104 THE STRUCTURE AND LIFE OF BIRDS chap. 



But they do not all centre in this, and the exact part 

 of the brain from which some of them come has yet to 

 be discovered. The vaso-motor nerves not only have 

 local power, but by combined action can affect the 

 temperature of the body generally. They lower it 

 by sending a large flow of blood to the surface ; 

 more heat will then be lost through radiation and con- 

 duction, and in man by evaporation. Thus exercise at 

 once raises and keeps down temperature — raises it by 

 muscular activity which always generates heat, keeps 

 it down by bringing blood to the surface. In exposure 

 to cold, the blood withdraws from the surface, and 

 protects the vital organs from chill. 



It is supposed by high authorities that there are yet 

 other nerves or nerve-fibres, which have more complete 

 power over temperature in that they control directly 

 the amount of oxidation. When a warm-blooded 

 animal is dosed with the drug urari, it behaves like a 

 cold-blooded creature. If the nerves that arise from 

 the medulla oblongata are severed, the results are 

 the same in kind, though not so striking. This 

 cannot be due to the vaso-motor nerves, which 

 only regulate the amount of blood sent along the 

 arteries. 



The subject will be more intelligible, when I have 

 made clear what is meant by " behaving like a cold- 

 blooded animal." For purposes of distilling, a chemist 

 puts various substances in a retort and exposes them 

 to heat, and the greater the heat applied the faster 

 the process goes on. A cold-blooded animal has 

 been well compared to such a mixture of dead 

 substances in a chemist's retort. Heat increases and 



