ANIMAL HEAT. 603 



become poorer in fluid, so that the conduction of heat from the warmer inner 

 parts is lessened. The conductivity of the skin is further decreased by the 

 action of the pilo-motor muscles, which when in contraction or in a state of 

 greater tonicity render the skin tenser and thus press out the blood and tissue 

 juices. The secretion of sweat is diminished, so that the quantity of heat lost 

 in the vaporization of water is decreased. On the other hand, heat-dissipation 

 tends to be materially increased by the greater radiation of heat due to the 

 greater difference between the temperature of the body and of the douche, bath, 

 etc., and the tendency to an increase in this way is much greater than the 

 opposite tendency depending upon the factors above noted, therefore heat- 

 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 that 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 C. or more, and is followed by a re- 

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

 continue subnormal for some hours ; 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. 540) and 

 presumably similarly increases heat-production. 



Moderate external heat causes dilatation of the cutaneous vessels, excites 

 the general 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 that conditions are caused which are favorable to an 

 increased loss of heat by radiation. Increased activity of the respiratory 

 movements means 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 larger amount of water evaporated 

 from the skin. When, however, the external temperature is higher than that 

 of the body, loss of heat by radiation and conduction cannot occur, so that 

 heat not only accumulates as a result of the interference with heat-dissipation, 

 but by absorption. 



The chief reactions brought about by moderate external heat are a dilata- 

 tion of the cutaneous blood-vessels, excitement of the general circulation, an in- 

 crease in the number of respiratory movements, increase in the amount of sweat, 

 diminished tonicity of the muscles, and diminished thermogenesis which is prob- 

 ably due to a lessening of the activity of the chemical changes in the muscles. 



