VI FORM AND FUNCTION 115 
while the aeration of the bones has obviated the natural 
increase of weight which would have been a serious 
hindrance. But there remains the perplexing physio- 
logical problem: what organ of the body does the 
work that, in mammals, and, presumably, in birds that 
have solid bones, is done by the marrow ?! 
The Kidneys. 
In man, as remarked above, three organs—the skin, 
the lungs, and the kidneys—divide between them the 
work of eliminating waste products from the body. 
The skin disposes of a great deal of water and a little 
carbonic acid ; the lungs of carbonic acid and water, 
but water to a much less extent than the skin; the 
kidneys of urea, uric acid (much nitrogen in both, the 
débris of the tissues) and water. As these three are 
allied organs, doing work that is similar, to some extent 
actually the same, it might be expected that in birds, 
since their skin is not an excretory organ, the other 
two would be unusually active. With the lungs we 
have seen that this is the case. And the kidneys are 
very large; they will be found lying behind the lungs 
against the pelvis—long dark bodies. Yet they do not 
undertake all the work that they do in mammals. 
They are very active in excreting urea and uric acid ; 
but, as is the case with snakes, it is in a nearly solid 
form, the product of their activity being easily dis- 
1 On “Aeration of Bones” see Fiirbringer, Morphologie und 
Systematik der Végel, especially pp. 47 and 133; Strasser, 
Morphologisches Jahrbuch (Leipzig, 1877); Dr. Crisp, Proc. 
Zool. Society, 1857. 
IZ 
