io- THE STRUCTURE AND LIFE OF BIRDS chap. 



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 all the heat communicated to the air is withdrawn 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 is in 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 



