THE MECHANICAL PHENOMENA OF DIGESTION 



327 



II A.M 



the sphincter relaxing from time to time by a reflex inhibition to 

 admit the better-digested portions into the duodenum, but tighten- 

 ing more stubbornly at the impact of a hard and undigested morsel. 

 The nature, as well as the consistence of the food, influences the 

 length of its sojourn in the stomach. Carbo-hydrate food passes 

 more rapidly through the pylorus than fatty food, and fat more 

 rapidly than protein. The reason is that 

 the acidity of the gastric juice varies 

 with the different kinds of food, hydro- 

 chloric acid being secreted in abundance 

 in the presence of proteins, and to a 

 much smaller extent in the presence of 

 fats and carbo-hydrates. Now, dilute 

 hydrochloric acid when introduced into 

 the stomach remains there for a much 

 longer time than water. This depends 

 upon the fact that such portions of the 

 acid as get into the duodenum stimulate 

 afferent fibres in its mucous membrane, 

 and so cause reflex spasm of the pyloric 

 sphincter. When the acid chyme be- 

 comes neutralized to a certain point by 

 the bile and pancreatic juice, inhibitory 

 impulses pass up from the duodenum 

 and cause the sphincter to relax. The 

 cardiac division of the stomach, with 

 the exception of the portion that borders 

 the transverse band, takes no share in 

 the peristaltic movements. And, indeed, 

 it is far more difficult to cause such con- 

 tractions by artificial stimulation in the 

 fundus* than in the pylorus. The two 

 portions of the stomach are partially, or 

 in certain animals from time to time 

 completely, cut off from each other by 

 the contraction of the sphincter of the 

 antrum. The fundus, so far as its 

 mechanical functions are concerned, acts 

 chiefly as a reservoir for the food, which, 

 like a hopper, it gradually passes into 

 the pyloric mill as digestion goes on by a tonic contraction of its 

 walls. The existence of this reservoir enables larger quantities of 

 food to be taken at one meal, which can then be digested gradually. 



* The common idea, however, that the fundus is completely inactive is 

 erroneous. Rhythmical variations in tone with a period of i to 3 minutes have 

 been observed during digestion. In hunger vigorous peristaltic contractions 

 sweep over the whole stomach, beginning at the cardiac end. 



Fig. 150. Cat's Stomach seen 

 by Rontgen Rays (Cannon). 

 The outlines of the stomach 

 containing food mixed with 

 bismuth subnitrate were 

 drawn at intervals from 

 ii a.m. to 4.30 p.m. 



