184 MAN --AN ADAPTIVE MECHANISM 



the liver. The volume of the thyroid is increased in 

 fear, in exertion, in sexual excitation, in infection, and 

 it is notably increased during 'periods of physiological 

 adjustment, such as adolescence and pregnancy. 



Negative evidence of the kinetic function of the thy- 

 roid is found in the fact that thyroid deficiency pro- 

 duces an adynamic state, in marked contrast to the 

 excessively dynamic state of hyperthyroidism. 



The Relation of the Muscles to Energy Transformation 



As for the role of the muscles in the transformation of 

 energy for both heat and motion, it is sufficient to point 

 out that in the muscles most of the motion and the heat 

 of the body is produced. In the muscles, then, we find 

 the fourth vital link in the kinetic chain. The muscles 

 move the body, circulate the blood, effect respiration 

 and govern the body temperature. 



Work Changes 



We have seen that every stimulus which produces 

 either heat or muscular activity in the animal body 

 produces morphologic changes in the brain, the ad- 

 renals and the liver, and probably changes the iodin 

 content of the thyroid ; that no like changes are pro- 

 duced in other organs or tissues of the body ; and 

 that these changes are produced primarily or seconda- 

 rily by the stimuli in question. It is thus a reasonable 

 conclusion that these changes represent work changes 

 in the organism. The evidence already presented, 

 however, is not sufficient to prove what that work is, 

 in what capacity each organ assists in its performance, 



