IN BIRDS. 79 



mammalia, as any one may infer from the greater 

 rigidity of their texture, and the whiteness of their 

 colour. Thus, there is not a blood-vessel of any con- 

 siderable size in the whole body of a bird, to the 

 coats of which the air has not access during the 

 greater part of their course ; and thus the real action 

 of breathing in birds is not concentrated into one 

 organ, to be toiling and panting there, as it would be 

 in the lungs of the mammalia, but distributed over 

 the whole circulation, and consequently diminished 

 in local intensity, in proportion as it is extended over 

 a greater surface. 



This is a subject which it is impossible to bring to 

 the test of numbers, so as to compare accurately the 

 diminution of local action by means of the general 

 access of air to the blood-vessels. There are two 

 difficulties, neither of which can be, from the great 

 nicety of observation which they would require, over- 

 come. In the first place, we know not, and we cannot 

 ascertain, the relative surfaces of the blood-vessels 

 exposed to the air in lungs only, and in the whole 

 system, as in birds ; and, in the second place, we 

 know not the difference of action which the air may 

 have on the coat of a very small blood-vessel, such 

 as those in the lungs, and that of a larger one. We 

 do know that the exposed surface of the vessels in 

 the lungs must form but a small portion of that of 

 the whole vessels in the system, because in the freest 

 breathers, that is, what is usually styled the " longest 

 winded" of the mammalia, which have their blood 

 aerated in the lungs only, or chiefly, the portion 

 which passes through these organs at each respira- 

 tion of the breath, is only a small fraction of the 

 whole. We know also that the coats of the larger 

 blood-vessels must, in order that the vascular system 



