CONTROL OF BODY TEMPERATURE AND FEVER 745 



it has recently been shown that the blood becomes diluted with tissue 

 fluid when more surface cooling has to occur. 



Although afferent impulses from the skin are therefore of great im- 

 portance in adjusting the cutaneous blood supply according to the 

 amount of surface cooling that has to occur, a further effect is also pro- 

 duced on them by the action on the nerve centers of temperature dif- 

 ferences in the blood itself. Thus, when the temperature of blood going 

 to the brain is raised by placing the carotid arteries on some heating de- 

 vice or when the region of the corpora striata is directly warmed, the 

 skin vessels become dilated as if the animal had been exposed to general 

 warmth, and the rectal temperature tends to fall (Barbour 56 ). 



When the loss of heat by radiation and conduction is no longer ade- 

 quate to prevent a rise in body temperature, or when the processes can 

 not operate on account of a high temperature in the environment, the 

 loss of heat from the skin is mainly dependent upon the evaporation of 

 sweat. Under ordinary conditions this evaporation takes place at such 

 a rate that there is no visible perspiration on the surface of the body 

 the so-called insensible perspiration. When the heat loss by this channel 

 must become greater, the perspiration is produced in larger amount, so 

 that it collects on the surface of the body ; and, provided the conditions of 

 the environment are such that evaporation can readily take place (low 

 relative humidity), the amount of cooling of the body that can be effected 

 becomes very great. A man may exist without any marked rise in body 

 temperature in a very hot environment even when he is exposed to an out- 

 side temperature that is the same as that of his body, or even greater. To 

 encourage evaporation, however, he should be naked or very lightly clad, 

 and the air should be kept in constant motion so that the layers of air 

 next to the skin, which ordinarily very quickly become saturated with 

 vapor, are transferred and replaced by dryer air. Movement of the air 

 also increases the heat loss by convection, provided the temperature of 

 the air is not too near that of the body. 



The importance of the movement of air in the regulation of heat loss 

 has been clearly demonstrated by Leonard Hill, 54 F. S. Lee, and others, 

 and will be fully discussed in the chapter on ventilation (page 754). 



The stimulus to increased sweating has been thought to be dependent 

 mainly on changes in the temperature of the blood since sweating does 

 not immediately set in when the body is subjected to heat, as by a warm 

 bath or a hot pack. It usually takes from ten to twenty minutes after 

 the person has been placed in the bath or surrounded by the warm blan- 

 kets of the pack before sweating becomes pronounced. It has been 

 shown, however, that the body temperature does not require to be raised 

 before sweating sets in. In a hot bath sweating starts on the forehead 



