DIGESTION IN THE STOMACH 383 



but slightly dissociated. Hydrochloric acid readily combines with 

 proteins, and this is known as " combined acid," for it dissociates 

 very slightly. Under these circumstances, it will not give the reac- 

 tions of free acid. Considerable importance is attached clinically to 

 the proportion of the free acid to the combined. A small amount of 

 acidity may also be due to the presence of acid salts. 



The Mechanism of Secretion. After a period of secretion, the 

 granules of the chief cells have greatly decreased in number, and the 

 colls have become shrunken. These granules have given rise to the 

 peptic ferment of the juice, for less ferment can be obtained from cells 

 poor in granules than from those rich in granules. 



The granules are the precursor or zymogen. This can be shown 

 as follows: Sodium carbonate destroys the active enzyme pepsin. 

 Nevertheless, a sodium carbonate extract of the fundus, when rendered 

 faintly acid with hydrochloric acid, manifests all the digestive actions 

 of pepsin. The precursor is not destroyed by sodium carbonate, and 

 the extract of zymogen, when rendered acid by HC1, is converted into 

 the enzyme. If the fundus is first treated with hydrochloric acid, 

 and then with sodium carbonate, no active enzyme is obtained, for 

 the zymogen is converted by the acid and the enzyme destroyed by 

 the sodium carbonate. 



The parietal or oxyntic cells secrete the hydrochloric acid, since 

 the acidity only appears in the juice of that portion of the stomach 

 where they occur. This is not the only example of free mineral acid 

 being excreted by glands. In a giant mollusc, Dolium, the salivary 

 glands secrete HgSO,! (about 2 per cent, solution). This juice effer- 

 vesces when it falls on a marble floor. It is strange that living cells 

 should secrete so potent an acid, which is destructive to life. The 

 hydrochloric acid is probably secreted in a combined state, which 

 becomes active after secretion. 



The gastric juice is liberated by a double mechanism nervous and 

 chemical. 



The Nervous Mechanism. The gastric glands have a double nerve- 

 supply from the autonomic system: (1) the vagus, (2) the sympathetic. 

 It is not easy to excite directly the secretory fibres to the stomach 

 and produce secretion, as can be done in the case of the sali vary gland. 

 On the other hand, the secretion can be excited reflexly, the afferent 

 paths being the same as those which excite the secretion of saliva. 

 The effect of psychic and other stimuli has been fully studied by 

 means of sham feeding on the dog, in which oesophageal and gastric 

 fistulse have been established. Secretion results from (1) the psychic 

 element, (2) contact with buccal mucous membrane and the act of 

 mastication, (3) the taste of food (see Fig. 192). The sight of food 

 causes a secretion; in a hungry dog this continues for as long as one 

 and a half hours. If the animal is then given a sham meal, it is 

 found that the amount of secretion obtained by the psychical stimu- 

 lation is rather greater than that obtained by the introduction of 

 the food into the mouth. 



