2<)4 ANATOMY FOR NURSES [Chap. XV 



forced against the pylorus tend to keep it closed, but hydrochloric 

 acid in the stomach seems to favor or produce relaxation of the 

 pyloric sphincter. In the intestines hydrochloric acid has a con- 

 trary effect, as it causes a contraction of the sphincter, which 

 remains closed after each ejection until the acidity has been 

 neutralized. 



Secretion of gastric juice. — During the intervals of digestion 

 the stomach is bathed in an alkaline mucus. The entrance of 

 food acts as a stimulant to the whole organ. The blood-vessels 

 dilate, and the glands pour out an abundant secretion upon the 

 mucous lining. This secretion continues as long as food remains 

 in the stomach, and is caused and maintained by two factors : 

 (1) psychical, the sensations of eating, the taste and odor of food 

 stimulate the sensory nerves situated in the mouth and nose. 

 These afferent impulses are transferred through nerve centres 

 to efferent fibres of the pneumogastric nerve, and thus are carried 

 to the stomach. (2) Chemical, (a) by secretogogues contained in 

 certain foods and (b) by secretogogues contained in the products 

 of digestion. Certain foods, such as meat juices and extracts, con- 

 tain substances called secretogogues or hormones which are sup- 

 posed to act directly upon the nerves of the pyloric mucous 

 membrane and form a substance called gastrin or gastric secretin, 

 which is absorbed into the blood and carried to the gastric glands. 

 This substance stimulates the glands to secretion. Other foods, 

 such as milk, bread, white of egg, etc., do not appear to contain 

 secretogogues. When such foods are eaten, a psychical secretion 

 is started and when this has acted, some products of their digestion 

 in turn become capable of stimulating a further secretion of gastric 

 juice. 



Gastric juice. — Gastric juice, -secreted by the peptic and 

 pyloric glands in the mucous lining of the stomach, is a thin, 

 colorless, or pale yellow fluid, of an acid reaction. The amount 

 secreted in twenty-four hours has never been accurately measured, 

 but has been estimated to be about fifteen pints (7.1 litres). It 

 contains few solids, and is dependent for its specific action upon 

 two enz^'mes called (1) pepsin and (2) rennin. Pepsin is only 

 properly active in an acid solution, and we therefore find that free 

 hydrochloric acid in the proportion of 0.2 to 0.4 per cent is always 

 present in normal gastric juice. 



