66 OF VITAL MOTION. 



far as we may perceive, why the food should pass in 

 one direction rather than another, or why the act of 

 contraction, which is supposed to be excited by the 

 food, should, in the natural course of things, be 

 behind^ and not before the morsel. 



The entire history of the assimilative function, how- 

 ever, is at variance with the idea that the food induces 

 contraction in the alimentary canal : indeed the con- 

 stant lesson is that the food remains within the diges- 

 tive apparatus so long as any molecular changes 

 are unaccomplished, and that expulsion takes place, 

 and the effete matter is expelled, only when these 

 changes are at an end. The ingestion of food into 

 the stomach does not cause this organ to contract, 

 nor does any contraction interrupt the process of 

 digestion ; and it is only when the gastric juices have 

 effected their office and the food is dissolved, that the 

 contraction takes place, by which the food is trans- 

 posed to the duodenum. In the small intestines, the 

 food remains during the continuance of the processes 

 of ulterior digestion, and it is transmitted onwards, 

 only when these are at an end And lastly, there is 

 no contraction in the large intestines, until the di- 

 gestive changes are completed. So long, therefore, 

 as there are any evidences of the continuance of 

 molecular changes, by which the food could act as a 

 stimulus to the muscles of the alimentary canal, so 



