26 ANIMAL MECHANISM. 



render the temperature of the organism uniform. But this 

 uniformity is never complete; in fact, except in the case of the 

 animal's being in a vapour bath at 38, and losing none of 

 its heat, the surface of the body is always colder than the 

 interior, but no ill effect is produced by the chill, which does 

 not act upon the essential organs. 



If the circulation of the blood were of equal swiftness in every 

 part, such a uniformity would not result in the preservation of 

 the uniform temperature necessary for the internal regions of 

 the body; we should then merely see it exposed to more general 

 elevations and depressions of temperature, according to the re- 

 spective predominance of causes of heat or the loss of it. To 

 produce uniformity of central heat it is indispensable that some 

 influence should augment the rapidity of the circulation 

 each time that the organism produces more heat, or that the 

 elevation of the surroundiug temperature diminishes the causes 

 of cooling. Circulation in the superficial portions of the body 

 is extremely variable, as we may ascertain by observing the 

 varying aspects of those portions, which are sometimes red. 

 hot, and swollen, sometimes pale, cold, and shrunken, accord- 

 ing to the more or less abundance of the blood which circu- 

 lates in them. This variability depends upon the contraction 

 or the relaxation of the little arteries, whose muscular sheaths 

 obey special nerves. When, under the influence of these nerves, 

 named vaso-motors, the vessels contract, circulation slackens, 

 while by a contrary action, the relaxation of the vessels ac- 

 celerates the course of the blood. Now, it is the tempera- 

 ture itself "which most generally acts in regulating this state 

 of contraction or relaxation of the vessels, so that the animal 

 temperature possesses in reality an autmiuitif regulator. 



Every one has observed the influence of heat and cold on 

 the circulation in the skin. If we dip one hand in hot, and 

 the other in cold water, the first will grow red and the second 

 pale; heat has, therefore, the effect of relaxing, and cold of 

 contracting the vessels. In other words, according to what we 

 have already seen, heat, by its action upon the circulation, 

 favours the loss of heat ; while cold acts in an inverse sense, 

 and tends to diminish the intensity of tin: chilling process. 

 And it is not only under the influence of the variations of the 



