1o2 THE STRUCTURE AND LIFE OF BIRDS cuap. 
very nearly the same temperature as the body, and 
all the heat communicated to the air is withdrawn 
from the bird. The rapid breathing, therefore, that is 
natural in flight, will of. itself counteract the heating 
effect of violent exercise. In the same way, since 
they perspire only through the tongue and feet, dogs 
maintain an equable temperature when running fast, 
by means of quickened respiration. It is, probably, 
as regulators of temperature that the air-sacks have 
been developed till their cubic capacity surpasses that 
of the lungs many times: how many, it is difficult to 
estimate; probably ten times at least. They cannot, as 
some writers have supposed, do the work of lungs, 
since the blood vessels in them are so minute as to 
be of little use, whereas, by exposing their very 
large surfaces constantly to fresh indraughts of air, 
they cause a large withdrawal of heat from the body, 
and for this no other effectual machinery exists. It 
would be very interesting to discover exactly what 
amount of aqueous vapour is given off by a bird in 
breathing, so that we might know whether, in pro- 
‘portion to the size of body, it is more than it isin man. 
Among other reasons for regarding this as probable, is 
the fact that a bird’s kidneys secrete little or no 
water, so that of the three organs which get rid of the 
waste products of the body—the skin, the lungs, and 
the kidneys—the lungs alone are available for dis- 
posing of any great amount of what is fluid. 
Unfortunately, it is impossible to give any exact 
figures. As far as I am aware, no evidence on this 
point has been obtained by experiment. 
Books on comparative anatomy are common, and 
