454 TEXT-BOOK OF PHYSIOLOGY 



as it does during the summer months, the vaso-constrictor centers will be 

 inhibited, as a result of the action of heat on the cutaneous nerve endings, 

 the cutaneous vessels will dilate and bring to the surface a larger volume of 

 blood, a condition favorable to increased radiation. This, however, is to 

 some extent prevented by reason of a diminution of the difference in 

 temperature between that of the atmosphere and that of the body. 



The sweat-glands at the same time are reflexly 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 

 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 a rise in the temperature 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 move- 

 ments 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. 



When the external temperature falls as, it does in the autumn and winter 

 months, 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 due to increased activity of the general 

 vaso-motor center. The blood, being prevented from coming to the surface, 

 is retained in the deeper portion of the body, and in consequence the con- 

 ditions for radiation are diminished. Nevertheless by reason of the in- 

 crease in the difference of temperature between that of the air and that of the 

 body, radiation takes place to a greater extent than during the summer 

 months. These variations in the cutaneous circulation in response to varia- 

 tions 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 tem- 

 perature maintained. 



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

 excess of clothing diminishes, a diminution of clothing increases radiation. 

 The quality of clothing is also an important factor. Wool is a poor 

 conductor of heat but a good absorber and retainer of moisture, and hence 

 is adapted for cold weather. Linen and cotton possess the opposite quali- 

 ties, and hence are adapted for warm weather. Radiation from the skin is 

 somewhat interfered with by subcutaneous fat, the extent of the interference 

 being dependent on its amount. 



Inasmuch as the mean temperature of the body remains practically 

 constant, notwithstanding seasonal variations, it is apparent that heat-dis- 

 sipation must be exactly balanced by heat-production. Should there 

 be any want of correspondence between the two processes, there would 

 arise either an increase or a decrease in the mean temperature. As both 

 heat-production and heat-dissipation are variable factors, dependent on a 

 variety of internal and external conditions, their adjustment is accomplished 



