Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/731

NUTRITION 673 by a gland though the pressure within it is many times higher than that of the blood circulating through the arteries which supply it. On many grounds it may be positively asserted that the secreting cells are the primary agents in the withdrawal from the blood of the water necessary for the secretion, though the exact nature of the process is yet unknown ; similarly, on the grounds stated below, we know that within the protoplasm of the gland- cells are formed the characteristic soluble constituents of the secretion. The researches made during the last few years by Heiden- hain, and fully confirmed by a large number of observers, have demonstrated that in the salivary glands, as indeed in all secreting glands, structural and perfectly obvious microscopic changes occur, which stand in close relation to the different conditions of functional activity. The resting gland-cell is large, but possesses compara tively little protoplasm, and therefore comparatively little matter which can be stained by colouring bodies, especially by carmine ; it contains instead a store of material which has been elaborated in, or at the expense of, the protoplasm. This material does not constitute the specific matter of the secretion, but is its antecedent. That it differs chemically in the case of the mucous glands is proved by the fact cited by Heidenhain, and discovered by Watney and Klein, viz., that, whilst mucin is stained by haematoxylin, its antecedent (mucigen) is not affected by that colouring matter ; in all other respects the two bodies are identical. When, however, a gland passes into a state of activity, as, for example, by the irritation of its so-called secretory (trophic?) nerves, the gland-cells undergo the following changes, which may proceed simultaneously though not necessarily so : the stored-up matter previously referred to is converted into soluble constituents of the secretion, and at the same time there occurs a growth of the proto plasm of the cells, at the expense doubtless of the richer supply of lymph which during the secretory act bathes the gland. The period of activity in so far as the gland-cell is concerned is indeed a period of removal of ready-made constituents of secretion, in some cases, as in the mucus- bearing cells of the mucous glands, a period of destruc tion of cells laden with such constituents ; but at the same time in all cases a period in which the protoplasmic constituents of the cells generally increase, and active pro liferation of secreting cells occurs. Whilst we have in a few sentences sketched the general characters of the changes which glands undergo during secretion, our picture is wanting in all details, and the reader is referred for further information to works on histology and physiology. Glands of the Stomach and Secretion of Gastric J uife. Tn vol. vii. p. 225 a brief description has been given of two kinds of glands which are found embedded in the mucous membrane of the stomach, and which are concerned in the preparation of the secretion of this organ, the so-called f/astric juice. The first kind of glands, the "mucous glands of the stomach," so called from their having been formerly erroneously supposed to be engaged in the secre tion of mucus only, are situated chiefly at the pyloric end (figs. 4, 5) of the stomach. The second kind, the so- called "peptic glands," owe their name to the view that they alone secreted the digestive gastric juice, and are found in most animals in the mucous membrane of other parts of the stomach than the pylorus. The pyloric glands are tubular glands, simple or compound, composed of a basement membrane lined by two kinds of epithelium- cells, (1) cylindrical epithelium -cells confined to the neck of the gland, (2) cylindrical or cubical epithelium -cells lining the part of the tube below the neck, and which, though not very different in form from the cells of the neck, possess an entirely different function. The peptic glands of the older writers, now usually called " glands of the fundus," present three varieties of epithelium -cells. Their "neck" is lined by cylindrical epithelium similar to that of the neck of the pyloric glands. The deeper part of the glands is lined by small cubical " central " cells, below which and interposed between them and the base ment membrane is a discon tinuous layer of large ovoid " border " cells, the so-called " peptic cells " of older writers. During active digestion the cubical cells of both py loric glands and glands of Fig. 5. Fio. 4. Pyloric Gland, from a section of the Dog s stomach (Ebstein). m, mouth; n, neck; tr, deep portion of a tubule. cut transversely. (From Quain s Anatomy, iig. 515.) Fio. 5. Cardiac Gland, from the Dog s stomach (Klein and Noble Smith). Highly magnified, b, base or fundus of a tubule ; rf, duct or mouth of gland: c, central cells; p, parietal cells. (From Quain s Anatomy, tig. 517.) the fundus become swollen and granular ; after a period of rest they return to their original size and assume a com paratively clear appearance. The ovoid cells also swell during digestion, but in other respects remain unchanged. When the stomach is inactive, i.e., in the fasting con dition, its mucous membrane is pale and the organ contains no fluid ; the mucous membrane in the pyloric region presents an alkaline reaction. When, however, food enters the stomach, or the mucous membrane is subjected to mechanical stimulation, an acid juice, the so-called gastric juice, is poured out, the act of secretion being accompanied by a reddening of the mucous membrane. . . ,. Evidence appears to be most conclusive in support ot the view that the three kinds of epithelium -cells found in the stomach and its glands possess entirely separate func tions. The epithelium -cells which line the interior of the stomach, and which, like those of the neck of both kinds of glands, are cylindrical in shape, appear to be mucus- forming cells. The cubical cells which are found in the deeper parts of the pyloric glands appear functionally to be identical with the central cells of the glands of the fundus, both being engaged in forming the chief ferment of the gastric juice, viz., pepsin, whilst the ovoid cells of the glands of the fundus are the sole elaborators of the acid of the gastric juice. The glands which possess these XVIT. 85