HEAT-INCOME. 431 



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 tem- 

 perature. 



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 me- 

 dium be in motion, so that the relatively colder fluid is constantly 

 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. 



As has been said, the exact income of heat is uncertain and 

 variable, because the data upon which the absolute amount can 



