454 HANDBOOK OF PHYSIOLOGY. 



to a much less degree (3) from the air-passages; in each act of respira- 

 tion, 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 Wood. We must remember too that (4) all food 

 and drink which enter the body at a lower temperature than itself ab- 

 stract a small measure of heat; (5) 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.) From the Surface of the Body.Ry far the most important loss 

 of heat from the body, probably 90 per cent and upward of the whole 

 amount, is that which takes place by radiation, conduction, and evapora- 

 tion from the skin. The actual figures are as follows: of 100 calories 

 of heat produced, 2.G are lost in heating food and drink; 2.6 in heating 

 air inspired; 14.7 in evaporation; and 80.1 by radiation and conduction. 

 The means by which the skin is able to act as one of the most impor- 

 tant organs for regulating the temperature 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 versa. For the circum- 

 stance 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 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 sweat- 

 ing; 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 similar effect may be produced through the vaso-motor cen- 

 tre in the medulla and spinal cord. 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 evapora- 

 tion; 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 mentioned. 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 



