402 GENERAL NUTRITION 



that muscular exercise usually is attended with increased action of the 

 skin, which keeps the heat of the body within restricted limits. In vio- 

 lent muscular exertion, as in fast running, the increased production 

 of heat may be so rapid that it can not be entirely compensated by 

 evaporation from the skin, and the temperature may rise to 104 Fahr. 

 (40 C). In about an hour and a half of repose the temperature falls 

 to the normal standard. 



The elevation in temperature that attends muscular work is produced 

 directly in the substance of the muscle (Becquerel and Breschet). In- 

 troducing a thermo-electric needle into the biceps of a man who used 

 the arm in sawing wood for five minutes, these physiologists noted an 

 elevation of temperature of nearly 2 Fahr. (i C.). 



Observations on the influence of mental exertion on the temperature 

 of the body have not been so many, but they are, apparently, no less 

 exact in their results. Davy observed a slight but constant elevation 

 during " excited and sustained attention." Lombard noted an elevation 

 of temperature in the head during mental exertion of various kinds, but 

 it was slight, the highest rise not exceeding 0.05 Fahr. (0.027 C.). 

 According to Burdach, the temperature of the body is increased by the 

 emotions of hope, joy, anger, and all exciting passions, while it is dimin- 

 ished by fear, fright and mental distress. 



It is evident that if animal heat be one of the necessary attendant 

 phenomena of nutrition, it must be greatly influenced by conditions of 

 the circulation. It has been a question, indeed, whether the modifica- 

 tions in temperature produced by operating on the vasomotor nerves 

 are not due entirely to changes in the supply 'of blood. It is certain 

 that whatever determines an increased supply of blood to any part raises 

 the temperature ; and whenever the quantity of blood in any organ or 

 part is considerably diminished, the temperature is reduced. This fact 

 is illustrated in operations for the deligation of large arteries. It is 

 well known that after tying a large vessel, the utmost care is necessary 

 to keep up the temperature of the part to which its branches are dis- 

 tributed, until the anastomosing vessels become enlarged sufficiently to 

 supply the quantity of blood necessary for normal nutrition. 



Influence of the Nervous System on the Production of Animal Heat 

 (Heat-Centres}. The local influences of the vasomotor nerves on calori- 

 fication operate mainly if not entirely through changes in the nutrition 

 of parts, produced by variations in blood-supply. These influences will 

 be fully considered in connection with the physiology of the nervous 

 system. 



The temperature of the body may be modified through the nervous 

 system by reflex action, and this implies the existence of nerve-centres, 



