THE THYROID AND PARATHYROID GLANDS 



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and it can be produced in certain normal animals (particularly rats) by 

 placing them on an excessive meat diet. Important observations bearing on 

 this point have been made by Marine on brook trout, in which it has been 

 found that the so-called carcinoma that develops when the fish kept in 

 hatcheries are fed with unsuitable food and overcrowded, is really a 

 typical hyperplasia. In its second stage this develops into what is known 



A. 









Fig. 193. Microphotographs of thyroid gland of dog. A, normal; B, active hyperplasia; C, colloid 

 goiter. (From Marine and Lenhart.) 



as colloid goiter which is produced by a deposition of colloid material 

 between the rows of cells so as to cause an opening out again of the 

 vesicles (Fig. 193), with a consequent tendency to a reversion to the 

 normal histological structure, so far as this is possible. The vesicles in 

 such a gland are of enormous size, and the lining epithelium, low cubical, 

 or almost flat in shape. 



The outstanding characteristic feature of the colloid material is that 



