ECONOMY OF THE BODY 115 



system is analogous to that of a gas engine used to compress 

 air into a reservoir, from which it is taken to drive, by its 

 pressure, various machines and tools. The energy of the 

 oxidation of the fuel is not used from the engine directly " 

 (1915, p. 462). 



The muscles of birds are specialised in many ways, 

 e.g. in moving the wings and neck, in contracting the gizzard 

 if there is one, in clinching the toes in perching, but careful 

 investigation has shown that the raw materials, as it were, 

 are in the main represented in reptiles. Very remarkable, 

 however, is the fact that the muscles concerned with flight 

 may be so strongly developed that they amount to half the 

 whole weight of the bird. 



It must be kept in mind that the muscles, besides being 

 the engines of the body, still rather mysterious in their 

 operations, are also the producers of the animal heat which 

 plays an important part in the internal economy. The 

 production of animal heat compensates the body for what 

 is lost to the outer world — especially in the hot breath and 

 by radiation from the skin, feathered though it is, and it 

 thus saves the bird from cooling down so far in cold weather 

 that vital processes would be stopped by the freezing of 

 fluids. But it has also a positive importance in facilitating 

 various chemical reactions which are always going on in 

 the routine of metabolism ; thus heat accelerates an oxida- 

 tion process. The animal heat has also a part to play in 

 the physical processes that go on in the body. Thus it 

 lessens the friction of body-fluids like the blood, and makes 

 diffusion easier. 



§ 3. Nervous Activity 



The nervous system consists of the brain and the spinal 

 cord and the nerves issuing from these, with in addition the 

 sympathetic nervous system, which has particularly to do 

 with the superintendence of the viscera. The chief 

 functions of the nervous system are (i) to receive messages 

 from the outer world through the sense-organs, so that 



