86 THE STRUCTURE AND. LIFE OF BIRDS cunap. 
It forms on either 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 thc 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, ze, 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 does not 
count for much. Nor is the fact that the diaphragm is 
muscular in mammals and membranous in birds in any 
way conclusive. Inthe 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.! 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. 
