16 PROFESSOR OWEN ON THE GENUS DINORNIS. 
Sternum of Palapteryx and Aptornis. 
The most simple form of sternum in the class of Birds is that which is presented by 
the terrestrial species deprived of the power of flight, in which, however, the size and 
especially the breadth of the bone surpass those of the sternum of any of the terrestrial 
mammals, and relate to the peculiar mode of respiration in the class of Birds. The 
mechanical part of this function is effected by alternately bringing the sternum nearer 
to the back and pushing it farther from it, these movements of elevation and depression 
being performed chiefly upon the synovial joints between the sternal and vertebral ribs ; 
by these movements the large air-cells interposed between the concave surface of the ster- 
num and the lungs, which lungs are fixed in intercostal cavities at the back of the thorax, 
are alternately expanded and contracted, receiving the air in expansion from the orifices 
on the sternal aspect of the lungs, and expelling it on contraction through the same 
apertures back into the lungs ; or, if, as is commonly the case, other air-cells be developed 
beyond the sternum, into those extrasternal cells. The suprasternal or thoracic air-cells 
being those which are most essential to this mode of respiration, are constantly developed 
in Birds, and are present in the Apteryx’, where no other extra-pulmonary air-cells exist ; 
in which bird accordingly we find the sternum of greater relative breadth? than in any 
Mammalian animal, notwithstanding the wings are reduced to mere rudiments; the 
primary and essential relations of the sternum being to the ornithic mode of respira- 
tion above described. The other modifications of the sternum in Birds relate to the 
functions and actions of the anterior extremities. The great extent, however, of its di- 
versity of shape and proportion has not, as yet, been fully or satisfactorily explained on 
the principle of final causes ; but they are characteristic, to a certain degree, of natural 
groups, and are useful as accessory guides to the natural arrangement and affinities of 
the class. 
The relation of particular forms of sternum to particular genera of Birds is illustrated 
by those which characterize the different genera of the Struthious family, in which the 
secondary modifications are superinduced upon a common family type of the bone ex- 
emplified by its resemblance to a buckler and the total absence of the keel. They are 
so constant and well-marked, that the Comparative Osteologist, who had had the oppor- 
tunity of comparing them, would afterwards readily distinguish the genera Séruthio, 
Rhea, Casuarius and Dromaius, by the sternum alone. That bone in the Apteryz is still 
more characteristic of the genus, and it is to this particular modification of the keel-less 
sternum that the sternum of one or both genera of the gigantic wingless birds of New 
Zealand makes the nearest approach. This is exemplified in the attempted restoration 
of the sternum of a large species* referred to Dinornis prior to the reception of the evi- 
dence afforded by the cranium and beak of two genera of large wingless birds in New 
" Zool. Trans. vol. ii. p. 278. pl. 51. fig. 4. * Ib. vol. iii. p. 318. pl. 43. fig. 8. 
* Ib. vol. iii. p. 316. pl. 43. figs. 1, 2 & 3. 
