ANIMAL HEAT. 495 



motor, respiratory, sweat, and pilo-motor mechanisms. All these are affected 

 directly or indirectly by the temperature of the blood and skin. An increase 

 in the temperature of the blood and skin excites all of them so that changes 

 are brought about which favor heat-loss. The respiratory movements especially 

 may be rendered intensely active, and in certain animals to such a marked 

 degree that they may become more frequent than the heart-beats. 



Thermotaxis. — Thermotaxis or heat-regulation is effected by reciprocal 

 changes in heat-production and heat-dissipation brought about by the inter- 

 vention of the thermogenic and thermolytic centres, just as the regulation 

 of arterial pressure is effected by the reciprocal relations of the cardiac 

 and vaso-motor mechanisms. If heat-production is more active than heat- 

 dissipation, thermolysis is so affected that the heat-loss is increased, and thus 

 the mean bodily temperature maintained ; if heat-production is subnormal, 

 heat-dissipation also falls. Similarly, if heat-dissipation is increased, the 

 heat-producing processes are excited to greater activity to make up the loss ; 

 conversely, if heat-dissipation is decreased, heat-production also tends to be 

 decreased. These reciprocal actions depend essentially or wholly upon the 

 influence of cutaneous impulses and the temperature of the blood. For 

 instance, an increase of the temperature of the blood increases the activity 

 of the thermolytic processes, thus effecting a compensation. If we subject an 

 animal to a moderately cold atmosphere, as in the winter, heat-dissipation is 

 increased, but cutaneous impulses are generated which excite the thermogenic 

 centres so that heat-production is also increased, and thus the bodily temperature 

 is maintained practically unaffected. It is only under abnormal conditions or 

 under conditions of intense muscular activity that this reciprocal relation- 

 ship is so disturbed that changes in one process are not quickly compensated 

 for by changes in the other. 



Thermotaxis is effected in a large measure reflexly, especially by cutaneous 

 impulses generated by external cold and heat, both thermogenic and thermo- 

 lytic processes being affected. Cold applied temporarily, as in the form of a 

 douche, bath, sponging, etc., causes constriction of the cutaneous capillaries. 

 This lessens both the quantity and temperature of the blood passing through 

 the skin, the effect of which tends to decrease the dissipation of heat by radia- 

 tion and conduction. Moreover, a lessened blood-supply causes the skin to 

 become poorer in fluid, so that the conduction of heat from the warmer inner 

 parts is lessened. The conductivity of the skin is further decreased by the 

 action of the pilo-motor muscles, which when in contraction or in a state of 

 greater tonicity render the skin tenser and thus press out the blood and tissue 

 juices. The secretion of sweat is diminished, so that the quantity of heat lost 

 in the vaporization of water is decreased. On the other hand, heat-dissipation 

 tends to be materially increased by the greater radiation of heat due to the 

 greater difference between the temperature of the body and of the douche, bath, 

 etc., and the tendency to an increase in this way is much greater than the 

 opposite tendency depending upon the factors above noted, therefore heat- 



