DIGESTION. 181 



the case in his experiments. A similar inhibition may be brought about 

 reflexly by stimulation of the central end of a divided vagus. This result 

 will not be produced if the opposite vagus has previously been divided. The 

 vagi, therefore, contain both inhibitor and augmentor nerve-fibers for the 

 gastric musculature, though the augmentor nerves largely preponderate. 



Stimulation of the peripheral end of a divided splanchnic is followed by 

 an inhibition of the peristalsis and a loss of tone. Morat, however, has 

 observed a primary opposite effect. The splanchnic nerves, therefore, 

 also contain both inhibitor and augmentor fibers for the gastric musculature 

 though the inhibitor fibers largely preponderate. From these facts it would 

 appear that the gastric muscles receive both inhibitor and augmentor fibers 

 from two different sources. 



The conditions necessary for the development of the gastric peristalsis 

 are (i) a condition of tonicity of the musculature, i.e., a slight degree of con- 

 traction whereby the muscle is shortened; (2) intragastric pressure. When 

 these two conditions are mutually adapted fche musculature acquires a cer- 

 tain degree of tension whereupon the peristalsis arises. An excess or de- 

 ficiency of internal pressure as well as a loss of tonicity prevents peristalsis. 

 The peristalsis has no necessary fixed point of origin but arises at that 

 portion of the stomach in which the two factors previously mentioned bear 

 a certain relation one to the other. From their origin the peristaltic waves 

 pass toward the pylorus as a result of increased internal pressure. The 

 necessary degree of the preliminary tonus is imparted to the musculature 

 by nerve impulses descending the vagi. If these nerves are cut, the tonus is 

 lost and peristalsis fails to develop. When once the peristalsis is well 

 developed in digestion, division of the vagi has no effect. (Cannon.) 



INTESTINAL DIGESTION. 



The physical and chemic changes which the food principles undergo in 

 the small intestine, and which collectively constitute intestinal digestion, 

 are probably more important and complex than those taking place in the 

 stomach, for the food is, in this situation, subjected to the solvent action of 

 the pancreatic and intestinal juices, as well as to the action of the bile, each 

 of which exerts a transforming influence on one or more substances and 

 still further prepares them for absorption into the blood. 



To rightly appreciate the physiologic actions of the digestive juices 

 poured into the intestine, the nature of the partially digested food as it 

 comes from the stomach must be kept in mind. This consists of water, 

 inorganic salts, acidified proteins, proteoses, peptones, starch, maltose, 

 liquefied fat, saccharose, lactose, dextrose, cellulose, and the indigestible 

 portion of meats, cereals, and fruits. Collectively they are known as chyme. 

 As this acidified mass passes through the duodenum its contained acids 

 excite a secretion and discharge of the intestinal fluids: e.g., pancreatic 

 juice, bile, and intestinal juice. Inasmuch as these fluids are alkaline in 

 reaction they exert a neutralizing and precipitating influence on various 

 constituents of the chyme. As soon as this has taken place, gastric diges- 

 tion ceases and those chemic changes are inaugurated which eventuate in 

 the transformation of all the remaining undigested nutritive materials into 



