596 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The surface of the body as a radiating surface cannot be regarded in the 

 same light as an indifferent, inanimate surface, such as metal or wood. The 

 coefficient of radiation (the quantity of heat emitted during a unit of time at a 

 standard temperature from a given area) in an inanimate body remains fixed, 

 because the surface itself is virtually unchangeable ; but the coefficient for the 

 living organism is subject to material alterations. These alterations depend 

 chiefly (1) upon the actions of the pilo-motor mechanism whereby the relation 

 of the natural covering (hair or feathers in the lower animals) of the body to 

 the skin is effected ; (2) upon changes in the conductivity of the skin owing to 

 variations of the blood-supply ; (3) upon the varying thickness of the skin in 

 different species, in different individuals, and in different parts of the body ; 

 (4) upon the temperature of the surroundings ; (5) upon the extent of the 

 body-surface exposed; (6) upon the character of the clothing. When the 

 arrector pili muscles contract the skin is made tense and the cutaneous blood- 

 vessels are pressed upon and rendered anaemic, thus lessening the quantity of 

 fluid in the skin and as a consequence lowering the coefficient of dissipation ; 

 moreover, in animals whose natural covering is fur or feathers, these fibres 

 cause an erection of one or the other, as the case may be, and in this way 

 affect the radiating coefficient. The coefficient is enormously increased by 

 removing the natural covering, such as the fur of the rabbit, under which cir- 

 cumstances, even though the animal be subjected to a relatively high external 

 temperature, heat-dissipation is so enormously increased that death ensues within 

 two or three days. When one side of the body of a horse was shaved and the 

 animal subjected to an atmosphere having a temperature of C., the tem- 

 perature of the skin of the shaven side fell 8 in forty minutes, while the 

 temperature of the unshaven side fell only 0.5. 



The coefficient is diminished where there is excessive sebaceous secretion, 

 and where grease is artificially applied, and by an accumulation of subcutaneous 

 fat ; it is increased by wetting the skin, as by sweat or bathing ; and it is 

 affected by many other circumstances. 



Through the operations of the nervous system heat-dissipation may be 

 affected directly or indirectly by action upon the heat-dissipating and heat- 

 producing processes circulation, respiration, sudorific and sebaceous glands, 

 and arrector pili muscles. 



There are many drugs which directly or indirectly affect heat-dissipation. 

 Drugs which cause dilatation of the cutaneous vessels tend to increase heat- 

 dissipation ; conversely, those which cause contraction of the blood-vessels 

 hinder dissipation. Diaphoretics increase . heat-loss essentially by increasing 

 the amount of sweat. Respiratory excitants increase the loss of heat by means 

 of the increased volume of air respired. Drugs which increase heat-production 

 tend to indirectly increase heat-dissipation. 



All pathological states which affect heat-production tend to similarly disturb 

 heat-dissipation. Conditions of malnutrition favor heat-dissipation by causing 

 a loss of subcutaneous fat, but this is to a greater or less extent compensated 

 or by the enfeebletnent of the circulation, respiration, and metabolic processes 



