122 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



abdominal muscles. The lungs are filled, "the breath is held" 

 (forcing down and holding the diaphragm), 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 emotional 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 separate 

 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 absorption. 



While it is known that the laws of diffusion and osmosis out- 

 side 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 



