vI FORM AND FUNCTION 10! 
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 thelungs. 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 
inhaled, by the time it emerges from the lungs it has 
1 Dr. Michael Foster (Zertbook of Physiology, p. 464, 1883 
ed.) writes : “It has been calculated that the relative amounts 
of the losses by these several channels are as follows : in warm- 
ing the urine and feces about 3, or according to others, 6 per 
cent.; by respiration about 20, or according to others, about 9 
only per cent., leaving 77, or, alternatively, 85 per cent., for con- 
duction and radiation and evaporation from the skin.” 
