ANIMAL HEAT. 393 



through stimulation of the nerve-centers. When all these conditions, 

 increased muscular activity, increased amount of food of high poten- 

 tial energy, and a low external temperature coexist, heat-production 

 attains its maximum, amounting to as much as 4726 calories daily 

 (Hultgren). 



Heat-dissipation varies in rapidity in accordance with variations 

 of a number of factors, but principally with variations in the external 

 temperature and the activity of the perspiratory apparatus. The 

 heat is dissipated mainly by the skin, 69 per cent., in consequence of 

 radiation and conduction and by the evaporation of the sweat. The 

 loss by this channel as well as from the lungs is dependent for the 

 most part on a difference of temperature of the surrounding air and 

 of the body. If the surrounding temperature is high, there is an 

 increase in the activity of both the circulatory and respiratory mechan- 

 isms, brought about by the central nervous system. In addition to 

 an increased action of the heart, the blood-vessels of the skin dilate 

 and deliver to the surface a larger volume of blood in a given time, 

 thus increasing the Conditions favorable to radiation. The sweat- 

 glands at the same time are stimulated to increased activity, and 

 in consequence of the additional volumes of blood brought to the skin 

 a larger amount of sweat is secreted, which speedily undergoes evap- 

 oration. As each gram of water for its evaporation requires 0.582 of 

 a calorie, it is evident that increased secretion of sweat favors heat- 

 dissipation. The nerve- centers influencing the activity of the sweat- 

 glands may be stimulated not only reflexly, but directly by an excess 

 of heat in the blood. If, however, the atmosphere itself possesses a 

 high percentage of moisture, evaporation from the body is much 

 diminished and the value of sweating as a means of lowering the 

 body-temperature is much impaired. Evaporation is hastened by 

 air in motion. Hastened respiratory movements and the dilatation 

 of blood-vessels of the respiratory surface also increase the evaporation 

 of water from the lungs and thus occasion a greater loss of heat. 



If the external temperature falls there is a decrease in the physio- 

 logic activity of the skin from a rnntrartfo^ frf *^ Kl^-yg Cg gl C| a 

 diminutlon^oTl^tiBo^^i^y, and a cessation in the secretion of 

 sweat, i'he blood, being prevented from coming to the surface, is 

 retained in the deeper portion of the body, and in consequence the 

 conditions for radiation are diminished. These variations in the 

 cutaneous circulation in response to variations in the external tem- 

 perature are brought about by the vasomotor nerve mechanism ; and 

 as they take place with extreme promptness heat-dissipation and heat- 

 production are quickly adjusted and the mean temperature main- 

 tained. 



Radiation from the skin is modified to some extent by clothing. 

 An excess of clothing diminishes, a diminution of clothing increases 



