Page:The New International Encyclopædia 1st ed. v. 06.djvu/285

* DIGESTION. 2il DIGESTION. or duodenal end of the stomach, these tuhes (at least in the dosj and several other animals whose stomaclis have Ihhmi carefully examined in a per- fectly fresh state) are con^iderahly wider than those which we have descrihed. and differ from them also in other respects; and hence some ^9* • Fig. 2. A, inner surface of the stonmcli, showing the cells after the mucus has been washed out, nia^uitied 25 diameters. B. st<imax-h-tube from the middle of the human stomach] ma»rniTled about 150 diameters: a, wall of the tube, lined with lar^-e oval nucleated cells: b. the same cells isolated; e. nucleated cells of columnar epithelium, occupvingthe upper parts of the tubes; d, blind extremity of the tube. physiologists believe that while they collectively secrete the gastric juice, one set may secrete the acid fluid and the organic matter termed pepsin, and the other mucus: the free acid and the pep- sin are, as we shall shortly see, the two essential constituents of the gastric juice. When food is introduced into the stomach, tliree special phenomena are induced in that viscus: ( 1 ) Certain movements dependent on the muscu- lar coat: (2) the mucous membrane is altered in appearance; and (3) gastric juice is secreted. Each of these phenomena requires a brief notice. On killing an animal while the act of digestion is going on, and at once laying open its abdomen, we find that the stomach is in a contracted state, firmly embracing its contents, and with both its orifices so closed as to prevent the escape of the food, this contraction being due to the stimula- tion of the muscular coat by the food. If we examine the movements of the stomach during digestion, which we can do either by exposing the stomach of a living animal, or by sending a n.agneto-elcctric current through this organ in an animal just killed, we perceive that, in the cardiac half or two-thirds, the movements are e.xtremely slow, the muscular coat apparently contracting on the food, and progressively send- ing it toward the pylorus; while in the pyloric end of the stomach the movements are more energetic and rapid, resembling the peristaltic or vermicular movement, which we shair presently describe as occurring in the intestinal canal. V'hen the transverse constriction has reached the firmly .shut pylorus, a relaxation lasting about a minute ensues, followed by a repetition of the circular contractions. The movements which these contractions impress upon the food have been described in the following terms: 'The food entering the cardiac end of the stomach, r, turns to the left, descends into the splenic extremity and follows the great 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 Be.umoxt.) Dr. Urinton, how- ever, adopts a modified view, which is probably the correct one. He 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 "curvafure: and that these two currents of food meet at the closed pylorus, when they are both reflected into the direction 6, forming a cen- tral or axial current, occupying the real axis of the stomach, which unites" the two apertures. The mutual interference of these currents at their borders causes a uniform admixture of the arious 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. Fig. 3. Diagram to show the general direction of movement impressed on the semi-fluid food in the stomach. na, the hemispherical or surface current, carrying the semi-fluid food towani the closed pylorus, where' it is de. fleeted into h. the centra! current, which unites the cardiac (c) and pyloric {d) openings. The changes in the mucous membrane are mainly the following: The inner surface of the healthy fasting stomach is of a paler pink tint than after the introduction of food, and while in the latter case the reaction of the moisture on the surface is very acid, in the former it is neutral, or even alkaline. Dr. Beaumont found (in the case of Alexis Saint Martin) that, on the introduction of food into the stomach, the ves- sels of the nuicous membrane became more in- jected, and that its color changed from a pale pink to a deep red. A pure, colorless, and slightly viscid fluid, with a well-marked acid reaction, was then observed to distill from the surface of the membrane, and to collect in drops, which trickled down the walls, and mixed with the food. That the gastric juice, which is the term ap- plied to the acid fluid which Dr. Beaumont saw exuding from the mucous membrane, and which is secreted or formed in the gastric tubes, which we liave already described, is capable of exerting a solvent action on food, is proved by numerous experiments. It was first ascertained by Reau- mur (1752), wlio obtained some of this fluid by making animals swallow sponges with a string attached, by which he could withdraw them. He thus showed that alimentary substances out of the body were altered by this fluid in the same manner as they arc changed in the stomach, and