THE THYROID AND PARATHYROIDS 283 



Professor Cameron and myself have recently described an 

 enlarged thyroid in an Elasmobranch fish Squalus Mecklii. 

 The structure showed active hyperplasia and in certain parts 

 all the appearances of true carcinoma. We are not aware of 

 any previous observations on thyroid growths inElasmobranchs. 

 Teleostean thyroid enlargements have been described by 

 Marine and others. One of the chief points of interest about 

 the Teleostean growths is that the gland is unencapsuled and 

 consists of scattered vesicles and groups of vesicles. The 

 Elasmobranch thyroid, on the other hand, is definitely encap- 

 suled, and forms a compact organ. Again the change we 

 have described occurred in a wild fish, and one for which a 

 constant supply of iodine is readily accessible. Marine finds 

 that in fresh- water fishes iodine is a valuable curative agent. 



It is usual to classify goitres as parenchymatous, vascular, 

 and cystic. But Marine has emphasized the importance of 

 a physiological cycle as explaining the innumerable forms of 

 goitre. 



In interpreting the thyroid changes in goitre, according to 

 Marine, one must bear in mind the simple thyroid cycle which, 

 beginning with the normal cell, consists of progressive changes 

 through all degrees of hypertrophy and hyperplasia and regres- 

 sive changes either (1) throughout all degrees of atrophy, or 

 (2) throughout all degrees of involution to the resting or colloid 

 or nearest normal state that such thyroids can again assume 

 which have once undergone hyperplasia. 



The cycle may be represented schematically as follows : 



/'atrophy 



Normal thyroid hypertrophy hyperplasia < involution ;to 



\colloid state 



/atrophy 

 Colloid state hypertrophy hyperplasia j involutiqn to 



Vcolloid state 

 _ I 



Colloid .... 



This conception has made it possible to interpret the many 

 gradations of thyroid reactions found in diffuse overgrowths 

 of the thyroid (goitre), and now one looks upon any given 



