74 PHYSIOLOGY CHAP. 



drive, though the air was not very cold. According to Jiirgensen, 

 muscular work causes a rise in the temperature of about 1'2, at 

 which point it remains as long as the muscular activity continues. 

 He found a runner's temperature to be 39'5. Half an hour's 

 quick walk caused the temperature to rise 0'5 above normal. 



Even when the temperature of the air is very low, muscular 

 exercise will cause a considerable rise in the temperature of the 

 body. But according to Vernet's numerous observations, the 

 hyperthermia disappears after about twenty minutes' rest, and 

 the temperature becomes normal again. 



Even a slight muscular exertion which only lasts a few 

 moments is enough to produce a perceptible thermic effect. 

 Even going upstairs without hurrying particularly will raise the 

 temperature by several hundredths of a degree; it falls to normal 

 after a few minutes' rest. 



These phenomena prove that the mechanism regulating the 

 temperature is not sufficiently perfect to re-establish the normal 

 equilibrium between the production and the loss of heat whilst 

 muscular activity is going on. We must admit, as Bichet 

 correctly remarks, that it is always behindhand, and that some 

 moments of rest are therefore necessary in order to restore the 

 normal temperature. 



Psychical activity also raises the temperature about 0*1 above 

 the normal. J. Davy noticed this rise after two to five hours of 

 concentrated attention. Speck, Eumpf, and Gley confirmed the 

 thermic influence of intellectual work lasting some time. 



With regard to the cause of this slight hyperthermia, it 

 is difficult to believe that it is directly due to an increase in 

 the process of oxidation ; it is more probable, as Richet suggests, 

 that it depends on stimulation of the nerve-centres, resulting 

 in the regulation of the temperature at a level above the normal. 

 This rise of temperature caused by psychical activity is accom- 

 panied by a local warming of the brain, which was noticed for the 

 first time by M. Schiff, later by Dorta (1890), and more especially 

 by A. Mosso (1894). The daily curve of temperature, in so far 

 as it is to a certain extent independent of muscular activity, but 

 related to the alternation of wakefulness and sleep, is a proof 

 of a relative hyperthermia caused by the activity of the nerve- 

 centres and a relative hypothermia produced by their state of rest. 

 It is a question of a rise or fall in the level at which the regulation 

 of temperature occurs at different hours of the day and night. 



IV. We have pointed out several times that the constancy of 

 the average temperature of homoiothermic animals, mammals and 

 birds, involves the possession by these animals of special means 

 to regulate their temperature in such a way that the amount of 

 heat produced equals that lost in the same time. We must now 

 go more fully into the question of these means of regulation. 



