496 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



dissipation is increased. Bathing the skin with cold water increases heat-loss 

 by the vaporization of water as well as by conduction. 



The excitation of the cutaneous nerves by cold reflexly increases thermo- 

 genesis, and to such an extent thai heat-production may even exceed the 

 quantity dissipated, thus causing an increase of bodily temperature. This rise, 

 which is transient, may amount to 0.2' Cor more, and is followed by a re- 

 action in which the temperature may tall 0.2° C. or more below the normal, and 

 continue subnormal for some hum-; this fall in turn is succeeded by a supple- 

 mentary reaction in which the temperature may rise slightly above the normal. 



The chief reactions brought about by moderate external cold are constriction 

 of the cutaneous blood-vessels, a diminution of the quantity of sweat secreted, 

 increased tonicity of the pilo-motor muscles, and increased tonicity of the 

 skeletal muscles. The action upon the latter muscles may be so marked as 

 to cause shivering, which increases respiratory activity (see p. 432) and 

 presumably similarly increases heat-production. 



Moderate external heat causes dilatation of the cutaneous vessels, excites 

 the genera] circulation and thus increases the blood-supply to the skin, excites 

 respiratory movements and the sweat-glands, but decreases thermogenesis. 

 Owing to the dilatation of the blood-vessels of the skin and the excitation of 

 the circulation the temperature and the quantity of the blood supplied to the 

 skin are increased, so thai condition- are caused which are favorable to an 

 increased loss of heat by radiation, [ncreased activity of the respiratory 

 movements mean- a larger volume of air respired, and consequently a greater 

 loss of heat in warming the air and in the evaporation of the larger quantity 

 of water from the lungs. The increase in the quantity of sweat formed also 

 favors heat-dissipation by means of the lamer amount of water evaporated 

 from the skin. External heat also causes diminished tonicity of the muscles, 

 and consequent diminished thermogenesis which is probably due to a lessening 

 of the activity of the chemical changes in the muscles. 



When external temperature is excessive and continued, heat-regulation is 

 rendered impossible : if extreme cold, heat-dissipation takes place more rapidly 

 than heat-production, so that bodily temperature falls until death results; if 

 extreme heat, heat-dissipation is so interfered with that heat accumulates 

 within the organism, causing a continuous rise of temperature which finally 

 causes death. 



Abnormal Thermoiaxis. — By this term is meant the regulation of the heat- 

 processes under conditions in which the mean bodily temperature is maintained 

 :it ;i standard above or below the normal, a- in fever and in animals from 

 which the hair has been shaved. It is assumed that under normal conditions 

 the beat-centres are "set," as it were, for a given temperature of the blood, 

 and that when the temperature of the blood goes above or below this standard 

 a compensatory reaction occur.-, so that thermogenesis and thermolysis are 

 properly affected to bring about an adjustment. In fever it may be considered 

 that the centres are set for a higher temperature than the normal ; the higher 

 the fever, the higher the adjustment. The centre- may beset for subnormal 



