VI. 



Animal Temperature. 



'T*HE higher animals have within their bodies some source of heat 

 -■■ and some mechanism to regulate the production and loss of that 

 heat, for, equally in the height of summer as in the depth of winter, 

 their mean temperature is constant. This fact was known to the 

 Ancients, but imperfectly ; they had no thermometers, no exact 

 methods of determining temperature. They judged from their 

 sensations, but here sensations are imperfect guides. 



A patient attacked by an ague feels chilly, miserably cold ; he 

 huddles up by a fire, he shivers, and his teeth chatter ; he says that 

 he is cold, but at this precise moment he is in a fever-heat. How 

 does this contradiction arise ? The feeling of heat or cold arises in 

 the nervous structures of the skin. When these nerve-endings are 

 flushed by a rapid stream of blood, there is a sensation of warmth ;. 

 should the blood supply be small, not enough to compensate for the 

 loss of heat by radiation and conduction, there arises a sensation of 

 cold. During the rigor, for such is the name given to the early stage 

 of ague, the vessels of the skin are contracted, the skin is pale and 

 cold, its temperature twelve or sixteen degrees below the normal blood- 

 heat ; but, at the same time, the temperature of the internal parts has 

 risen six or seven degrees above the normal. 



Let the converse condition be studied in the same disease during 

 its third or siveating stage. The skin is now red and flushed, its 

 blood-vessels are dilated ; the patient is bathed in perspiration ; he 

 says that he is intensely hot, although his internal temperature is 

 rapidly falling to the normal. 



Animal heat the Ancients considered to be beyond the reach of 

 physical or chemical laws. They could assign no cause for it, and 

 therefore looked upon it as some innate quality, something essentially 

 " vital." This " vital " heat was supposed to be concentrated in the 

 heart, and to be distributed to the body by the blood in the veins ; it 

 was prevented from accumulating by respiration, the chief function of 

 which was to cool or temper the blood. 



After the year 1595 it was first possible to determine the tempera- 

 ture of the body more exactly, and to make observations upon the 

 independence of the temperature on external conditions. It was 

 about this time that Galileo invented the thermometer. As observa- 



