80 PHYSIOLOGY CHAP. 



The dilatation of the vessels which takes place in these cases 

 is greater than that which results from simple decrease in the 

 tone of the vaso- constrictor nerves caused by reflex action. It is 

 therefore probable that the excessive external heat, acting on the 

 periphery and on the nerve-centres, stimulates the vaso -dilator 

 system both reflexly and directly. It is certain that a rise in the 

 internal temperature of the body is enough to cause an active 

 dilatation of the cutaneous vessels. There is a marked increase 

 in the production of heat after taking hot food or drinks, during 

 the process of digestion, and during hard muscular work. This 

 increase is proved by the larger quantity of carbon dioxide dis- 

 charged and of oxygen absorbed by the lungs. This increased 

 production of heat, however, only causes a very slight rise in the 

 internal temperature, because there is at the same time an increase 

 in the loss of heat owing to the flushed condition of the skin, pro- 

 duced by the action of the vaso-dilator system. The efficacy of 

 this heat-regulating mechanism is clearly proved by the fact that 

 the taking of food and its digestion have little or no influence on 

 the regular course of the daily curve of temperature. 



Whenever the causes of the heating of the body become more 

 intense, as for instance when the* external temperature rises above 

 the internal temperature of the body, we notice, in addition to the 

 diffuse reddening of the skin, a more or less great secretion of sweat, 

 the evaporation of which is fche only efficient means of cooling the 

 skin. In such cases the dilatation of the cutaneous blood-vessels 

 not only fails to cool the body, but actually tends to heat it. The 

 evaporation of sweat under such circumstances causes a rapid loss 

 of a large quantity of heat. One grm. of water at 38, the 

 average temperature of the blood, absorbs during vaporisation 

 about 580 small calories, i.e. the amount of heat required to raise 

 the temperature of 580 grms. of water 1. The heat-regulating 

 efficacy of the evaporation of sweat is clearly proved by the fact 

 that a man can remain without distress for some time in the so- 

 called laconium of the new Eoman baths, where the air is dry and 

 heated to 60. He could not bear such a temperature even for a 

 minute if the air were damp instead of dry, because the moisture 

 would prevent the rapid evaporation of water from the skin and 

 the pulmonary passages, which is essential if the temperature is to 

 be prevented from rising to a point incompatible with life. For 

 the heat-regulating function of the skin, especially in relation to 

 the protection of the organism against heat, and for the dependence 

 of the secretion of sweat on the nervous system, the reader should 

 consult Vol. II. Chap. IX. pp. 480-499. In man and in all animals 

 in which the capacity to sweat is well developed, such as monkeys, 

 horses, and other mammals, cutaneous perspiration, both insensible 

 and sensible, is the main protection against heat ; in animals 

 which do not sweat, such as the larger number of mammals and 



