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 
P Vor. IV. 1 95 
at 
