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 

 debris 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, Morphologic und 

 Systematic der Vogel, especially pp. 47 and 133 ; Strasser, 

 Morphologisches Jahrbuch (Leipzig, 1877) ; Dr. Crisp, Proc. 

 Zoo I. Society ; 1S57. 



I 2 



