460 COMPARATIVE PHYSIOLOGY. 



sented finally as carbonic anhydride and water principally, 

 proteids as urea. 



Nitrogenous foods may be regarded as . accelerating the 

 metabolic processes generally and proteid metabolism in par- 

 ticular, while fats have the reverse effect ; hence fat in the diet 

 renders a less quantity of proteid sufficient. Gelatin seems to 

 act when mixed with proteid food either like an additional 

 quantity of proteid, or possibly like fat, at all events under such 

 circumstances less proteid sufa.ces. 



These facts have a bearing not only on health but on econ- 

 omy, in the expenditure for food. 



Salts hold a very important place in every diet, though 

 their exact influence is in great part unknown. The heat of 

 the body is the resultant of all the metabolic processes of the 

 organism, especially the oxidative ones. Certain food-stuffs 

 have greater potential capacity for heat formation than others ; 

 but, finally, the result depends on whether the organism can 

 best utilize one or the other. 



A certain body temperature, varying only within narrow 

 limits, is maintained, partly by regulation of the supply and 

 partly by the regulation of the loss. 



Both these are, in health, under the direction of the nervous 

 system, and both are co-ordinated by the same. Loss is chiefly 

 through the skin and lungs ; gain chiefly through the organs 

 of most active metabolism, as the muscles and glands. 



Vaso-motor effects play a great part in the escape of heat. 



Animals may be divided into poikilothermers and homoio- 

 thermers, or cold-blooded and warm-blooded animals, accord- 

 ing as their body heat varies with or is independent of the ex- 

 ternal changes of temperature. All the facts go to show that 

 in mammals the processes of the body (metabolism) can con- 

 tinue only within a slight range of variations in temperature, 

 though the upward limit is narrower than the downward. 



Upon the whole, the evidence justifies the conclusion that 

 the nervous system is concerned in all the metabolic processes 

 of the body in mammals including man, and that, as we descend 

 the scale, the dominion of the nervous system becomes less tiU 

 we reach a point when protoplasm goes through the whole 

 cycle of its changes by virtue of its own properties uninfluenced 

 bv any modification of itself in the form of a nervous system. 



