122 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



phragm), and the abdominal muscles likewise contract 

 powerfully to compress the viscera and force the feces into 

 the rectum. Pressure on the afferent nerves of the rectum 

 probably sets up the desire to defecate, and the contraction of 

 its walls, as well as the relaxation of the internal sphincter is 

 a reflex act. The center is in the lower segment of the cord, 

 but it is connected with the cerebrum, as is shown by emo- 

 tional influences on the act. 



The average time occupied in the passage of the residue 

 of an ordinary meal from the mouth to the rectum is about 

 24 hours. Something like 12 hours of this is thought to be 

 spent in the large intestine. 



While it has been endeavored to establish clearly the sep- 

 arate action of each fluid with which the aliment comes in 

 contact, it is to be remembered that they form a mixture, the 

 combined activity of whose component parts results in the 

 extraction of all the nutritive material from the bolus in its 

 long journey through the gastro-intestinal tract. It can 

 hardly be said to be still at any time during that passage, 

 the continual peristalsis to which it is subjected facilitating 

 both the chemical action of the enzymes and the physical 

 phenomenon of absorption. 



ABSORPTION IN GENERAL. 



Obviously digested materials are of no service in the vital 

 economy until they are absorbed first by the circulation and 

 then by the tissues themselves. Here we will consider only 

 their absorption from the alimentary canal, which process, in 

 contradistinction to the other, may be termed external ab- 

 sorption. 



While it is known that the laws of diffusion and osmosis 

 outside the body are largely responsible for absorption within 

 the organism there are many phenomena in connection with 

 that process which cannot be explained under these laws, 

 and which are indeed, in some cases, at variance with them. 



