Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/732

Rh 674 NUTRITION acid -forming cells have of late been termed (Langley) oxyntic glands (ogvvcir, to render acid). No accurate estimate can be formed of the amount of gastric juice secreted during twenty-four hours. It has been calculated to be between 20 and 30 pints. Secretion of the gastric juice may occur after all the nerves going to the stomach have been divided, though it is for a time arrested after division of the pneumogastric nerves. It is probable that the process of gastric diges tion is essentially under the control of an intrinsic nervous mechanism situated in the mucous membrane, though this is normally influenced by the higher nerve centres. Liver and Secretion of Bile. The structure of the liver has already been sufficiently described (vol. vii. p. 229 sq.), and need not be further alluded to in this place. The secretion of the liver, the bile, is being continually formed, though not always at the same rate. In animals possessed of a gall-bladder it is stored in the intervals of digestion in this reservoir. When food is taken into the stomach the bile begins to be secreted in larger quantities, the maximum being reached about six hours after the meal. As the so-called acid &quot; chyme,&quot; to be afterwards referred to, passes the opening of the bile-duct in the duodenum, it is probable that a stream of bile is poured upon it by the reflex contraction of the gall-bladder. During the time when bile is being secreted much heat is evolved in the liver. The bile, as will be shown in the sequel, should be looked upon as a liquid containing only certain, for the most part useless, by-products resulting from great chemical operations going on in the liver. Little is known as yet of the exact changes which occur in the liver-cells during activity, nor of the manner in which the nervous system influences the secretion of bile. Pancreas and the Secretion of Pancreatic Juice. The pan creas possesses a structure which presents great resemblance to that of serous salivary glands, and its alkaline secretion was until lately looked upon as closely resembling the saliva. The secreting cells of the pancreas, which differ but little microscopically from those of such a salivary gland as the parotid, exhibit very marked differences, which correspond to different states of functional acti vity. During a period of glandular repose the cells con tain innumerable granules, which are congregated at that side of the cell which lies towards the centre of the acini (fig. 6, B). The A B Outer Or peripheral FIG. 6. Pancreas of Rabbit. A, during rest; B, in i--fi r&amp;gt;v, rvf tVia nolle, activity, a, inner granular zone; 6, outer trans parent zone ; c, lumen ; d, indentation at junction the Smaller part of two cells - ( From Foster s Physiology, fig. 43.) of the resting cell is clear. After a period of glandular activity the granular half of each cell is found to have diminished greatly (fig. 6, A) ; the whole cell is clear and distinctly smaller than before, and its behaviour towards colouring matters is very different. The pancreas, when perfectly fresh and just removed from the yet warm body of an animal which is killed, does not contain, ready formed, all ferments which will in the sequel be referred to as characterizing the pancreatic juice. If we treat the gland, for instance, with glycerin, which possesses the power of extracting and dissolving all the ferments, we fail to obtain a solution which possesses the power of digesting proteids ; but, instead, we find a substance in the solution from which, by the addition of a little acetic acid, the pro- teolytic ferment may be formed. The cells of the pancreas thus elaborate a substance which is the antecedent of the proteolytic ferment, and which yields it when it passes into the pancreatic ducts ; it is customary to speak of this body as symoyen or ferment- former, because it gives rise to one of the chief enzymes or ferments of the juice. The secretion of pancreatic fluid is slight except during digestion. After the taking of a full meal the secretion is suddenly exalted, reaching its maxi mum two or three hours afterwards. The secretion then diminishes until a period which extends from the fifth to the seventh hours, when a rise occurs, which lasts to between the ninth and eleventh hours after food. The secretion then gradually sinks, until it absolutely ceases. Stimulation of the gastric mucous membrane starts the secretion of pancreatic juice ; it is arrested during nausea and vomiting, as also when the central end of the divided pneumogastric is stimulated. Intestinal Glands and their Secretion. The mucous mem brane of both the small and the large intestine contains embedded in its substance innumerable simple tubular glands, the so-called crypts or glands of Lieberkiihn, which are lined by a single layer of cylindrical epithelium con tinuous with and resembling that covering the exposed surface of the intestine. Very little is known as to the function of these glands, whose office is probably widely different in different sections of the intestinal tube, particu larly in the large as distinguished from the small intestine. We do know that the epithelium -cells of these glands are capable of forming mucus, and that in the small intes tine they are doubtless the source whence is derived the so-called intestinal juice or &quot;succus entericus&quot; which is poured into the intestine in large quantities under par ticular circumstances, as, for instance, when all the nerves supplying the gut are divided. Physical and Chemical Properties of the Several Alimentary Juices. The mixed saliva of man is, when perfectly fresh, a clear, transparent, viscid liquid, which, on microscopic exa mination, is found to contain cells of squamous epithelium derived from the mouth, besides certain globular cells derived from the salivary glands and called &quot; salivary cor puscles.&quot; Its reaction is alkaline; it has a specific gravity of about 1 003, and contains about five or six parts per thousand of solid matters. Its solid constituents consist ( 1 ) of certain saline matters transuded from the blood and, in addition, of a small quantity of a soluble sulphocyanide, detected by the reddish tinge caused by the addition of solution of ferric chloride to the saliva; (2) small quantities of soluble proteids, coagulable by heat ; (3) mucin, to which saliva owes its viscidity, and which is thrown down when the liquid is acidulated with acetic acid ; (4) a ferment or enzyme called ptyalin, salivary diastase, or the diastatic ferment of saliva. The last-mentioned is the most import ant constituent. It possesses the power of rapidly render ing cooked starch soluble, though its action on raw starch is but very slight. Not only does it dissolve starch, but it progressively decomposes the complex starch molecules into isomeric bodies of less complexity. Starch has a chemical composition which may be represented by the empirical formula (C^HjQOjo)^ the exact value of n being yet a matter of doubt. The salivary diastase, under suitable conditions, as has been said, first converts insoluble into soluble starch (hence it is sometimes termed an amylolytic ferment) which, it has been surmised, probably has the composition (C 12 H 10 O 10 ) 10. This soluble starch, like the insoluble body, gives a beautiful blue colour with iodine. By the further action of the ferment the soluble starch yields a series of dextrins which are isomeric with starch, though they possess smaller molecular weights and differ-