794 PHYSIOLOGY OF DIGESTION AND SECRETION. 



be regarded as examples of internal secretions. Chemical products 

 of this kind which stimulate the activity of special organs Starling 

 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 makes the suggestive 

 remark 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 coordination through the medium of a 

 nervous system is of later development. In the mammalian body 

 both methods, as we have seen, are employed. 



Liver. We do not usually regard the liver 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. 



The parathyroids are, according to most authors, quite different 

 structures. Four of these bodies are usually described, two on each 

 side, and their positions vary somewhat in different animals. In 

 man the superior (or internal) parathyroids are found upon the 

 posterior surface of the thyroid at the level of the junction of its 

 upper with its middle third. This portion of the organ may be 

 imbedded in the thyroid. The inferior (or external) parathyroids 

 lie near the lower margin of the thyroid on its posterior surface, 

 and in some cases lower down on the sides of the trachea. The 



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

 Physiology of Digestion," Chicago, 1906. 



