88 PRINCIPLES OF PHYSIOLOGY 



proteins, carbo-hydrates, and fats must be 

 modified by digestive processes, So far the 

 animal protoplasm, like that of the plant, 

 has been storing energy There is now a 

 reversal of the operation Under various 

 stimuli, which may be the nervous impulse, 

 or a mechanical, thermal, or electrical 

 stimulation, the protoplasm either contracts 

 (as in muscle) or is the seat of chemical 

 operations (as in a secreting cell or an electrical 

 organ), and energy is set free as motion and 

 heat, or, it may be, luminous or electrical 

 energy This implies decompositions and 

 oxidations, and requires oxygen from the air. 

 The splitting up of the complex protoplasm 

 causes the formation of simpler bodies. These 

 may be again and again further oxidized into 

 simpler compounds, always with the evolution 

 of energy, as heat, until ultimately we reach 

 simple ammonia-like bodies, urea, and water. 

 Thus the protoplasm of animal cells is chiefly 

 engaged in the liberation of potential into 

 kinetic energy. Both plant and animal take 

 part in both processes. In the plant there 

 are oxidations as well as reductions, but 



