80 HUMAN PHYSIOLOGY 



be kept up only by the generation of heat in their bodies 

 and this evolution goes on chiefly in the skeletal muscles. 

 A contribution to the total is also made by the heart 

 and the glands. When the outside temperature equals 

 or exceeds that of the body the unavoidable continuance 

 of internal heat production does add to the difficulties 

 of the situation. On the other hand, an animal whose 

 muscles have been removed from the control of the 

 nervous system, so that they cannot be called into play 

 on occasion, has little resistance to cold. We save 

 ourselves from freezing to death partly, to be sure, by 

 extra clothing but largely because when we feel cold 

 we are instinctively active. If we try not to be so we soon 

 shiver or grow tense in the attempt to remain still. 

 Either shivering or tension means muscle contraction; 

 so nature refuses to be cheated. 



