76 PHYSIOLOGY CHAP. 



shaving, there will be, according to the temperature of the environ- 

 ment, a more or less marked drop in their internal temperature. 

 A man stripped of his clothing and in a state of rest cannot 

 maintain his normal temperature unless the temperature of the 

 environment be brought up to at least 27 (Senator). Eubner made 

 special calori metric researches in order to obtain direct confirma- 

 tion of the importance of the cutaneous covering for reducing 

 the loss of heat and keeping up the average temperature of the 

 body. He found, for instance, that a normal guinea-pig lost by 

 conduction and radiation 3*35 cal. per hour on an average ; the 

 same guinea-pig when shaved lost in the same time 447 cal., i.e. 

 about 30 per cent more. He found the same held good for man : 

 a bare arm at the ordinary temperature of the room lost about 

 30 per cent more heat than the arm covered with clothing. 



When the temperature of the environment in which we live 

 oscillates between 15 and 20, as is the case in the temperate 

 seasons and in the house, warm-blooded animals with their 

 natural covering and man protected by suitable garments can 

 maintain their average normal temperature at the same level ; 

 they do not need to make use of any special apparatus for 

 regulating the heat, because the natural and artificial coverings 

 moderate continually the loss of heat, and thus preserve an almost 

 perfect equilibrium in the thermic balance. But when the outer 

 temperature goes down or rises beyond those limits, as happens in 

 winter and summer, the means for regulation are set in motion in 

 order to maintain the normal temperature, the loss of heat is 

 diminished or increased, and the production is varied in the 

 reverse direction according to the need. 



One of the chief means of regulating the loss of heat is the 

 vasomotor nervous system ; it is able to vary either directly or 

 indirectly the supply of blood to the cutaneous vessels. The 

 temperature of the skin depends on the quantity of blood supplied 

 to it; it increases when the peripheral vessels dilate, and falls 

 when they contract. In the former case the loss of heat at the 

 surface of the body is increased, in the latter decreased. 



Cold air stimulates the sensory organs of the skin, which 

 transmit the stimulus to the vaso-constrictor centres and by reflex 

 action cause the contraction of the cutaneous vessels, and thus the 

 loss of heat by conduction and by radiation is diminished. Hot 

 air, on the contrary, reflexly reduces the tonic action of the 

 vaso-constrictor nerves of the skin, and causes a dilatation of the 

 cutaneous vessels, so that the flow of blood in the periphery is 

 increased ; the increased conduction and radiation of heat to the 

 environment cool the blood. We must also take into account 

 another fact which makes the mechanism of regulation by means 

 of the vasomotor system more perfect. When the peripheral 

 vessels contract, a larger quantity of blood circulates in the deeper 



