It might be asked, 'What is the object of this psychic secretion?* for Pawlow has clearly es tablished the existence of two kinds of gastric secretion, the chemic and the psychic. This question applied to the human physiology would be the same as inquiring, *What is the object of the appetite?* The answer is, that under the influence of the psychic secretion a gastric juice is furnished which is much more effective than that which is secreted under purely chem ical stimulation of the food, that is, when food is taken without any special appetite. Further more, under the influence of psychic secretion foods which would otherwise not stimulate the gastric mucosa to secretion become converted by the already present psychic secretion into something else which constitutes a further stim ulant to the secretion of gastric juice. For in stance, if a solution of albumen be administered to a dog upon which a Pawlow operation has been performed, that is, splitting off part of the stomach, with all the vessels and nerves in tact, and making this second smaller stomach communicate with the abdominal wall, but not with the general cavity of the large stomach from which it is dissected (see Hemmeter's Practical Physiology) there will be no secretion from the small stomach, for albumen by itself does not excite chemical secretion. But if the psychic secretion is previously set up by some other means, before the albumen is placed in the large stomach — for instance, by waving a piece of meat before the dog's eyes — then, following the introduction of albumen, a secre tion will be found in the small (artificial) stomach which is qualitatively and quantitat ively greater than the psychic secretion alone, or when albumen is given alone, it is evident that, while albumen in itself does not excite secretion, the products of albumen do cause this secretion. The same is true of pieces of bread which, when placed in the large stomach through the fistula, will not promote a secre tion; but if the dog is allowed to swallow the bread, secretion begins and continues for sev eral hours. Psychic secretion therefore is a preparatory secretion, transforming substances, which otherwise would not stimulate the stom ach into such forms as can accomplish this stimulation. The fact that bread will cause a secretion when chewed and swallowed and not when placed directly in the stomach through the fistula, may be interpreted (as by Pawlow and Peter Borisoff) as proving the secretion of a gastric juice under psychic influence; but —as will be shown presently— it may be due to a special body formed in the salivary glands and reaching the blood eventually circulates through the glands of the stomach and stimu lates gastric secretion.
Edkin has demonstrated that the pyloric mucosa forms a chemic substance during diges tion which also stimulates gastric secretion af ter it has once been started by other mechan isms. Further revelations from Pawlow's labor atory disclose very important relations between the various classes of food, permitting the con clusion that they may mutually advance or in terfere with the digestion of their various con stitutents in the gastric chyme. For instance, starch paste does not by itself promote gastric secretion, but when mixed with meat it was found to accelerate the action of the gastric juice, increasing its digestive power. Further more, it was demonstrated that the stomach is capable of distinguishing between lactic, butyric and hydrochloric acids and responded to each of these acids with a varying quantitative secretion. As lactic and butyric acids are either contained in the food as such or are prod ucts of gastric fermentation, their stimulating influence on gastric secretion is of therapeutic importance. It is evident, therefore, not only that the stomach is extremely delicate in de tecting the composition of foods and regulat ing the composition of its secretion correspond ingly, but that it can distinguish between vari ous organic acids. These experiments further more gave the clue to the treatment of gastric secretory disorders not by drags merely but by dietetic measures.
Hemmeter found that extracts made from salivary glands that were removed from ani mals during the act of chewing, when injected into the general circulation of dogs caused a flow of gastric juice. This occurred when the animal's stomach was empty. All of these chemic agents that correlate the function of physio logically and anatomically distant organs are called Hormones by Starling (from the Greek (5ppat.) (I stimulate).
The human stomach accomplishes its work by the means of three essential functions, secre tion, absorption and peristalsis (this term re fers to the movements of the stomach). The secretion of the gastric glands owes its digestive power to hydrochloric acid and three ferments. The quantity of hydrochloric acid amounts to two parts in the 1,000 of gastric juice. The ferments are pepsin, which acts mainly on the proteid constituents of food like meat and egg, rennin or chyrnosin, which acts principally up on milk, precipitating the casein; lipase, which is a fat-digesting ferment; and the new gastric ferment, gchyrnaze,s which is not a digestant of food, but an accelerator of the digesting action of the ferments of the pancreatic juice. (International Clinics, Vol. II, 12th Series, p. 276, article by Peter Borisof).
Dependence of Intestinal upon Gastric It is a prevalent opinion among the laity that the stomach has a marked ab sorptive power. This is a natural consequence of an older error, according to which the stom ach is the chief digestive organ, it having been formerly believed that by far the greater part of the digestive act takes place in the stomach.
As already intimated, the actual digestion which takes place in the stomach is insignificant com pared with that which takes place in the in testine. Later it will be shown here that the secretion of pancreatic juice depends upon the liberation from the membrane of duodenum, or first part of the bowel, of an agent called duodenal secretin, another hormone (discovered by Bayliss and Starling), and that the liberation and secretion of this agent depends upon the presence of hydrochloric acid in the gastric chyme. So that if this acid is not present in the chyme as it enters the upper bowel, there can be no normal performance of intestinal di gestion. Just as normal gastric digestion de pends upon a normal condition of the mouth and normal salivary secretion, so normal in testinal digestion depends upon a normal stom ach.
Absorption from the The amount of absorption that takes place front the stomach is surprisingly small. Water is prac tically not absorbed at all, for it appears that fully 95 to 100 per cent of all water taken into the stomach is passed out into the duodenum and absorbed from the intestine. Alcohol and sub stances in solution of alcohol are readily' ab sorbed. Grape-sugar, milk, cane-sugar and mal tose are absorbed in moderate amounts when they are in aqueous solution. When they are in alcoholic solution larger amounts are absorbed. Dextrin and peptones are also taken up from the stomach, but in smaller amounts than sugar. The amount of these substances absorbed in creases with the concentration of the solution. But simultaneously with this absorption oc curs a more or less active secretion of water into the stomach, which secretion increases or diminishes directly in proportion to the amounts of the substances absorbed. So that under cer tain conditions it is possible to draw more water out of the stomach by means of a stom ach-tube than has been drunk half an hour previously, which certainly refutes the assump tion that water could be absorbed from the stomach, and favors the view that water is not only not absorbed, but that it is actually se creted into the stomach under certain conditions. This conclusion is of great value in the treat ment of certain dyspepsias characterized by a weakened condition of the gastric muscula ture; for in these the contractile power of the stomach is not sufficient to expel the water into the bowel in the proper time. And as water is, bulk for bulk, the heaviest substance which the human being takes into the stomach, its reten tion in the stomach beyond a certain length of time drags down and exhausts the already attenuated gastric muscularis and dilutes the al ready weakened gastric secretion. Therefore in some forms of gastric an essential part of the treatment consists in restricting the amount of water to the lowest possible requisite or to give it by means of colon-injections, if necessary, for a time at least ; or if some .of it must be taken by way of mouth, to induce the patient to lie on his right side and by means of massage to facilitate the expulsion of the water from the stomach into the intestine.