DIGESTION 185 



of the stomach (the vagi). From the widely separated origin of these 

 various nerves it would seem probable that a coordinating mechanism 

 would be necessitated, otherwise their coordinate activity would be 

 difficult of explanation. Such a center has been located in the floor of 

 the fourth ventricle in close relation with the afferent part of this mechan- 

 ism. It is a well-known fact that vomiting can be excited by the hypo- 

 dermatic injection of apomorphin which while stimulating the entire 

 nerve system, stimulates most actively the efferent cells of the medulla 

 oblongata. If even the medulla be brushed over with a solution of apo- 

 morphin vomiting promptly follows. 



The afferent nerves are those distributed to the mucous lining of the 

 stomach the afferent fibers of the vagus which centrally are in relation 

 with the vomiting center. Hence peripheral stimulation of the vagi 

 by the products of gastric fermentation, or by pharmacologic agents 

 the emetics promptly excites the vomiting center, which in turn, excites 

 the efferent nerves and their related muscles. Stimulation of the af- 

 ferent portion of the glosso-pharyngeal will have a similar effect. Mental 

 emotions will also at times give rise to nausea and vomiting. 



INTESTINAL DIGESTION 



The physical and chemic changes which the. food principles undergo in 

 the small intestine, and which collectively constitute intestinal digestion, 

 are complex and probably more important 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 

 absorbable and assimilable compounds which collectively constitute intes- 

 tinal digestion. 



THE SMALL INTESTINE 



The Small Intestine. This portion of the alimentary canal is a convo- 

 luted tube, measuring about seven meters in length and 3.5 cm. in diameter, 

 and extending from the pyloric orifice of the stomach to the beginning of 

 the large intestine. 



The Walls of the Small Intestine. The walls of the intestine consist 

 of four coats: viz., serous, muscle, submucous, and mucous. 



