vi FORM AND FUNCTION 101 



place when heat passes out into the surrounding 

 atmosphere, not into the particular molecules in 

 contact with the body, just as the earth radiates out 

 its heat on a clear starlight night. In man the skin 

 does most of this work, a much less but still a con- 

 siderable amount being done by the lungs. Estimates 

 by different authorities vary considerably, some credit- 

 ing the lungs with 20 per cent., others with as little as 9. 1 

 In birds, the skin undertakes comparatively little of 

 the work, the lungs and air-sacks far the greater share. 

 Conduction is much checked by the feathers, though 

 the bare tracts, called apteria, make the coating much 

 less impervious than might be supposed. It must also 

 be borne in mind that owing to the high temperature 

 of the bird's body, the air will be colder to him than to 

 ourselves, and, so far, the conditions for loss of heat 

 by conduction are more favourable. Making all 

 allowances, the heat given off in this way cannot be 

 very great ; and, as I have said, owing to the absence of 

 sweat glands, there is no appreciable amount of evap- 

 oration. The work is, therefore, of necessity, thrown 

 upon the lungs and air-sacks. It is these organs that 

 by means of evaporation and conduction regulate 

 the temperature of the bird's body. It must be re- 

 membered that, however hot or however cold the air 

 inhalcd, by the time it emerges from the lungs it has 



1 Dr. Michael Foster {Textbook of Physiology, p. 464, 1SS3 

 ed.) writes : " It has been calculated that the relative amounts 

 of the losses by these several channels are as follows : in wann- 

 ing the urine and faeces about 3, or according to others, 6 per 

 cent. ; by respiration about 20, or according to others, about 9 

 only per cent., leaving yy, or, alternatively, 85 per cent., for con- 

 duction and radiation and evaporation from the skin." 



