240 ANIMAL HEAT. 



a greater or less tension of the muscular element in the 

 walls of the arteries (see p. 141), and, in correspondence 

 with this, a lessening or increase of the calibre of the vessel 

 accompanied by a less or greater current of blood. A 

 warm or hot atmosphere so acts on the nerve fibres of the 

 skin, as to lead them to cause in turn a relaxation of the 

 muscular fibre of the blood-vessels ; and, as a result, the 

 skin becomes full-blooded, hot, and sweating ; and much 

 heat is lost. With a low temperature, on the other hand, 

 the blood-vessels shrink, and in accordance with the con- 

 sequently diminished blood-supply, the skin becomes pale, 

 and cold, and dry. Thus, by means of a self-regulating 

 apparatus, the skin becomes the most important of the 

 means by which the temperature of the body is regulated. 



In connection with loss of heat by the skin, reference 

 has been made to that which occurs both by radiation 

 and conduction, and by evaporation; and the subject of 

 animal heat has been considered almost solely with regard 

 to the ordinary case of man living in a medium colder than 

 his body, and therefore losing heat in all the ways men- 

 tioned. The importance of the means, however, adopted, 

 so to speak, by the skin for regulating the temperature of 

 the body, will depend on the conditions by which it is sur- 

 rounded; an inverse proportion existing in most cases 

 between the loss by radiation and conduction on the one 

 hand, and by evaporation on the other. Indeed, the small 

 loss of heat by evaporation in cold climates may go far to 

 compensate for the greater loss by radiation ; as, on the 

 other hand, the great amount of fluid evaporated in hot 

 air may remove nearly as much heat as is commonly lost 

 by both radiation and evaporation in ordinary tempera- 

 tures ; and thus, it is possible, that the quantities of heat 

 required for the maintenance of an uniform proper tem- 

 perature in various climates and seasons are not so different 

 as they, at first thought, seem. 



Many examples might be given of the power which the 



