in other regions of the digestive tract — 

 namely, at the point where the small intestine 

 leads into the large intestine, and at the 

 anus. Essentially, a sphincter represents a 

 local thickening of the circular layer of the 

 muscularis, which contracts and closes the 

 passage, except at certain times, when the 

 valve relaxes and allows the food mass to 

 pass. 



Nerve Supply of the Stomach. Like all the 

 other viscera, or internal organs of the body, 

 the stomach is supplied by two sets of nerves, 

 both belonging to the autonomic nervous 

 system (Chap. 25). These two sets of auto- 

 nomic nerves have an antagonistic effect 

 upon the gastric musculature and upon the 

 secretion of the gastric glands. The parasym- 

 pathetic fibers (p. 465) are carried to the 

 stomach by the vagus nerves, which originate 

 from the brain stem and pass along the sides 

 of the esophagus to the stomach and small 

 intestine. Impulses from these parasympa- 

 thetic fibers augment — that is, strengthen 

 and accelerate — the gastric movements and 

 increase the secretion of the gastric glands. 

 But in addition, the stomach receives a num- 

 ber of delicate sympathetic nerves from the 

 spinal cord (p. 465); and these sympathetic 

 fibers depress the mobility of the gastric 

 musculature and reduce the flow of gastric 

 juice. 



The Gastric Juice. The gastric juice is a 

 powerful digestive fluid secreted by the gas- 

 tric glands, of which there are about 35,000,- 

 000 in the human stomach. Each gastric 

 gland is a minute tubular outfolding of the 

 mucosa, although several glands may be asso- 

 ciated with each of the small pits that can be 

 observed with a hand lens, when the inner 

 lining of the stomach is inspected (Fig. 16-13). 



Two early studies on human gastric juice 

 deserve to be mentioned specifically. In 1776 

 Spallanzani demonstrated the chemical po- 

 tency of the juice; this was important be- 

 cause previously the action of the stomach 

 upon the food was thought to be entirely 

 mechanical. Spallanzani swallowed porous 

 wooden capsules filled with meat and other 



The Digestive System - 305 



protein foods; and when the capsules were 

 reclaimed and examined, it was found that 

 the proteins gradually had dissolved and 

 escaped from the capsule — despite the fact 

 that the food was entirely protected from 

 mechanical influences by the rigidity of the 

 capsule walls. Then in 1819 William Beau- 

 mont, an American Army surgeon, began a 

 long series of studies on a patient with an 

 unusual gastric fistula. A fistula is an arti- 

 ficial channel communicating between a part 

 of the digestive tract and the exterior of the 

 body. In the present case, the patient was a 

 Canadian trapper named Alexis St. Martin, 

 who had exploded a shotgun into the pit of 

 his stomach. The charge penetrated not only 

 the body wall, but also the stomach wall; 

 and when the wound healed, the stomach and 

 body walls adhered together, leaving a per- 

 manent fistula that led directly into the 

 gastric cavity. A flaplike overgrowth of the 

 body wall covered the external opening so 

 that food could be retained in the stomach 

 and normal gastric digestion could proceed. 

 At any given time, however, the valvelike 

 flap could be lifted, permitting Beaumont to 

 obtain a sample of the semidigested food 

 mass or to collect pure gastric juice — uncon- 

 taminated with food. 



Composition of Gastric Juice. The collection 

 of gastric juice by means of a stomach tube 

 is now a routine procedure, carried out by 

 the physician in the clinic or by the medical 

 student in the physiology laboratory. An out- 

 Table 16-2— Composition of Gastric Juice 



Percent 

 Components by Weight 



Inorganic 



Water 98.02 



Hydrochloric acid (HC1) 0.50 



Salts 1.03 



Organic 



Mucin "I 



Enzymes (pepsin, rennin, lipase) I 0.45 

 Other proteins j 



rooToo 



