INTERNAL SECRETIONS OR HORMONES 407 



a substance with a marked power of combining with iodine. 

 This has been called iodothyrin. It contains about 3 '6 per 

 cent, of iodine, and it seems to be the active constituent of 

 the internal secretion of the gland. 



The removal of the thyroid usually leads to a train of 

 symptoms which varies somewhat in different animals, but is 

 essentially the same in nearly all. The connective tissues 

 tend to revert to the embryonic conditions, and the amount 

 of mucin increases. The temperature falls, muscular tremors 

 appear, and in dogs these may go on to convulsions. They do 



T, 



FIG. 164. Section through part of the Thyroid (Th.) and a Parathyroid (P.) 



of a Mammal. 



not disappear on removing the cortex cerebri, but are stopped 

 by section of the nerves to the muscles, and thus they 

 appear to be spinal in origin. The functions of the higher 

 nervous system become sluggish, and the animal frequently 

 dies. By administering the substance of the thyroid, or by 

 giving extracts of the thyroid, most of these symptoms may be 

 delayed or prevented. When thyroid gland or extract is given 

 to healthy animals in moderate doses it causes an increased 

 metabolism of both fats and proteins, and may thus induce 

 emaciation. These phenomena seem to indicate that one 

 function of the organ is to produce an internal secretion 



