THE ACTIVE FORCES OF LIVING ORGANISMS 85 



matter which contains relatively but little oxygen has 

 already been pointed out, and it would seem that a 

 real gain in this respect can only be brought about 

 when oxidation goes on after each fresh stimulus in a 

 slightly decreasing rather than increasing manner. 



In the effects produced upon the nervous system 

 by sudden variations of temperature one may obtain 

 further evidence, though of an indirect kind, of the 

 modification which the state of contraction or ex- 

 pansion of parts of the nerve-cells undergoes as the 

 nervous rhythm varies. The contracting influence 

 of cold and the expanding influence of heat, it is 

 scarcely necessary to say, are by no means limited to 

 physiological processes, for with the exception of 

 liquids in a freezing state, which are probably not 

 true exceptions, all substances, whether organic or 

 inorganic, are affected by them in the same way, 

 though in different degrees. The molecules of living 

 organisms behave, therefore, very much as do those of 

 a piece of copper or iron ; they separate under the in- 

 fluence of heat, though of course much more readily, 

 and they draw together under that of cold. When 

 the whole cutaneous surface of the body is suddenly 

 exposed to the influence of cold, as in a bath, there is 

 a general movement of contraction. That this move- 

 ment is mainly vaso-motor may be granted, but that 

 it is limited to the bloodvessels is apparently the 

 case. The phenomenon known as goose-skin shows 

 that muscular, and probably other elements such as 

 the nerve-endings, take part in it. If the latter are 



