434 MANUAL OF PHYSIOLOGY. 



tional activity, we do not know, but many facts seem to point to 

 such a possibility. 



The effect of nerve influence on the production of heat is 

 greatly complicated, also, by the enormous power exercised by the 

 nerves over the blood supply through the means of the vaso- 

 motor mechanisms, for the temperature of any given part is so 

 intimately related to the amount of blood flowing through it that 

 the former has been commonly accepted as an adequate measure 

 of the latter. 



For the present, therefore, we are not in a position to speak 

 with decision of nerves with a purely thermic action. 



The expenditure of the heat may be classed under the follow- 

 ing headings : 



1. In warming ingesta : As a rule, all the food and drink we 

 make use of, as well as the oxygen we breathe, are colder than 

 the body, and before they pass out they are raised to the body 

 temperature. 



2. Radiation and Conduction : From the surface of the body 

 a quantity of heat is being expended in warming the surround- 

 ing medium, which is habitually colder than our bodies. The 

 colder the medium the greater its capacity for heat, and the more 

 quickly it comes in contact with new portions of the surface the 

 more warmth it robs us of. Water or damp air takes up much 

 more heat from our surface than dry air of the same temperature, 

 and the quantity of heat lost is still further increased if the 

 medium be in motion, so that the relatively colder fluid is con- 

 stantly renewed. 



3. Evaporation : (a) From the air passages : a quantity of 

 water passes into the vaporous state and saturates the tidal air, 

 and this change of condition, from liquid to that of vapor, ab- 

 sorbs much heat, (b) From the skin : surface evaporation is 

 always going on, even when no moisture is perceptible on the 

 skin, and much fluid of which we are not sensible is lost in this 

 way. The quantity of heat lost by evaporation from the skin will 

 depend on the temperature and the degree of moisture of the air 

 in proportion to that of the surface of the body. 



