162 PRINCIPLES OF PHYSIOLOGY 



in muscular tissue, energy is latent when the 

 muscle is at rest, but when the nervous impulse 

 reaches it by travelling in a nerve the muscle 

 contracts, becomes warmer and does work by 

 motion, in one form or another. The nervous 

 impulse is the liberator, the muscle substance, 

 along with the chemical phenomena we have 

 considered, liberates energy as heat and motion. 

 In like manner, energy is stored up in all 

 living matter, in the secreting cell, in the tissues 

 of the nervous system, and probably in other 

 living tissues, and it is set free by a nervous 

 impulse. This explains why heat is developed 

 in a secreting gland during its activity, and 

 if we had adequate experimental appliances, 

 we should find evidence of heat in all vital 

 activities. 



88. Heat is also produced in the body in 

 other ways. The friction of the blood on the 

 walls of the vessels as it is driven along in the 

 circulation is resolved into heat, in other 

 words all the energy of the circulation becomes 

 heat. In like manner the movements of 

 organs causing friction produce heat. But 

 the great sources of heat are the phenomena 



