436 TEXT-BOOK OF PHYSIOLOGY. 



potential energy and quantity consumed be larger than the average daily 

 requirements, there will be an increase in heat-production. A lowering 

 of the external temperature, as in the winter season, leads to increased heat- 

 production through stimulation of the nerve-centers. When all these con- 

 ditions, viz : increased muscle activity, increased amount of food with high 

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

 attains its maximum, amounting to as much as4726 Calories daily (Hultgren). 

 In winter time, the lowering of the external temperature leads through 

 reflex stimulation of the spinal motor centers to a larger discharge of 

 nerve impulses to muscles leading to increased activity and increased heat- 

 production. 



Heat-Dissipation. 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, about 85 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 mechanisms, brought about by the central 

 nerve 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 by the central nerve system (the sweat centers in the 

 spinal cord) by the action of nerve impulses transmitted from the skin 

 developed by the action of the higher temperature on the nerve 

 endings; hence, in consequence of the additional volumes of blood brought to 

 the skin a larger amount of sweat is secreted, which speedily undergoes 

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

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

 tion. 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 evapora- 

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



If on the contrary, the external temperature falls there is a decrease in 

 the physiologic activity of the skin from a contraction of the blood-vessels, 

 a diminution of the blood-supply, and a cessation in the secretion of sweat. 

 The 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 reponse 

 to variations in the external temperature are brought about by the vaso- 

 motor nerve mechanism; and as they take place with extreme promptness 

 heat-dissipation and heat-production are quickly adjusted and the mean 

 temperature maintained. 



