NATURAL HISTORY 



NERVOUS SYSTEM AND SENSE- 

 ORGANS 



CHAPTER LVII 



GENERAL PRINCIPLES NERVOUS SYSTEMS OF 

 INVERTEBRATES AND VERTEBRATES 



GENERAL PRINCIPLES 



Some of the properties of living matter or protoplasm have 

 already been pretty fully considered, in sections which may be 

 regarded as expansions of part of the brief sketch of Human 

 Physiology given in vol. i, pp. 24-59. We have seen that 

 protoplasm is a very complex and eminently unstable substance, 

 which is continually breaking down into simpler compounds, 

 with the result that stored or potential energy is transformed 

 into actual or kinetic energy, without which movement and 

 other life-manifestations would be impossible. The breaking- 

 down process ultimately results in the formation of waste pro- 

 ducts, which being physiologically useless are cast out of the 

 body. One such product is carbonic acid gas or carbon dioxide, 

 and a primary object of Breathing or Respiration is to get rid 

 of this. But Breathing also includes the taking in of free oxygen, 

 without which the breaking down of the complex body-substance 

 would not take place at the rate necessary for liberating the 

 energy required. We have also seen that the gradual wasting 

 of the body associated with the breaking-down process requires 

 to be made good; hence the necessity for Food, which is built 

 up into fresh protoplasm. In cases where Growth is taking 



^VOL. IV. 1 95 



