408 ANIMAL HEAT AND ITS REGULATION 



Experience proves that a man can maintain his body temperature without 

 increase in an atmosphere whose temperature is much higher than that of his 

 body. This appears the more remarkable when we consider that the metabo- 

 lism and heat production of the body never cease, however high the surround- 

 ing temperature may be. The fact, as was first observed by Benjamin Frank- 

 lin, is to be explained by the secretion of sweat. At a higher atmospheric 

 temperature the sweat glands are stimulated, and evaporation of the sweat 

 thus poured out upon the skin absorbs a large quantity of heat from the body. 

 In this way the body is cooled and maintains its temperature unchanged, 

 whether the outside temperature exceeds or only approaches that of the body. 



But the amount of sweat secreted depends not only upon the temperature 

 of the air, but also upon the amount of heat being produced in the body at 

 the time. If the heat production of the body be considerably increased as the 

 result of severe muscular work, the body will sweat even at an atmospheric 

 temperature of C. After a full meal, owing to the increased heat produc- 

 tion a greater quantity of sweat is secreted than when the metabolism is 

 reduced for lack of food. 



B. CENTERS FOR HEAT REGULATION 



Among the many so-called " heat centers," located in different parts of 

 the central nervous system, which have been mentioned by different authors, 

 only a single one seem's to be fairly entitled to the name. If a fine needle be 

 thrust into the brain from above downward in such a direction as to strike 

 the medial edge of the corpus striatum, a rise in temperature appears in the 

 skin, in the muscles and in the rectum; likewise an increase of metabolism 

 and of heat loss as determined by the calorimeter (Aronsohn and Sachs, 

 Richet). The increase of temperature amounts to more than 2 C., the in- 

 crease of metabolism and of heat loss to about 20 per cent. The maximum 

 effect appears within twenty-four to seventy-three hours after the puncture, 

 unless the needle be pressed through to the base of the cranium, in which case 

 it appears within two to seven hours. The results of electrical stimulation by 

 means of electrodes insulated to the ends show that the effect of puncture 

 is due to stimulation and not to destruction of the parts encountered. 



We cannot form any definite opinion at present, as to the significance 

 which this and other " heat centers " have in the regulation of this important 

 function. 



How the centers for heat regulation (wherever they may be located) are stimu- 

 lated, is another question which cannot be conclusively answered as yet. It is 

 indeed fairly certain that the cold and heat nerves of the skin play a great part, 

 since heat production and heat loss are reflexly influenced in one direction or 

 the other according to conditions reported by these nerves. Changes in the 

 temperature of the blood also might play a part; that is, cold might by direct 

 action on the heat centers bring about an increase of metabolism and a con- 

 striction of the cutaneous vessels, or warmed blood might rouse the sweat centers 

 to increased activity. This mechanism does actually participate in some such 

 way in the regulation of heat, for in muscular work the sweat breaks out only 

 when the body temperature has increased 0.3-0.5 C. (Fredericq). Likewise 



