CHAPTER XVII. 

 WARMTH, LIGHT AND VENTILATION. 



CONTROL OF TEMPERATURE. 



The life activities manifested in the animal body involve 

 the continuous maintenance of a train of chemical changes 

 which give rise to or maintain them. These chemical 

 changes, like all others, can only begin at a certain tem- 

 perature; below this they cease; within a certain range they 

 go forward at normal rates; above this temperature reac- 

 tions occur which interfere with the life activities, making 

 them abnormal or causing them to cease. 



420. Automatic Control of Temperature. The animal 

 body is so constituted that within certain limits the normal 

 temperature of the body may be maintained automatically, 

 if only sufficient food is supplied. If outside conditions are 

 such as to lower the temperature of the body the nervous 

 system reacts, setting in operation a train of changes which 

 evolve heat fast enough to meet the greater loss. If on the 

 other hand the surrounding temperatures are too high and 

 the body is becoming too warm the heat producing reac- 

 tions are inhibited or perspiration is stimulated to reduce 

 the too high temperature by bringing the blood to the skin, 

 where the temperature may be lowered by the evaporation 

 of water in the same manner that the wet bulb of a ther- 

 mometer is cooled by the loss of heat which does the work 

 of evaporation. 



421. Normal Animal Temperatures. The normal temper- 

 atures which must be maintained within the animal body 



