Formerly, attempts were made to obtain gastric juice by inducing a dog, while fasting, to swallow a sponge, and withdrawing it when saturated with the gastric secretion; or a fasting dog, allowed to swallow insoluble materials, was killed, and the secretion collected from the stomach.
It is best obtained directly from a fistulous opening in the abdominal wall communicating with the stomach. A gastric fistula was first made accidentally in a man by injury. A case in which the surgical treatment of a gunshot wound of the stomach left a permanent fistula, allowed the gastric secretion to be carefully investigated, and proved a valuable subject for experimental research.
It is not a difficult matter to reach the stomach by making an artificial opening through the wall of the abdomen, and, having brought the serous surface of the gastric wall into firm connection with the serous lining of the abdominal wall, to open the stomach. The juxtaposition of the parts, as well as the patency of the fistula, can be secured by a suitable flanged cannula closed with a well-fitting cork. By removing the cork the gastric juice may be obtained in small quantities, and various kinds of food may be introduced through the cannula, and the changes occurring in them studied.
For experimental purposes an artificial gastric juice may be used. This can be made from the gastric mucous membrane of a dead animal (pig) by extracting the pepsin from the finely-divided glandular membrane, with a weak acid (less than.2 per cent.) or, better, with a large quantity of glycerine, and subsequently adding HC1 to the extent of.2 per cent.
The gastric juice is not secreted in large quantity when the stomach is empty, but only when the mucous membrane is irritated with some chemical or mechanical stimulus. The swallowing of alkaline saliva acts as a gentle stimulus and causes secretion, so that the surface of the stomach becomes acid. When the lining membrane of the stomach is mechanically stimulated through a fistula it becomes red, and drops of secretion appear at the point of stimulation, but the amount of secretion thus produced is very scanty when compared with that called forth by chemical irritants.
Thus, ether, alcohol and pungent condiments produce copious secretion. Weak alkaline solutions also cause secretion, but the most perfect form of stimulant seems to be a mass of food saturated with alkaline saliva.
In all probability the secretion of the gastric juice is under the control of a special nerve mechanism, and the way in which the state of activity follows stimulation of the part seems to point to its being a simple reflex act. However, the nervous connections (vagi and splanchnics) between the stomach and central nervous system may all be severed without any marked effect on the secretion, other than that which would naturally follow the changes in the amount of blood supply, which, of course, is greatly altered by cutting the vasomotor nerves - the splanchnics. Whether this be so or not, there must be some connection with the nerve centres, for sudden emotions check the secretions, and the sensations caused by the sight or smell of food give rise to gastric secretion.
It has been suggested that Meissner's submucous ganglionic network may act as a reflex centre and regulate the secretion. But as the reflection from local ganglionic centres has not yet been definitely demonstrated, we are hardly entitled to assume that it occurs here, and since the stimulus comes into close contiguity with the secreting cells, it seems quite as probable that these elements are excited to activity by direct stimulation of their protoplasm.
As in the salivary glands, so in the gastric tubes, the cells show some structural changes which accompany with great regularity their periods of rest and activity, and therefore may be concluded to be the indications of the internal processes belonging to the production of the specific materials of the secretion.
It appears probable that the chief secretory activity resides in the small central cells, and not in the large ovoid border cells, since no distinct changes can be seen in the latter, and the smaller gland cells seem to contain the pepsin; for if the mucous membrane be treated with weak hydrochloric acid, these central gland cells are rapidly dissolved by a process of digestion, while the border cells simply swell up and become more transparent. So that the outer ovoid cells have no title to their former name of " peptic cells".
The central cells of the gastric glands are finely granular, pale, protoplasmic masses, and continue so during the time when the stomach is empty and the glands not secreting. In the earlier stages of digestion these cells swell up and become turbid and coarsely granular, and stain more readily with the aniline dyes. As the digestive process goes on the cells again diminish in size, but are found to contain a large quantity of peculiar granules, which are discharged from the cell before its return to the ordinary state of rest. The cells are said to be rich in pepsin in proportion to their size; when swollen during active digestion they contain much pepsin, when small, during hunger, they contain but little.
It would therefore appear that the pepsin of the gastric juice is produced as a distinct and new manufacture by the central cells of the peptic glands, and not by the other cells. Structural changes have also been followed out in the so-called mucous glands and in glands without any of the ovoid border cells, which, taken with the fact that the alkaline secretion of the pyloric end of the stomach, where the mucous glands abound, is capable of rapidly digesting proteid if acid be added to it, tends to show that in these so-called mucous glands pepsin is also produced.
The acid is found chiefly on the surface of the stomach. The mode of its production seems distinct from that of pepsin, but is not well understood. Possibly the surface epithelial cells store up in their protoplasm and render inert the small quantities of HC1 which are constantly being set free from the NaCl by the action of the newly-formed weak organic acids (lactic, etc.). The amount of HC1 thus slowly accumulated in time becomes considerable and is discharged by the cells at appropriate periods.
Although the fact that the deeper part of the glands do not give an acid reaction, while the neck and orifices of the gland are distinctly acid, would support the former view, there is some reason for believing that the manufacture of acid from the alkaline blood is really an active process carried out by some glandular cells.
It has been suggested that the cell elements which produce the acid are the ovoid border cells, from whence it rapidly passes to the orifice of the glands. This view is supported by the alkalinity of the pyloric end of the stomach where the border cells are not found. In some animals the distinct distribution of the different cell elements and the accompanying reaction of the secretion are well marked.