230 INTERNAL SECRETION 



Methods of Investigation of tlie Organs of Internal Secretion 



Our knowledge of the'se organs has been derived — 



1. From their structure, development and comparative 

 anatomy ; 



2. From the efiects of extirpation; 



3. From the effects of administration of the glands 

 intravenously and orally both to normal animals and to 

 those from which the gland has been extirpated ; from the 

 action of the extract upon isolated organs and from the 

 effects of transplantation; 



4. From pathological conditions associated with changes 

 in these organs; 



5. From a comparative analysis of the blood entering, 

 and the blood leaving, the organ. 



THE THYROID AND PARATHYROID GLANDS 



Structure and Development of the Thyroid 



The thyroid consists of closed vesicles bounded by a 

 single layer of epithelium. There being no basement- 

 membrane, the vesicles are separated from one another 

 solely by areolar tissue, in which he the profuse blood- 

 vessels, lymphatics and nerve-filaments, the last-named 

 being derived from the superior and inferior laryngeal 

 branches of the vagus, and from the sympathetic. The 

 cavity of the vesicles is usually distended with a colloid 

 substance. In some animals the appearance of the vesicles 

 can be modified by changing the diet. When rats are fed 

 with lean meat the epithelium is cortical or even flattened, 

 and the vesicles loaded with colloid. When the diet 

 consists of bread and milk the epithelium is columnar, and 

 shows evidence of active secretion ; at the same time the 

 lumen, diminished in size by the protrusion of the cells, 

 contains a serous fluid but httle or no colloid. The colloid 

 thus appears to represent a store of secretion, which exists 

 when the gland is relatively inactive (Figs. 31 and 32). 



