ANIMAL HEAT. 313 



their quota, and this in direct proportion to their activity. The blood 

 itself is also the seat of metabolism, and, therefore, of the production of 

 heat; but the share which it takes in this respect, apart from the tissues 

 in which it circulates, is very inconsiderable. 



Regulation of the Temperature of the Human Body. The 

 average temperature of the body is maintained under different conditions 

 of external circumstances by mechanisms which permit of (1) variation 

 in the amount of heat got rid of, and (2) variations in the amount of heat 

 produced or introduced into the body. In healthy warm-blooded animals 

 the loss and gain of heat are so nearly balanced one by the other that, 

 under all ordinary circumstances, a uniform temperature, within two or 

 three degrees, is preserved. 



I. Methods of Variation in the amount of Heat got rid of. 

 The loss of heat from the human body is principally regulated by the 

 amount lost by radiation and conduction from its surface, and by means 

 of the constant evaporation of water from the same part, and (2) to a 

 much less degree from the air -passages; in each act of respiration, heat 

 is lost to a greater or less extent according to the temperature of the 

 atmosphere; unless indeed the temperature of the surrounding air exceed 

 that of the blood. We must remember too that all food and drink which 

 enter the body at a lower temperature than itself abstract a small measure 

 of heat: while the urine and faeces which leave the body at about its own 

 temperature are also means by which a small amount is lost. 



(a.) Loss of Heat from the /Surface of the Body : the Skin. By far 

 the most important loss of heat from the body, probably 70 or 80 per V / 

 cent, of the whole amount, is that which takes place by radiation, c$n- )( 

 duction, and evaporation from the skin. The means by which the skin 

 is able to act as one of the most important organs for regulating the tem- 

 perature of the blood, are (1), that it offers a large surface for radiation, 

 conduction, 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 

 /"w?. 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, and, in correspondence with this, a lessening or increase of 

 the calibre of the vessels, 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 consequently diminished 

 blood-supply, the skin becomes pale, and cold, and dry; and no doubt a 



