ANIMAL HEAT. 489 



removing the natural covering, such as the fur of the rabbit, under which cir- 

 cumstances, even though the animal be subjected to a relatively high external 

 temperature, heat-dissipation is so enormously increased that death ensues within 

 two or three days. When one side of the body of a horse was shaved and the 

 animal subjected to an atmosphere having a temperature of 0° C, the tem- 

 perature of the skin of the shaven side fell 8° in forty minutes, while the 

 temperature of the unshaven side fell only 0.5°. 



The coefficient is diminished where there is excessive sebaceous secretion, 

 and where grease is artificially applied, and by an accumulation of subcutaneous 

 fat ; it is increased by wetting the skin, as by sweat or bathing ; and it is 

 affected by many other circumstances. 



Through the operations of the nervous system heat-dissipation may be 

 affected directly or indirectly by action upon the heat-dissipating and heat- 

 producing processes — circulation, respiration, sudorific and sebaceous glands, 

 and arrector pili muscles. 



There are many drugs which directly or indirectly affect heat-dissipation. 

 Drugs which cause dilatation of the cutaneous vessels tend to increase heat- 

 dissipation ; conversely, those which cause contraction of the blood-vessels 

 hinder dissipation. Diaphoretics increase heat-loss essentially by increasing 

 the amount of sweat. Respiratory excitants increase the loss of heat by means 

 of the increased volume of air respired. Drugs which increase heat-production 

 tend to indirectly increase heat-dissipation. 



All pathological states which affect heat-production tend to similarly disturb 

 heat-dissipation. Conditions of malnutrition favor heat-dissipation by causing 

 a loss of subcutaneous fat, but this is to a greater or less extent compensated 

 for by the enfeeblement of the circulation, respiration, and metabolic processes 

 in general. In fever, both heat-production and heat-dissipation are generally 

 increased, the former being affected more than the latter, so that the bodily 

 temperature rises. In some forms of fever the rise of temperature is essentially 

 due to diminished heat-dissipation. 



D. The Heat-mechanism. 



The heat-mechanism consists of two fundamental parts, one being concerned 

 in heat-production, and the other in heat-dissipation. Heat-production is 

 briefly expressed as thermogenesis ; and heat-dissipation, as thermolysis. The 

 operations of these mechanisms are so intimately related that fluctuations in the 

 activity of one are rapidly compensated for by reciprocal changes in the other, 

 so that under normal conditions heat-production and heat-dissipation so nearly 

 balance that the mean bodily temperature is maintained within narrow limits. 



The regulation of the relations between heat-production and heat-dissipation 

 is termed thermotaxis, which regulation may be effected by alterations in either 

 thermogenesis or thermolysis. 



The Mechanism concerned in Thermogenesis. — The portion of the heat- 

 mechanism concerned in heat-production consists of (1) thermogenic tissues, 

 (2) thermogenic nerves, and (3) thermogenic centres. 



