434 MANUAL OF PHYSIOLOGY. 



are placed in colder surroundings such as a cold bath ; still, as 

 will presently appear, it is probable that the variations in heat- 

 income have but little regulating influence on the body tempera- 

 ture, and if they have any we are certainly ignorant of the man- 

 ner in which such influence is carried out. 



On the other hand, we know that the amount of heat-expenditure 

 may be varied by mechanisms which are almost self-regulating. 

 It has already been stated that the great majority of the heat is 

 lost by the parts in contact with the air, namely, the skin and air 

 passages. In these places the warm blood is exposed to the cool 

 air, and therefore loses much of its heat by radiation, conduction, 

 and evaporation. It is obvious that the greater the quantity of 

 blood thus exposed for cooling, the greater will be the amount of 

 heat lost in a given time by the body as a whole. 



If we review the circumstances which tend to interfere with the 

 uniformity of the temperature of the body, we shall see that each 

 one is accompanied by certain physiological actions which tend 

 to compensate for the disturbing influences. 



The chief common events tending to make our temperature ex- 

 ceed or fall short of its normal standard may be enumerated as 

 follows, and the explanation of their modes of compensation will 

 at the same time be given : 



COMPENSATION FOR INTERNAL VARIATIONS. 



A casual increase in the heat income may be induced by any 

 increased chemical activity in the tissues, notably the action of 

 the muscles and large glands. The moment this increased heat 

 is communicated to the blood, the warm blood brings about the 

 following results (partly through the stimulation of certain nerve 

 centres): (a.) An acceleration of respiratory movement, which 

 increases the amount of cold air to be warmed and saturated with 

 moisture by the air passages, and thus facilitates the escape of the 

 surplus caloric. (6.) Relaxation of the cutaneous arterioles, so 

 that a greater quantity of blood is exposed to the cooling influence 

 of the air. (c.) Greater rapidity of the heart-beat, by which a 

 greater quantity of blood is supplied to the air passages and to 



