THE THYROID AND ACCESSARY THYROIDS 



455 



This disease is known to be due to disease of the thyroid, whereby its func- 

 tion is interfered with. Moreover, if a piece of thyroid of sufficient size 

 be grafted into an animal from which the glands have been removed, and 

 the graft takes, the symptoms of thyroid removal are lessened in intensity 

 or disappear altogether. Thyroid feeding or the administration of thyroid 

 extracts relieves the symptoms of the disease myxedema. 



FIG. 311. Part of a Section of the Human Thyroid, a, Fibrous capsule; 6, thyroid 

 vesicles filled with, e, colloid substances; c, supporting fibrous tissue; d, short columnar cells 

 lining vesicles;/, arteries; g, veins filled with blood; h, lymphatic vessel filled with colloid 

 substance. (S. K. Alcock.) 



The above facts show that the thyroid gland must perform some im- 

 portant function in the animal economy, and it is believed that this is accom- 

 plished by virtue of its internal secretion. The colloid material of the gland 

 has been submitted to much chemical study, and a substance called iodo- 

 thyrin has been isolated as its active principle. Baumann and Roos state 

 that iodothyrin exists in the gland in combination with protein bodies. lodo- 

 thyrin relieves the symptoms of thyroid removal much to the same extent 

 as thyroid feeding. It is a very resistant substance, and is not injured by the 

 action of the gastric juice or by boiling with 10 per cent, sulphuric acid for 

 a long time. 



Total extirpation of the thyroids carries extirpation also of the para- 

 thyroids. The question is thus raised as to which part of this total appara- 



