ANIMAL HEAT. 25 



But these are not the true regulators of the animal 

 temperature. Our will commands all those actions whose 

 influence may be favourable to the regulation of our tempe- 

 rature ; but, in general, Nature, in order to secure the indis- 

 pensable functions of life, removes them from the control of 

 our will. It is in an automatic apparatus that we shall find 

 the real regulator of temperature. 



This apparatus must obey external and internal influences 

 at the same time, it must retain heat when it tends to be 

 dissipated too rapidly, and, on the other hand, it must facili- 

 tate its decrease when it is produced too abundantly within 

 the organism. 



This double end is achieved by a property of the circu- 

 latory system : the blood vessels, animated by nerves whose 

 action has beea revealed by M. Cl. Bernard, close under the 

 influence of cold, and open under the effect of heat. This 

 property regulates the course of the blood in each of the 

 organs, and at the same time the temperature throughout the 

 entire economy. 



Let us take an animal which has just been killed; the 

 circulation of the blood is stopped, and with it all the 

 functions. This animal, if placed in a low temperature, 

 becomes cold. According' to physical laws, the extremities of 

 the limbs and the surface of the body will lose heat in. the 

 first instance, while the central portions will still remain very 

 hot, being sheltered by the more superficial layers against the 

 causes of loss of heat. This corpse will resemble an inert 

 body which has been heated, and is growing cold. The cir- 

 culation of the blood opposes itself, during life, to the 

 unequal partition of heat over the various poiuts of the 

 organism ; bringing the arterial blood, at a temperature 

 of nearly 38 (centigrade), to the superficial portions, it warms 

 them when the external temperature tends to chill them. On 

 the other hand, if, in the living animal, the production of 

 heat has been augmented, the circulation opposes the inde- 

 finite heating of the central regions of the body ; it brings 

 that heat to the surface, where it is lost in contact with the 

 external colder medium. 



The effect of the circulation of the blood is therefore to 



