DIGESTION 



185 



opposite effect. The splanchnic nerves therefore apparently contain both 

 inhibitor and augmentor fibers for the gastric musculature though the inhibi- 

 tor fibers largely predominate. From these facts it would appear that the 

 gastric muscles receive both inhibitor and augmentor fibers from two differ- 

 ent sources. 



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. 



The serous coat is the most external and is formed by a reflection of the general 

 peritoneal membrane. It is, however, wanting in the duodenal portion. 



The muscle coat, situated just beneath the former, surrounds the entire intestine. 

 It is composed of non-striated fibers, which are more abundant and better de- 

 veloped in the upper than in the lower portions of the intestine. The muscle coat 

 consists of two layers of fibers: (i) an external or longitudinal, and (2) an internal 

 or circular layer. The longitudinal fibers are most marked at that border of 

 the intestine free from peritoneal attachment, though they form a thin layer all 

 over the intestine. The circular fibers are much more numerous, and completely 

 encircle the intestine throughout its entire extent. It has been demonstrated 

 that at the junction of the ileum and colon, and surrounding the orifice, the ileo- 

 colic, common to both, the muscle-fibers are arranged in the form of, and play the 

 part of, a sphincter muscle, which has been termed the ileo-cohc sphincter. It is 

 usually in a state of tonic contraction and regulates the passage of materials from 



