320 HANDBOOK OF PHYSIOLOGY. 



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

 amoujfc 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, an uniform tempera- 

 ture, 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 at- 

 mosphere; 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 Surf ace of the Body: the Skin. By far 

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

 cent of the whole amount, is that which takes place by radiation, con- 

 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 

 temperature of the blood are (1), that it offers a large surface for radi- 

 ation, conduction, and evaporation ; (2), that it contains 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 tissue 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 lessen- 

 ing 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 mucular 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 accord- 

 ance 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 centre in the medulla and spinal cord. Thus, 

 by means of a self -regulating apparatus, the skin becomes the most im- 

 portant 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- 



