1320 PHYSIOLOGY 



agency of the acid of the gastric juice (the specific stimulus) of 

 a substance secretin. This substance, which is heat stable and 

 diffusible, but is easily destroyed by oxidation, is absorbed into the 

 blood and carried to the pancreas, where it acts as a specific stimulus 

 for the secretory cells. The same substance excites also the secretion 

 of bile by the liver-cells and the secretion of intestinal juice by the 

 glands of the small intestine. These are perhaps the two best examples 

 of chemical reflexes, i.e. adaptations effected by chemical means rather 

 than by impulses passing along the nerve-channels. There are, 

 however, many other examples of a chemical influence exerted by one 

 organ on another, in which the interaction probably depends on the 

 production of minute quantities of some substance acting in virtue of its 

 excitatory qualities rather than of its value as a source of energy. For 

 many of the organs of the body we know, in fact, no other function 

 than the production of some substance the presence of which in the 

 blood is a necessary condition for the carrying out of the normal 

 functions either of growth or activity of many other parts of the body. 

 In other cases an organ may have a twofold function. Thus the 

 pancreas gives an external secretion which is used for the prepara- 

 tion of the food for absorption, and an internal secretion which, 

 passing into the blood, exercises an important influence on the meta- 

 bolism of the food-stuffs after absorption. Other instances of 

 these chemical correlations may be cited. The secretion of gastric 

 juice, which results from the presence of peptones or other substances 

 in the stomach, has been ascribed by Edkins to the production in the 

 pyloric mucous membrane of a gastric hormone, which travels by the 

 blood to the glands of the fundus, where it excites secretion of gastric 

 juice. According to Frouin the injection of boiled succus entericus, 

 free from secretin, provokes the secretion of intestinal juice. In the 

 reproductive system we have many examples of such chemical correla- 

 tions. The pancreas, by its internal secretion, probably influences 

 not only the oxidation of the carbohydrates but also the assimilation 

 of the food-stuffs by all parts of the small intestine. All these examples 

 are discussed in fuller detail in other parts of this book. There is one 

 class of organs in which a chemical influence exerted on the rest of the 

 body is the only function with which we are acquainted. These are 

 included under the term ductless glands. As examples, we may cite 

 the suprarenal bodies, the thyroid and parathyroids, the thymus and 

 the pituitary body. 



THE SUPRARENAL BODIES 



The suprarenal capsules in mammals are two small masses lying 

 on the upper or oral side of the kidneys. They consist of two parts, 

 the cortex and the medulla. The cortex is composed of 



