448 COMPARATIVE PHYSIOLOGY. 



Tlie production of feeai, however, seems to be equally under 

 the influence of the nervous system, though we know less about 

 the details of the matter. 



A cold-blooded animal difiPers from a warm-blooded one in 

 that its temperature varies more with the surrounding medium: 

 hence the terms poikilothermer and homoiothermer for cold- 

 blooded and warm-blooded, would be appropriate. 



Such an animal, as a frog or turtle, may have its chemical 

 processes slowed or quickened, almost like those going on in a 

 test-tube or crucible, by altering the temperature. Very differ- 

 ent is it, as we have seen, in the normal state of the animal with 

 any mammal. Hence hibernation or an allied state has be- 

 come a necessary protection for poikilothermers, otherwise they 

 would perish outright, and the groups become extinct in north- 

 ern latitudes. 



It is plain that vaso-motor changes alone can not explain 

 these effects ; and, though possibly a part of the rise of tem- 

 perature, following exposure of the naked body in a cool air, 

 may be accounted for by the increased metabolism of internal 

 organs, accompanying the influx of blood caused by constric- 

 tion of the cutaneous capillaries, it is probable that in this as in 

 so many other instances the blood and circulation have been 

 credited with too much, and the direct influence of the nervous 

 system on nutrition and heat production overlooked or under- 

 estimated. The thermogenic center has not yet been definitely 

 located, though some recent investigations seem to favor a spot 

 in or near the corpus striatum for certain mammals. Some in- 

 vestigators also recognize a cortical heat-center. It has been 

 suggested that we may to advantage speak of a thermotaxic 

 (regulative of loss) and a thermogenic mechanism (regulative 

 of production), and even a thermolytic or discharging mechan- 

 ism. It has been further suggested that different nerve-flbers 

 may be concerned in the actual work of conveying the different 

 impulses of these respective mechanisms to the tissues ; and the 

 whole theory has been framed in accordance with the prevalent 

 conception of raetabolism as consisting of anabolism and ca- 

 tabolism, or constructive and destructive processes. But these 

 theories have not yet been confirmed by experiments on ani- 

 mals, though they are, in the opinion of their authors, in har- 

 mony with the facts of fever. Certainly, any theory that wUl 

 imply that vital processes are more under the control of the 

 nervous system than has hitherto been taught, will, we think, 



