6go ANIMAL HEAT 



even with that of resting muscles (Hill). Nor is it possible to demon- 

 strate any marked or constant increase when the cerebral cortex is 

 roused to such an active discharge of impulses as leads to general 

 epileptiform convulsions. The rise of temperature of certain regions, 

 especially the occipital portion, of the scalp, which some observers have 

 stated to take place during mental activity, cannot be due to con- 

 duction of heat from the brain through the skull. It might be caused 

 by vaso-motor changes in the scalp, associated with corresponding 

 changes in related areas of the cortex. The alleged increase in the 

 temperature of the brain during intense psychical activity, sometimes 

 to 0-2 C., or 0*3 C. above the rectal temperature (Mosso), may also, 

 if genuine, be due to vascular changes. And if we remember how 

 large a proportion of the central nervous system is made up of nerve- 

 fibres, in which no sensible production of heat has ever been demon- 

 strated, it will not appear surprising if even a considerable increase 

 in the metabolism of the really active elements should fail to make 

 itself felt. 



SECTION III. REGULATION OF TEMPERATURE OR THERMOTAXIS- 



What, now, is the mechanism by which the balance is maintained 

 in the homoiothermal animal between heat-production and heat- 

 loss ? In answering this question we have to recognize that both 

 of these quantities are variable, that a fall in the production of heat 

 may be compensated by a diminution of heat-loss, and an increase 

 in the loss of heat balanced by a greater heat-production. 



The loss of heat from the surfaces of the body may be regulated 

 both by involuntary and by voluntary means. It is greatly affected 

 by the state of the cutaneous vessels, and these vessels are under the 

 influence of nerves. A cold skin is pale, and its vessels are con- 

 tracted. In a warm atmosphere the skin is flushed with blood, its 

 vessels are dilated, its temperature is increased; an effort, so to 

 speak, is being made by the organism to maintain the difference of 

 temperature between its surface and its surroundings on which the 

 rate of heat-loss by radiation and conduction depends. A still 

 more important factor in man, and in animals like the horse, which 

 sweat over their whole surface, is the increase and decrease in the 

 quantity of water evaporated and of heat rendered latent. It is 

 owing to the wonderful elasticity of the sweat -secreting mechanism, 

 and to the increase of respiratory activity and the consequent 

 increase in the amount of watery vapour given off by the lungs, 

 that men are able to endure for days an atmosphere hotter than the 

 blood, and even for a short time a temperature above that of boiling 

 water. The temperature of a Turkish bath may be as high as 

 65 to 80 C. Blagden and Fordyce exposed themselves for a few 

 minutes to a temperature of nearly 127 C. Although meat was 

 being cooked in the same chamber by the heat of the air, they 

 experienced no ill-effects, nor was their body-temperature even 

 increased. But a far lower temperature than this, if long con- 



