ANIMAL HEAT. 601 



any influence on thermogenesis after section of the spinal cord at its junction 

 with the medulla oblongata. 



Theoretically, these centres are associated in this way : The general thermo- 

 genic centres are in the spinal cord, and while they are perhaps impressionable 

 to impulses coming to them through various sensory nerves, they are not 

 apparently in the least influenced by cutaneous impulses caused by changes in 

 external temperature nor by changes of the temperature of the blood. It is 

 not improbable that these centres are in the anterior cornua of the spinal cord. 

 The thermo-accelerator and thermo-inhibitory centres are connected with the 

 general centres by nerve-fibres, the former influencing the general centres to 

 increased activity, and the latter to diminished activity. The thermo-accel- 

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

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

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

 through these centres 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 sent 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 thermpgenic 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 



