32 PRINCIPLES OF PHYSIOLOGY 



heat regulating mechanisms ? These ques- 

 tions will be afterwards discussed. It is to 

 be noted that living matter can only perform 

 its junctions within a narrow range of tem- 

 perature. 



(2) Motion. Animals move and parts of 

 their body move. They leap, run, or walk, 

 the chest moves in respiration, and the heart 

 beats. These movements are accomplished 

 r by the muscular tissues, which are the seat of 

 intense activities. Energy is here liberated 

 as motion. The physiologist has to study the 

 structure, nutrition, and the contractile func- 

 tions of muscular tissue. He finds that every 

 contraction of muscular tissue is associated 

 with active chemical changes occurring in the 

 tissue, by which there is a breaking down of 

 muscle substance, and the formation of 

 chemical substances of a simpler nature. 

 These chemical changes are intimately con- 

 nected with the breathing of the living muscle 

 substance, that is to say, they depend on 

 chemical phenomena in the muscle substance 

 which require the presence of oxygen and lead 

 to the elimination of carbonic acid and of other 



