[94 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



erator and thermo-inhibitory centres seem to be especially affected by cuta- 

 neous impulses which arc generated by changes in external temperature, and 

 to be influenced by alterations of the temperature of the blood. It is doubtless 



through thes intres that changes in external and internal temperature are 



able to affect the heat-producing processes. Presumably both an increase of 

 temperature of the blood and cutaneous impulses generated by an increase of 

 external temperature excite the thermo-inhibitory centres, and thus inhibitory 

 impulses are senl to the general centres, lessening their activity; on the other 

 hand, both a fall of temperature of the blood and cutaneous impulses gener- 

 ated by cold presumably excite the thermo-accelerator centres and thus cause 

 impulses to be sent to the general centres, exciting them to greater activity. 



The Mechanism concerned in Thermolysis. — The loss of heat by the body 

 is in a large measure incidental to attendant conditions and is not a reflex 

 result of the activity of a thermolytic mechanism; in other words, the loss 

 occurs essentially by virtue of the same conditions as would cause inanimate 

 bodies to lose heat. The living homothermous organism differs as regards the 

 loss of heat from dead matter, chiefly in that the rapidity with which heat- 

 dissipation occurs is regulated to a material extent by vital processes. The 

 regulation of the loss of heat is effected by the operations of a complex mech- 

 anism — that is, one consisting of a number of distinct although correlated parts. 

 A study of this mechanism, which is designated the thermolytic mechanism, 

 includes a consideration of all of the processes by which heat is lost, of the 

 nervous mechanisms which govern them, and of the conditions which affect 

 them, but especially of those processes and mechanisms which act reciprocally 

 in conjunction with the thermogenic mechanism to maintain the mean bodily 

 temperature. Practically all of the heat lost by the organism occurs by radia- 

 tion and conduction from the skin, by the evaporation of water from the skin 

 and lungs, and in warming the food, drink, and inspired air. From these facts we 

 believe that mechanisms which affect the blood-supply to the skin, the quantity 

 of sweat secreted, the condition of the surface of the skin, and the quantity of 

 air inspired must in a large measure regulate thermolysis. For instance, if the 

 temperature of the organism be materially increased there occur increased activ- 

 ity of the heart, peripheral vascular dilatation, increased respiratory activity, and 

 (except in fever) an increase in the secretion of sweat. The increase of the 

 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 loan 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- 



