REGULATION OF THE TEMPERATURE. 195 



indirect means for accomplishing the same end are, variations 

 in the amount and quality of the food and drink taken, varia- 

 tions in clothing, and in exposure to external heat or cold. 



In order to understand the means by which the heat of the 

 body is regulated, it is necessary to take into consideration the 

 following facts : First, the immediate source of heat in the 

 body is the presence of a large quantity of a warm fluid the 

 blood, the temperature of which is, in health, about 100 F. 

 In the second place, the blood, while constantly moving in a 

 multitude of different streams, is, every minute or so, gathered 

 up in the heart into one large stream, before being again dis- 

 persed to all parts of the body. In this way, the temperature 

 of the blood remains almost exactly the same in all parts ; for 

 while a portion of it in passing through one organ, as the skin, 

 may become cooler, and through another organ, as the liver, 

 may become warmer, the effect on each separate stream is more 

 or less neutralized when it mingles with another, and an aver- 

 age is struck, so to speak, for all the streams when they form 

 one, in passing through the heart. 



The means, by which the skin is able to act as one of the 

 most important organs for regulating the temperature of the 

 blood, are (1) that it offers a large surface for radiation, con- 

 duction, and evaporation ; (2) that it contains a large amount 

 of blood ; (3) that the quantity of blood contained in it is the 

 greater under those circumstances which demand a loss of heat 

 from the body, and vice versa. For the circumstance which 

 directly determines the quantity of blood in the skin, is that 

 which governs the supply of blood to all the tissues and organs 

 of the body, namely, the power of the vaso-motor nerves to 

 cause a greater or less tension of the muscular element in the 

 walls of the arteries (see p. 121), and, in correspondence with 

 this, a lessening or increase of the calibre of the vessel accom- 

 panied 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 

 bloodvessels ; and, as a result, the skin becomes full-blooded, 

 hot, and sweating ; and much heat is lost. With a low tem- 

 perature, on the other hand, the bloodvessels shrink, and in 

 accordance with the consequently 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 regu- 

 lated. 



In connection with loss of heat by the skin, reference has 

 been made to that which occurs both by radiation and conduc- 

 tion, and by evaporation ; and the subject of animal heat has 



