88 GENERAL BIOLOGY 



to be a white, crystalline, somewhat bitter powder, 

 to which the name adrenalin has been given. Prac- 

 tical use of this substance has been made in sur- 

 gery. By its application small hemorrhages may be 

 entirely done away with, particularly in delicate 

 operations on the nose and eye. There has been 

 synthesized recently a substance similar to adrenalin, 

 if not identical with it, which produces the same 

 effect. 



Another ductless gland, the thyroid, by its se- 

 cretion, influences the normal phenomena of differ- 

 ential ve growth in the higher animals. When the 

 thyroid is diseased, the whole system is affected. 

 In extreme cases degenerative conditions known 

 as " cretinism " and myxedema result. If the gland 

 be extirpated in a very young animal, death in- 

 evitably follows, but if small pieces be introduced 

 elsewhere in the body by grafting, or especially if 

 an extract of the gland be fed, the evil results dis- 

 appear, or are greatly mitigated. The extract has 

 been shown to owe its efficacy to the presence therein 

 of a chemical compound (thyroiodin) containing a 

 high percentage of iodine.' Another somewhat sim- 

 ilar ductless gland, the thymus, is also found in the 

 throat. If tadpoles be fed thyroid, their metamor- 

 phosis is greatly hastened, and they turn into tiny 

 frogs before they have had time to grow to normal 

 size. On the other hand, if fed thymus, differen- 

 tiation is inhibited and growth accelerated, with the 

 result that they grow into large tadpoles, but do not 

 metamorphose at all. 



