436 MANUAL OF PHYSIOLOGY. 



Since a variation of more than one or two degrees in the tem- 

 perature of our bodies interferes with the vital activities of the 

 controlling tissue in the nervous centres, it is, of course, of the 

 utmost importance that adequate means for the nice regulation 

 of the mean temperature of our bodies should exist. 



The temperature of an animal's body must depend on the 

 relations existing between the amount of heat generated in the 

 tissues and organs and the amount allowed to escape at the 

 surface, and these must closely correspond, in order that the 

 heat of the body remain uniform. Both these factors are found 

 to be very variable. Every increase in the activity of the mus- 

 cles, liver, etc., causes a greater production of heat, while a fall 

 in external temperature or increase in the moisture of the air 

 causes a greater escape of heat from the surface. 



The maintenance of uniform temperature may then be accom- 

 plished by (1) variations in the heat income, so arranged as to 

 make up for the irregularities of expenditure, or (2) variations 

 in the loss to compensate for the differences of heat generated. 

 Since the temperature and moisture of our surroundings are con- 

 stantly varying between tolerably wide limits, the amount of heat 

 given off by our bodies must vary greatly at different times. In 

 cold, damp weather a great quantity of heat is lost in comparison 

 with that which escapes from the body when the air is dry and 

 warm. If the heat generated had to make up for the changes in 

 the heat lost, one would expect to find a correspondingly great 

 difference in the amount of heat generated at different times of 

 the year, and no doubt we have some evidence in the keener ap- 

 petite and consequent use of more fuel, and the natural tendency 

 to active muscular exertion during cold weather, to show that a 

 greater amount of combustion does take place in winter than in 

 warm summer months. If the preservation of a uniform body 

 temperature depended upon the variations in the amount of in- 

 come exactly following those of the expenditure, we should find 

 it impossible to set our muscular or glandular tissue in action 

 except when the external temperature were such as would enable 

 us easily to get rid of the increased heat following their activity. 

 It certainly would appear that the general tissue combustion, as 



