496 A TEXTBOOK OF PHYSIOLOGY 



The nervous sj'stem regulates the body temperature in two ways : 

 (1) By the control of the sites of production the muscles and the 

 large glands ; (2) by the control of the structures concerned in heat 

 loss the cutaneous bloodvessels, the sweat glands, etc. That heat 

 is developed in the muscles has been demonstrated (see p. 552). It 

 has not been shown in other organs, but the respiratory exchange 

 in the glands proves that heat is developed therein. The circulation 

 through a gland is so rapid that the heat formed therein is at once 

 distributed through the body; thus even the most delicate thermo- 

 meters fail to show that the gland is hotter than the blood. 



The Regulation of Heat Production is controlled reflexly through 

 the sensitivity of the skin to changes of temperature. The sensation 

 of cold increases the tone and activity of the muscles; shivering in- 

 creases muscular activity without displacing the layer of air which 

 is in contact with, and warmed by, the body. It may increase the 

 heat production of a man at rest from 50 to 90 per cent. The cooling 

 of one leg in a bath of cold water may provoke local shivering in 

 that leg. Exposure to cold leads us voluntarily to increase our 

 muscular movements. We move about, stamp our feet, and beat our 

 arms, in order to keep warm. 



A certain degree of exposure to cold is therefore valuable. It 

 stimulates the tone, metabolism, and activity, of the body, and in 

 this lies the healthiness of open-air life. The activity of the body 

 provoked by cold leads to an ampler ventilation of the lungs and a 

 more vigorous circulation of the blood. B} 7 raising the metabolism, 

 it increases the appetite and better digestion of food, thus lessening 

 the bacterial decomposition of food in the large bowel, and preventing 

 the absorption of toxic products therefrom. The cooling effect of the 

 wind is far more powerful than that of the surrounding temperature, 

 and is the most important quality of open fresh air. It not only 

 cools, but by its varying stimulation of the skin prevents monotony of 

 sensation and invigorates. Ideal outdoor conditions are the radiant 

 heat of the sun warming the ground and thereby the feet and those 

 parts of the body turned towards it, together with a cooling wind 

 blowing on the face. 



On the other hand, the sensation of heat produces relaxation of 

 the muscles, a lessened tone and activity, and diminished metabolism. 

 Too warm an atmosphere in particular, one that is windless and 

 monotonous is therefore disadvantageous, on account of its relaxing 

 effect. 



The Regulation of Heat Loss is accomplished by the nervous control 

 of the loss of heat from the skin by radiation, convection, and evapora- 

 tion. This regulation is brought about through the vase-motor 

 centre and the centre controlling sweating. The afferent channels 

 concerned in the regulation of the cutane n;s bood-supply are chiefly 

 those from the skin. The sensation of cold causes a constriction of 

 the cutaneous bloodvessels, and thus diminishes the loss of heat. 

 On the other hand, a sensation of warmth induces a flushing of the 



