14 



The Endocrine Organs 



ready exit from the vesicles, and thus fails to accumulate within them; 

 it is possible that it may also be different in consistence and quality. 

 So far as can be ascertained, these differences of appearance are seen in 

 animals which are otherwise normal. A glandular structure of this type, 

 but more pronounced, is noticeable in the thyroid in cases of exophthalmic 

 goitre in man. It is usually there interpreted to indicate the pro- 

 duction of excess of secretion (hyperthyroidism). The epithelium cells 

 of the vesicles contain fatty granules which are said to increase in 

 number with age; they are most numerous near the free border. The 



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Fig. 5. — Section of thyroid, human, injected. (Major.) Highly magnified. The close 

 relation of the capillaries to the vesicles is shown. 



colloid is insoluble in alcohol, water, or ether; when coagulated it is 

 readily stained by eosin, less easily by hematoxylin. In fixed sections 

 it often appears shrunken away from the epithelium. It is believed to 

 be formed from granules which are produced within the cells, and which 

 become changed and dissolved on extrusion. Doubtless it contains the 

 active principles of the secretion, and probably forms a storehouse whence 

 they can be extracted as required by the organism. 



A close relationship exists between the size and structure of the thyroid 

 and the state of general nutrition of the animal. The effect of variations 

 of diet upon the thyroid has been studied in rats by Chalmers Watson (see 

 figs. 6a and 6b), who has shown that all transitions from an active or super- 

 active organ with highly developed columnar epithelium and irregular 



