DIGESTION. 240 DIGESTION. than five or six parts of solid lonstitupnts to 995 or '.t!t4 parts of water, the most important in- gredient heint; an organic matter termed pti/aliii. and siilplux'vanide of potassium, neither of which substances occurs in any other solid or lluid of the body. The daily quantity of saliva secreted by an adult man is estimated at about 48 ounces, but the activity of the salivary glands is depen- dent upon various influences and conditions. Thus, movement of the lower jaw, as in masticat- ing, speaking, or singing, increases the secretion; as also do acrid and aromatic substances, and dry, hard foml: 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 serves the double purpose of adapting it foT deglutition and of separating the particles, and thus allowing them to be more freely acted on by the other digestive lluids; moreover, from its viscidity, it lubricates 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 its 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 mouth leads, takes so slight a part in the digestive process, that we need scarcely allude to any anatomical details connected wit^i it. It is sufficient to observe that between it and the mouth is the pendulous or soft palate, which is a movable uiuscular partition that separates the two cavi- ties during mastication. As soon, however, as the latter act is accomplished, and the bolus is pressed backward by the tongue, the soft palate is 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 fesophagus or gullet (which is continuous in- teriorly and posteriorly with the pharynx), is driven into it by the action of certain muscles, which almost surround the pharynx, and are termed its coitKlrictor 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 cognizance) into the stomach. On receiving the food forced into its upper extremity by the action of the constrictor muscles of the pharynx, the a'sopha- gis is dilated (for it usually lies in a collapsed state, with its walls in contact, or nearly so) ; tliis contact of the pellet with its mucous mem- brane causes its muscular walls to contract, and the food is thus driven, by a series of these con- tractions, into the stomach. The act of degluti- tion is now completed. (5) Stomachal digestion or ehymification is the next process to be considered. The whole of the alimentary canal (Fig. 1) below the diaphragm Iq.v. ). or great muscular partition which sepa- rates the cavity of the chest from that of the abdomen or Ik'IIv. possesses the following charac- teristics, in relation to structure: The stom- ach, the small intestine, and the large intes- tine arc all lined by mucous membrnne. have a muscular coat, consisting of two sets of distinct fibres — namely, circular fibres which surround the tube or viscus after the manner of a series of rings, and longitudinal fibres running in the same direction as the intestine il-df — and are invested with a serous menibrane, the peritoneum (.see Serous .Memiir.knks), which at the same time retains the viscera in their proper position, and permits their necessary movements. The human stom- ach is an elongiited curved pouch, lying almost immediately 1> el o w the d i a - phragin, and having the form of a bag- pipe. It is very dilatable and con- tractile, and its functiim is to retain the food until it is duly acted upon and dissolved by the gas- tric juice, which is secreted by glands lying in its inner or nuicous coat, and then to transmit it, in a semi-tluid or pulpy state, into the duodenum. Its average capacity is al)Ovit five pints. The parts of it which have received special names are the great- er curvature (Fig. 1 ) b, the lesser cur- vature, upon its up- per border, and the cardiac, c, and py- loric, d, extremities. The mucous mem- brane, or lining coat of tl C- FlO. 1. HCMAX ALIMEXTARV C.VNAL. tt. iPMiphn^iis; b, stnnu'.-li; c, eanltac nrlfli-rt ; tj, p.vloriitt: p, stonutch, is Hmiillintestiii*': /I biliary tjuct; y. thick and soft, and paiirreatir duct: //. asi-eiii)tn^ ,. . , rnloii: /. trauKVMPse colon; >. de- nes in irregular sieudinscolon; t. rectum, folds, in consequence of the contraction of the nuiscular coat, unless when the organ is distended with food. On opening the stomach, ami stretching it so as to remove the appearance of folds, we perceive even with the naked eye, but better with a lens, numerous irregular pits or de- pressions, irregular in shape, and averaging about l-200th of an inch in diameter. To see them properly, the mucus with which they are filled nmst be washed out (Fig. 2, A). These pits arc so shallow as not to dip into the mucous membrane to a greater extent than one-sixth or one-eighth of the thickness. The rest of the tliickness is chiefly made up of minute tubes, running parallel to one another, and vertically to the surface of the stomach ( Fig. 2, B ) . These are the gastric tul)Os or glands which secrete the gastric juice from the blood in the capillarfes which abound in the nuicous membrane. They pass in twos, threes, or fours from the bottom of each pit. and usually subdiviile into several tubes, which, after running a more or less tortu- ous course, terminate in blind or closed extremi- ties. These tubes are filled with epithelial celU, whose contents are composed of granules, with which oil-globules are often mixed, and each tul>e is invested with capillaries, which usually run in the direction of its long axis. In the pyloric
