1 90 NUTRITION, DIETETICS AND ANIMAL HEAT 



It is increased when the internal temperature is raised and when 

 the external temperature is lowered. Any general increase 

 in vascular or respiratory activity increases heat dissipation for 

 reasons already given. When the external temperature is high 

 and the air is dry evaporation is more abundant, and conse- 

 quently heat dissipation is greater than when the air is already 

 impregnated with moisture. Hence the oppressiveness of a 

 high external temperature with high humidity. In fever heat 

 dissipation is usually increased, but to a less degree than that 

 production. 



Thermotaxis. Thermotaxis is the regulation of heat produc- 

 tion and heat dissipation so that the temperature of the 'body 

 may remain the same. It is evident that there is frequently a 

 transient increase or decrease of thermogenetic activity; unless 

 there be a corresponding change in thermolytic activity the 

 temperature will be disturbed. 



The temperature of the body is not necessarily raised when 

 heat production is increased, or lowered when it is decreased; 

 for heat loss may be, and in health is, correspondingly increased 

 or diminished. Conversely, a change in heat loss does not nec- 

 essarily mean an opposite change in the body temperature. Al- 

 terations which do occur in the temperature are the result of the 

 improper regulation of the heat at hand. For instance, fever may 

 result from average heat production and deficient heat loss; 

 from increased heat production and heat loss when the latter is in- 

 creased less than the former; from diminished heat production 

 and heat loss when the latter is diminished less than the former, 

 etc. A subnormal temperature is caused by opposite condi- 

 tions. The temperature remains constant when heat production 

 and loss are normal, or when they are increased or decreased 

 correspondingly. 



Thermotactic activity is the result of changes in the tempera- 

 ture of the blood, or of cutaneous impressions. A rise in the tem- 

 perature of the blood excites heat loss, as indicated. A cold 



