THE KINETIC SYSTEM 161 



different seasons of the year, of the long cycles of 

 growth, reproduction and decay, of the unbalanced 

 demands of consciousness and of sleep, of health and 

 of disease, of activity and of rest. 



One of the most conspicuous periods of increased 

 activity is the breeding season of most animals, which 

 occurs in advance of the period of maximum food supply, 

 that the young may be brought forth at a time when 

 food is plentiful. In the spring most birds and mam- 

 mals mate, to the accompaniment of the increased 

 activities which are expressed in song and courting, in 

 fighting and flying, in quickening pulse and increased 

 temperature. In the spring the chief activating gland 

 of the kinetic system, the thyroid, shows a distinct 

 enlargement. Even more activation than is repre- 

 sented by the seasonal activities is represented by the 

 physical act of mating, when the thyroid is known 

 to enlarge, and life is on a sensuous edge. 



In like manner the expenditure of energy in the or- 

 ganism varies for the day and the night ; for youth and 

 age. The full tide of activity and consciousness is in 

 the daytime ; the ebb tide at night. The youth is 

 geared at high speed for growth, for becoming adapted 

 to innumerable injuries, to bacterial invasions, to het- 

 erogeneous food and to the various elements in his 

 external environment. 



The period of greatest energy output for physiologic 

 ends is the period of reproduction. In the female 

 especially there is a period of greatly increased activity 

 just prior to her development into the procreative age. 

 During . this time secondary sexual characters are 

 being developed. The pelvis expands ; the ovaries and 



M 



