602 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



activity of the heart together with the dilatation of the cutaneous blood-vessels 

 increases the quantity of blood supplied to the skin ; the cutaneous blood-vessels 

 are dilated, exposing a larger surface of blood to the cooler external surround- 

 ings, and thus materially favoring the loss of heat by radiation ; the increase in 

 the quantity of sweat is favorable to an increase in the amount of water evaporated, 

 and thus to a larger loss of heat in this way ; an increase of respiratory activity 

 means a larger volume of air respired, a greater expenditure of heat in warming 

 the air and in the evaporation of water from the lungs. In man the pilo-motor 

 mechanism plays a subsidiary and unimportant part in the regulation of heat- 

 dissipation, but in some lower animals, as in certain birds, it is of considerable 

 importance. The thermolytic mechanism therefore includes the cardiac, vaso- 

 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 cardio- 

 inhibitory 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 sub- 

 normal, 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 



