864 PHYSIOLOGY OF DIGESTION AND SECRETION. 



a fruitful application in the study of the digestive secretions. 

 The gastric and pancreatic secretins may be regarded as examples 

 of internal secretions. Chemical products of this kind which 

 stimulate the activity of other organs Starhng designates as 

 hormones* From this point of view the active substances formed 

 in the thyroids, adrenal glands, etc., may all be classified as specific 

 hormones. Starling suggests that this means of coordinating the 

 activities of the various parts of a complex organism may be 

 regarded as the most primitive, while the better-known coordina- 

 tion through the medium of a nervous system is of later develop- 

 ment. In the mammalian body both methods, as we have seen, 

 are employed. 



Schafer calls attention to the fact that some hormones inhibit functional 

 activity while others act as a chemical stimulant. The term "hormone" 

 {hormao, I excite), etymologically considered, is not applicable to the former 

 class. He suggests, therefore, the general term "autacoid substances" for both 

 groups to indicate their drug-like action {acos, a remedy), and subdivides 

 them into two groups according as they stimulate or inhibit. 



. , • J u + i Hormones — stimulating action. 



Autacoid substances I (.j^^i^^^^_.^^i^^.^^^y ^^^-^^ 



Liver. — We do not usually regard the Hver as furnishing an 

 internal secretion. As a matter of fact, it does form two products 

 within its cells — glycogen (sugar) and urea — which are subsequently 

 given off to the blood for purposes of general nutrition or for elim- 

 ination. The processes in this case fall under the general defini- 

 tion of internal secretion, and, in fact, may be used to illustrate 

 specifically the meaning of this term. The history of glycogen and 

 urea has been considered. 



Internal Secretion of the Thyroid Tissues. — ^The most im- 

 portant and definite outcome of the work on internal secretions has 

 been obtained with the thyroids. Recent experimental work on 

 this organ makes it necessary for us now to distinguish between the 

 thyroid and the parathyroid tissues. The thyroids proper form 

 two oval bodies lying on the sides of the trachea at its junction with 

 the larynx. They have no ducts, and are composed of vesicles of 

 different sizes, which are lined by a single layer of cuboidal epithe- 

 lium and contain in their interior a material known as colloid. A 

 number of histologists have traced the formation of this colloid to 

 the lining epithelial cells, and have stated, moreover, that the vesicles 

 finally rupture and discharge the colloid into the surrounding lym- 

 phatic spaces. Accessory thyroids varying in size and number may 

 be found along the trachea as far down as the heart. They possess 

 a vesicular structure and no doubt have a function similar to that 

 of the thyroid body. 



* For general discussion, consult Starling, "Recent Advances in the 

 Physiology of Digestion," Chicago, 1906. 



