cells. The characteristic constituents of the several juices which are specially concerned in the chemical changes of the alimentary canal are certain so-called "unorganized" ferments, which we shall, following the suggestion of Kiihne, denominate enzymes. Like all ferments, these are capable, under suitable circumstances, of initiating specific changes in certain bodies with which they are brought into contact, changes which may be incommen- surably great when contrasted with the magnitude of the mass of the ferment engaged. "Unorganized " or, as they have also been called, " unformed " ferments differ, how ever, from the " organized " or " formed " in that, whilst they are the products of the activity of living cells y when once formed they cease to be living, are unconnected with any organized form, and have no power of reproduction or increase. The enzymes for the most part exert their action un impaired in the presence of certain bodies which kill the great majority of organized ferments ; thus salicylic acid does not hinder peptic and tryptic digestion, whilst it prevents the putrefactive changes which are very apt to occur in the latter case, and which are connected with the development of organized ferments. Certain enzymes, as the diastatic ferment of saliva and pancreatic juice, are, however, destroyed by salicylic acid. As will be shown in detail in the sequel, the secreting cells of glands which produce enzymes exhibit marked differences corresponding to different states of activity. In the case of the secreting cells of the pancreas the cells appear to produce and store up for a time a body, a zymoyen, from which subsequently the tryptic enzyme trypsin is set free. The progress of research may perhaps establish the existence of zymogens corresponding to the other enzymes. Usually the glandular organs which produce the aliment ary juices contain stored up within them their character istic enzymes, which may be extracted by digesting the comminuted organ in water, or still better in glycerin, which dissolves them nearly all, and furnishes solutions which preserve their activity long unimpaired. Enzymes are all insoluble in strong alcohol, so that the tissues from which they are to be extracted may be first de hydrated by digestion in absolute alcohol and afterwards extracted with glycerin. Solutions of enzymes are rendered inactive by boiling or by exposure to a temperature above 70 C. The principal enzymes of the alimentary canal belong either to the group of proteolytic or to that of diastatic or amylolytic ferments : the enzymes of the first group (pepsin and trypsin) dissolve proteids and effect their more or less perfect decomposition ; the enzymes of the second class (as ptyalin and the diastatic ferment of the pancreas) dissolve starch and produce from it a series of bodies which will be discussed in reference to the action of saliva on starch. Besides the proteolytic and amylolytic ferments, there occur in the alimentary canal a curdling ferment, an inverting ferment, and perhaps a fat-decomposing ferment. All enzymes exert a more energetic action at a moder ately high than at a low temperature, though the influence of a rise in temperature is more marked in some cases than in others. The reaction of the medium in which they are placed influences remarkably the activity of cer tain enzymes ; thus pepsin, the proteolytic ferment of the stomach, is inactive in neutral or alkaline solutions, the pre sence of a free acid being essential to its activity, whilst trypsin, the proteolytic ferment of the pancreas, acts with feebleness in solutions which are neutral or faintly acid, since it needs for the full exercise of its powers a decidedly alkaline medium. The enzymes appear to possess the power of rapidly inducing, at the temperature of the animal body, in bodies subjected to them similar chemical operations to those which can be brought about with great slowness by prolonged heating with dilute mineral acids, or by the prolonged action of boiling water or superheated steam. These operations are of the nature of hydrolytic decomposi tions, that is to say, such as are connected with the union of the elements of water with the decomposing body. Salivary Glands and Secretion of Saliva. There are no secreting glands in the body which have been sub jected to so elaborate a study as the salivary glands, whether we consider their structure or the circumstances which influence or accompany the act of secretion. We shall therefore give the results of researches on these glands at much greater length than those referring to the other glands concerned in the preparation of the aliment ary juices. As has been already said, the saliva is secreted by several glands of which the ducts pour their secretion into the cavity of the mouth, where it is mingled and constitutes the "mixed saliva." The chief of these glands are the parotid, the submaxillary, and the sublingual glands, though their secretion is mixed with that of small glands (mucous and serous) scattered through the mucous membrane of the mouth and tongue, which are included under the term " buccal " glands. The salivary glands all belong to the group of acmous or compound racemose glands. According to the researches of Heidenhain, they may, however, be divided into two groups, which he has denominated serous or albuminous and mucous glands, according to the structure of the celb of their acini, their chemical characters, and the nature of the secretion which they elaborate.