ELECTRO-CHEMICAL PHENOMENA 365 



The one secretion which answers these requisites 

 is the secretion of the thyroid gland. The specific 

 chemical constituent of the thyroid is known. It 

 is iodin. Thyroid extract alone, or iodin alone, causes 

 a steady rise in the rate of energy transformation, a 

 sustained maximum and a gradual fall. 



With the brain, the muscles, the liver, the adrenals 

 and the thyroid, we have the essential parts of an 

 automaton, which stores energy in sleep and automati- 

 cally discharges energy during consciousness ; maintains 

 the difference in potential between brain and muscles ; 

 mobilizes and demobilizes organs by means of the 

 adrenals ; and adaptively varies the speed to accord 

 with seasonal and physiologic vicissitudes by means of 

 the thyroid. 



