NUTRITION 153 



contractions, peristalsis, of the digestive tract wall. The esoph- 

 agus is a muscular tube which passes posteriorly through the 

 thorax, and rapidly delivers the food without further digestion 

 to the STOMACH. 



B. Stomach 



The stomach, really the first stopping place of food that has 

 been swallowed, is a thick-walled sac situated just below the dia- 

 phragm in Mammals. In common with most of the viscera, the 

 stomach is suspended in the abdominal cavity by broad loops of a 

 membrane, the mesentery, which is continuous with the perito- 

 neal membrane lining the cavity. Within the mesentery, blood 

 vessels, nerves, etc., pass to the stomach. (Figs. 106-110.) 



Here the work of the digestive tract actively progresses by the 

 action of specific chemical substances present in the gastric juice 

 which is secreted by innumerable gastric glands. The latter 

 are tiny pits in the stomach wall lined with special glandular cells. 

 Human gastric juice is a complex fluid comprising water — over 

 99 per cent; a protein-splitting enzyme, pepsin; a milk-curdling 

 enzyme, rennin; common salt, NaCl; and a free acid, HC1. This 

 array of components of gastric juice softens the food mass, gives 

 it an acid reaction, curdles the milk, and simplifies the proteins — 

 transforms, with the assistance of slow churning movements of 

 the stomach wall, the average meal in the course of an hour or 

 so into chyme which gradually passes through the pyloric valve 

 into the intestine. 



C. Small Intestine 



The human intestine is a much coiled tube, nearly twenty-five 

 feet in length, that extends from the stomach to the anus, and it is 

 in the upper part, known as the small intestine, that not only the 

 most radical changes in the food take place — digestion is essen- 

 tially completed; but also most of the products of digestion pass 

 through its walls into the body proper — absorption occurs. Thus 

 we find various glands to elaborate and secrete the digestive fluids 

 — cells that take from the circulatory system not only the materials 

 necessary for their own life but also other substances which they 

 transform chemically for the use of the organism as a whole. Some 

 are unicellular or simple multicellular tubular glands embedded in 

 the intestinal wall; others are highly complex and far removed 



