200 ORIGIN AND NATURE OF LIFE 



and arranged, and the degree of contraction 

 of the constituent cells of this muscular layer 

 enables the stomach to adapt itself to the 

 amount of food within, and keep up a certain 

 pressure upon its contents. As the stomach 

 expands when food enters, the walls become 

 thinner by the elongation of the muscular 

 cells, and as the food is passed onwards towards 

 the intestine the muscle cells shorten again, 

 so as always to keep the stomach full at 

 different sizes. Thus a cavity is obtained 

 with an adaptation to the amount of its 

 contents, much more perfect than would be 

 given by a rubber bag which, of inanimate 

 receptacles, more nearly resembles it than 

 any other. In addition to this steady tone 

 of the stomach walls, increased contractions 

 occur in regular series with a slow rhythm, 

 and these serve the purpose of mixing up the 

 food and the digestive secretion. Further, 

 when digestion has proceeded far enough, 

 and the condition of the contents of the 

 intestine warrant it, these movements serve 

 the purpose of passing the food onwards 

 into the lower part of the alimentary tube. 

 Similar instances of adaptation of structure 

 and function might be multiplied by the 

 thousand all over the body. Such delicate 

 adaptation, which in some cases is demon- 



