86 THE STRUCTURE AND LIFE OF BIRDS chap. 



It forms on cither side a sheet which slopes outwards 

 and fastens to the sternum near to its junction with 

 the ribs. Cross-partitions divide the chambers formed 

 between it and the body wall : into these compartments 

 the air-sacks enter, their walls being so thin that it is 

 difficult to separate them from those of the chambers 

 which they line. The diaphragm does not, as in mam- 

 mals, separate the heart from the intestines. The dia- 

 gram on page 85 will help to make this description clear. 

 Obviously a membranous partition like this cannot 

 do the work of the diaphragm of mammals, but 

 that it is homologous to it, i.e., the same in origin, 

 may well be maintained. The difference of position 

 does not disprove this, for it is well known that a 

 muscle may shift its point of attachment so that upon 

 such a question as the nearness of the relationship of 

 reptiles to birds the evidence of muscles docs not 

 count for much. Nor is the fact that the diaphragm is 

 muscular in mammals and membranous in birds in any 

 way conclusive. In the apteryx it is strong and fibrous. 

 In a puffin Mr. Beddard found it muscular, and I 

 myself found it very highly so in another bird of the 

 same species. Tendon in fact often replaces muscle. 

 It is certainly possible, on the whole it seems probable, 

 that the diaphragm in mammals and birds may be the 

 same in origin though different in function. 1 It is in- 

 teresting to find that crocodiles that come near to 

 birds in so many points, are like them also in having 

 an oblique septum. 



1 See Huxley on " Breathing Apparatus of Apteryx," Proc. 

 Zool. Soc, 1882: article on the "Diaphragm" in Newton's 

 Dictionary of Birds. 



