772 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 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 tissue has a structure quite different 

 from that of the thyroids, being composed of solid masses or columns 

 of epithelial cells which are not arranged in vesicles and contain no 

 colloid. 



Extirpation of the Thyroids and Parathyroids. In 1856 

 Schiff showed that extirpation of the thyroids (complete thyroi- 

 dectomy) in dogs is followed usually by the death of the animal in 

 one to four weeks. The animal exhibits certain characteristic symp- 

 toms, such as muscular tremors, which may pass into convulsions, 

 cachexia, emaciation, and a condition of apathy. This result was 

 confirmed by subsequent observers, but many exceptions were noted. 

 Great interest was shown in these results, because on the surgical 

 side reports were made showing that after complete removal of the 

 thyroids in cases of goiter evil consequences might ensue, either 

 acute convulsive attacks or chonic malnutrition. On the other 

 hand, it became known that atrophy of the thyroids in the young 

 is responsible for the condition of arrested growth and deficient 



