96 MAN AN ADAPTIVE MECHANISM 



of an ever-changing world without the organism, so 

 the chemical ceptors, implanted deep within the 

 tissues, as well as to some extent on the surface, repre- 

 sent not only certain factors of that outer entourage, 

 but more particularly the events of a ceaselessly ac- 

 tive, ever-changing world within the organism. 



As we shall later attempt to show, the phenomenon 

 of metabolism may in a sense be regarded as an example 

 of adaptation through chemical ceptors. The sensa- 

 tion of hunger which impels man to eat has its origin 

 in chemical changes and is manifested by muscular 

 contractions in the stomach, which in turn are induced 

 by the condition of lowered nutrition in the body. 

 The presence of food in the mouth is the adequate 

 stimulus for the secretion of saliva, which, passing 

 with the food to the stomach, becomes there the ade- 

 quate stimulus for the outpouring of gastric juice. 

 The impingement of this acid mixture upon the pyloric 

 end of the stomach is the adequate stimulus for the 

 rhythmic opening and closing of this gate, through 

 which the partially digested mass passes into the 

 duodenum and in turn becomes the stimulus which 

 excites to activity the pancreas and biliary apparatus. 

 In the intestine other glands are excited to action 

 through the excitation of chemical ceptors and at 

 last, largely through a series of chemical ceptor stimu- 

 lations, the whole intake of food, unassimilable in the 

 beginning, is prepared for absorption and use in the 

 organism. 



By far the most interesting examples of the adapta- 

 tion of the organism through chemical ceptors, how- 

 ever, are offered in its mode of defense against foreign 



