DIGESTION THE STOMACH. 405 



CHAPTER III. 



DIGESTION. THE STOMACH. 



I. PHYSIOLOGICAL RELATIONS. 



GASTRIC digestion is mainly effected by the gastric juice, an 

 acid secretion which owes its solvent and chemical power to 

 pepsin and hydrochloric acid. The gastric secretion is stimulated 

 by the mechanical presence of food; by the products of digestion, 

 part of which is rapidly absorbed ; by impressions on the 

 nervous centres, such as tastes, which were referred to in the 

 previous chapter ; and by the presence of saliva and other 

 dilute alkaline fluids at the mouths of the tubules. During 

 digestion the gastric vessels actively dilate ; the muscles move 

 vigorously ; by the end of four hours much of the proteids 

 have become peptones ; the sugar, starch, and fats are broken 

 down or emulsified, but remain chemically unaltered ; and the 

 whole of the products, constituting the chyme, are transferred 

 to the duodenum. 



The nervous arrangement by which the stomach is stimu- 

 lated, or prepared to receive and digest food, is chiefly a local 

 one ; the contact of food, digested products, and dilute alkalies 

 acting on ganglia in the gastric wall itself. Besides this, the 

 stomach is connected with a centre in the medulla, and with the 

 cerebrum, by means of afferent and efferent nerves the vagus 

 and the sympathetic. The impressions which thus reach the 

 sensorium and the gastric centre are reflected as impulses to the 

 stomach, through the efferent nerves ; which also convey from 

 the cerebrum the impulses generated by sensations of taste, 

 as we saw in the last chapter, as well as by the smell, sight, or 

 idea of food. Besides these, numerous impressions from the 

 intestines, liver, kidneys, and generative organs, indeed from 

 all impressionable parts whatsoever, influence the stomach by 

 being reflected to it through its centre in the medulla. The in- 

 fluence of these nervous impulses upon the stomach is very 

 marked. They affect the secreting glands, the vessels, and the 

 muscles, exciting, arresting, or otherwise modifying, as the case 

 may be, the secretion of gastric juice ; and under certain circum- 

 stances they give rise to vomiting. 



H. PHARMACODYNAMICS. 



We have now to inquire how many of the conditions which 

 influence gastric digestion are under our control : how far we 

 can act physiologically on the stomach. 



