(3) Insalivation is effected by the admixture of the secretions of the three pairs of salivary glands Ithe parotids. the submaxillaries. and the sublinguals) and of the lateen] mucus with the triturated food. (See OLAND.) The common saliva is a colorless. turbid. viscid. •inodorous, and tasteless. fluid. which, after standing for sonic time, deposits a layer or pavement epithelium (see E Prrn r M ) and imams corpuseles. In the normal state. its reaction is alkaline, but the d•gree of alkalinity varies, and is greatest during and after meals. Saliva does not contain more than five or six part'. of constituent, to 993 or ti114 part, of water, the most important in gredient being an organic matter termed ptyntsn, and sidphocyanide of potassium, neither of which substances ()mfrs in any other solid or fluid of the belly. The daily quantity of saliva seereted by an adult man is estimated at about 4 ounces, but the activity of the salivary gland: is depen dent upon varion, influences and conditions.
nno ement of the lower jaw. as in mastieat ing, speaking, or singing, increases the secretion; as also do acrid and aromatic substances, and dry, hard food; while the use of moist and soft food is accompanied by a scanty secretion. The uses of saliva in reference to digestion are partly mechanical and partly chemical. The moistening of the dry food by the saliva serve. the double purpose of adapting it for deglutition and of separating the particle-, and thus allowing them to be more freely acted on by the other digestive fluids; moreover, from its viscidity, it lubricate, the bolus of food, and thus facilitates degluti tion; and it is probably also subservient to the sense of taste. The great chemical use of the saliva is to convert the amylaceous (or starchy) portion of the food into glucose or grape-sugar, and thus to promote it absorption.
(4) Deglutition is the act by which the food is transferred from the mouth to the stomach. The pharynx, or cavity into which the month leads, takes so slight a part in the digestive process, that we need scarcely allude to any anatomical details connected with it. It is sufficient to observe that between it and the mouth is the pendulous or soft palate, which is a movable muscular partition that separates the two cavi ties during mastication. As soon, however, as the latter act is aecomplished, and the bolus is pressed backward by the tongue, the soft palate i- drawn upward and backward, so as to permit the passage of the food into the pharynx. The bolus or pellet of food having arrived near the wsophagus or ( which is continuous in feriorly and posteriorly with the pharynx), is driven into it by the action of certain muscles, which almost surround the pharynx, and are termed its constrictor muscles. All voluntary action ceases as soon as the food is pressed back ward by the tongue into the pharynx. It is impossible to recall the pellet. and it is neces sarily carried on ( without even our eognizan•e) into tln• stomach. On receiving the food forced into its upper extremity by the action of the eonstrietor muscles of the pharynx, the oesopha gus is dilated (for it usually lies in a collapsed state, with its walls in contact, or nearly sot ; this contact of the pellet with its mucous mem brane causes its muscular walls to contract, and the food i- thus driven, by a series id these con tractions, into the stomach. The act of degluti tion is now completed.
(.i ) omaehal digestion or clivm i flea t ion is the next process to be considered. The whole of the alimentary canal (Fig. 1) below the diaphragm (q v.), or great museular partition which sepa rates the cavity of the chest from that of the abdomen or belly. possesses the following charae. teristies, in relation to structure: The stom ach, the small intestine, and the large intes tine arc all lined by mucous membrane, have a muscular (-oat. eonsistinz of two sets of distinct libres—namely. circular fibres which snrroun 1 the tube or aher the manner of a series of rings. and longitudinal fibre, running in the
same direction as the intestine it -elf-4111d are kith a serous membrane. the peritoneum (see N.itot's which at the same time ietains the viscera in their proper position, and permits their necessary movement,.
The human stom ach is an ehanrated curved pouch. Icing almost immediately helots the d i a and having the form of a bag pipe. it is very dilatable and emi t raet ile, and its function is to retain the food until it i duly acted upon and dissolved by the gas tric juice which is secreted by gland, lying in its inner or mucous coat, and then to transmit it, in a semi-fluid or pulpy state. into the duodenum. Its average capacity is about tire pints. The of it which have received spec ial names are the great er curvature 1 Fig. 1) b, the lesser cur vature, upon its up per border. and the ea diac. r, and py loric, d. extremities.
The raucous np.m. bran,. or lining coat of the stoma eh, is thick and soft, and lies i n irregular folds. in c(aiseoplellee of the contraction of the muscular coat. unless when the organ is distended with food. opening the stomach. an 1 stretching it so as- to remove the appearance of fold,. we perceive even with the naked eye, but better with a lens, numerous irregular pit, or de. pre--ions, irregular in shape, and averaging about I-201)th of an inelt in diameter. To Are them properly, the mucus with which they are filled must be washed out (Fig. 2. A). These pit, are so shallow as not to dip into the nmeoui membrane to a greater extent than one-sixth or one-eighth of the thickness. The rest of the thickness is chiefly made lip of minute tubes, runl1111g parallel to one another, and vertically to the surface of the stomach 2. Bl. These are the gastric tubes or glands which seerete the gastric juice from the blood in the capillaries which abound in the mucous membrane. They pa— in two-, thr(•es, or from the bottom of eaeli pit. and usually subdivide into several tubes, which, after running a more or less tortu ous course, terminate in blind or elo,ed extremi ties. These tubes are filled with epithelial eel's, whose eon tents are of granules. with which oil globules are often mixed, and each tube is invested with capillaries, which usually rim in the direction of its long axis. In the pyloric or duodenal end of the stomach, these tubes (at least in the dog and several other animals whose stomachs have been carefully in a per fectly fresh state) are considerably wider than those which we have described, and differ from them also in other respect.; and hence some and follows the curvature toward the pyloric end, d. It then returns in the course of the smaller curvature, and makes its appearance again at the cardiac aperture in its descent into the great curvature to perform similar revolu tions. These revolutions are effected in from one to three minutes." This account. given by Dr. Beaumont, is based on the observations which he made in the stomach of Alexis Saint Martin, a Canadian, with a fistulous opening into the stomach. (See I lEA UMON T. ) Dr. Bruit CM, how ever, adopts a modified view, which is probably the correct one, lie supposes that the semi-fluid food entering at c (Fig. 3), the cardiac orifice, goes in the directions marked a. partly along the greater and partly along the lesser curvature: and that these I WO currents of food meet at the closed pylorus. when they are both reflected into the direetiou b, forming a cen tral or axial current. occupying the real axis of the stomach, which Incites the two aperture-. The mutual interference of these currents at their borders causes a uniform admixture of the various substances composing them, while the reflection of the upper and lower currents into one another insures an equal contact of all the mass with the secreting surface of the mucous membrane.