has no action on cellulose; hence it is inoperative on uncooked starch grains, for in these the cellulose layers are intact.

FIG. 15.-Alveoli of serous gland; A, loaded before secretion; B, after a short period of active secretion; C, after a prolonged period. (Langley.)

a'

b

FIG. 16.-Mucous cells from a fresh submaxillary gland of dog: a, loaded with mucinogen granules before secretion; b, after secretion: the granules are fewer, especially at the attached border of the cell; a' and b' represent cells in a loaded and discharged condition respectively which have been irrigated with water or dilute acid. The mucous granules are swollen into a transparent mass of mucin traversed by a network of protoplasmic cell-substance. (Foster, after Langley.)

FIG. 17.-Section of part of the human submaxillary gland. (Heidenhain.) To the right is a group of mucous alveoli, to the left a group of serous alveoli.

Ptyalin acts best about the temperature of the body (35-40°), and in a neutral medium; a small amount of alkali makes but

little difference; a very small amount of acid stops its activity. The conversion of starch into sugar by saliva in the stomach continues for a considerable time, for the swallowed masses which fall into the fundus of the stomach are not subjected to peristalsis and admixture with gastric juice until a later stage in digestion; the hydrochloric acid which is poured out by the gastric glands first neutralises the saliva and combines with the proteins in the food; but immediately free hydrochloric acid appears the ptyalin is destroyed, so that it does not resume work even when the semi-digested food once more becomes alkaline in the duodenum.

THE SECRETION OF GASTRIC JUICE

The uice secreted by the glands in the mucous membrane of the stomach varies in composition in the different regions, but the mixed juice is a solution of a proteolytic ferment called pepsin in a saline solution, which also contains a little free hydrochloric acid.

The gastric juice can be obtained during the life of an animal by means of a gastric fistula. Gastric fistulæ have also been made in human beings, either by accidental injury or by surgical operations. The most celebrated case is that of Alexis St. Martin, a young Canadian who received a musket wound in the abdomen in 1822. Observations made on him by Dr. Beaumont formed the startingpoint for our correct knowledge of the physiology of the stomach and its secretion.

We now make artificial gastric juice by mixing weak hydrochloric acid (0.2 to 0.4 per cent.) with a glycerin or aqueous extract of the stomach of a recently killed animal. This acts like the normal juice.

Three kinds of glands are distinguished in the stomach, which differ from each other in their position, in the character of their epithelium, and in their secretion. The cardiac glands are simple tubular glands quite close to the cardiac orifice. The fundus glands are those situated in the remainder of the cardiac half of the stomach their ducts are short, their tubules long in proportion. The latter are filled with polyhedral cells, only a small lumen being left they are more closely granular than the corresponding cells in the pyloric glands. They are called principal or central cells. Between them and the basement membrane of the tubule are other cells which stain readily with aniline dyes. They are called parietal or oxyntic (i.e. acid-forming) cells. The pyloric glands, in the pyloric half of the stomach, have long ducts and short tubules lined with cubical cells. There are no parietal cells.

FIG. 18.-A fundus gland from the dog's stomach (Klein): d, duct or mouth of the gland; b, base of one of its tubules; on the right the base of a tubule is more highly magnified; c, central cell; p, parietal cell.

The central cells of the fundus glands and the cells of the pyloric glands are loaded with granules. During secretion they discharge their granules, those that remain being chiefly situated near the lumen, leaving in each cell a clear outer zone (see fig. 20). These are the cells that secrete the pepsin. Like secreting cells generally, they select certain materials from the lymph that bathes them : these materials are worked up by the protoplasmic activity of the

FIG. 20.-A fundus gland of simple

form from the bat's stomach.

Osmic acid preparation (Lang

ley): c, columnar epithelium of the surface; n, neck of the gland, with central and parietal

cells into the secretion, which is then discharged into the lumen of the gland. The most important substance in a digestive secretion is the ferment. In the case of a gastric juice this is pepsin. We can trace an intermediate step in this process by the presence of the granules. The granules are not, however, composed of pepsin, but of a mother-substance, which is readily converted into pepsin. We shall find a similar ferment precursor in the cells of the pancreas, and the term zymogen is applied to these ferment precursors. The zymogen in the gastric cells is called pepsinogen. The rennet-ferment or rennin that causes the curdling of milk is distinct from pepsin, and is preceded by another zymogen; it is, however, formed by the same cells.

1

The parietal cells are also called oxyntic cells, because they secrete the hydrochloric acid of the juice. Heidenhain succeeded in making in one dog a cul-de-sac of the fundus, in another of the pyloric region of the stomach; the former secreted a juice containing both acid and pepsin; the latter, parietal cells being absent, secreted a viscid alkaline juice containing pepsin. The formation of a free acid from the alkaline blood and lymph is an important but puzzling problem. There is no doubt that it is formed from the chlorides of the blood and lymph, and of the chemical theories advanced as to how this is done, Maly's is the most satisfactory. He considers that the

1 The individuality of rennin has been questioned by Pawlow, who regards its action as a phase of pepsin activity.

acid originates by the interaction of sodium chloride and sodium dihydrogen phosphate, as is shown in the following equation:

NaH2PO4 + NaCl=Na2HPO4+HCl

The sodium dihydrogen phosphate in the above equation is probably derived from the interaction of the disodium hydrogen phosphate and the carbonic acid of the blood, thus :

Na2HPO4+ CO2 + H2O=NaHCO3 + NaH,PO1.

Other theories have tried to explain the formation of such a strong acid as hydrochloric by the law of 'mass action.' We know that by the action of large quantities of carbonic acid on salts of the mineral acids the latter may be liberated in small quantities. We know, further, that small quantities of acid ions may be continually formed in the organism by ionisation. But in every case we can only make use of these explanations if we assume that the small quantities of acid are carried away as soon as they are formed, and thus give room for the formation of fresh acid. Even then it is impossible to explain the whole process. A specific action of the cells is no doubt exerted, for these reactions can hardly be considered to occur in the blood generally, but rather in the oxyntic cells, which possess the necessary selective powers in reference to the constituents of the blood, and the hydrochloric acid, as soon as it is formed, passes into the secretion of the gland in consequence of its high power of diffusion.

COMPOSITION OF GASTRIC JUICE

The following table gives the percentage composition of the gastric juice of man and dog :