THE CONQUEST OF NATURE 



conditions of muscular activity, is largely dependent 

 upon the control of the blood supply, in which involun- 

 tary muscles, similar to those of the heart, are concerned. 

 In times of great muscular activity, when the production 

 of heat is relatively enormous, the arterioles that supply 

 the surface of the body are rapidly dilated so that a 

 preponderance of blood circulates at the surface of 

 the body, where it may readily radiate its heat into 

 space; the vast system of perspiratory ducts, with 

 which the skin is everywhere supplied, aiding enor- 

 mously in facilitating this result, through the secretion 

 of a film of perspiration, which in evaporating takes 

 up large quantities of heat. 



The flushed, perspiring face of a person who has 

 violently exercised gives a familiar proof of these 

 physiological changes; and the contrary condition, 

 in which the peripheral circulation is restricted, and in 

 which the pores are closed, is equally familiar. More- 

 over, the same cutaneous mechanism is efficient in afford- 

 ing the organism protection from the changes of external 

 temperature; though the human machine, thanks to 

 the pampering influence of civilization, requires addi- 

 tional protection in the form of clothing. 



APPLICATIONS OF MUSCULAR ENERGY 



Having thus outlined the conditions under which the 

 muscular machine performs its work, we have now to 

 consider briefly the external mechanisms with the aid 

 of which muscular energy is utilized. Of course, the 

 simplest application of this power, and the one univer- 



