16 PROFESSOR OWEN ON THE GENUS DINORNIS. 



Sternum o/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 tlie 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 efl'ected 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 diflPerent genera of the Struthious family, in which the 

 secondary modifications are superinduced upon a common family type of the bone ex- 

 emphfied 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 Struthio, 

 Rhea, Casuarius and Dromaius, by the sternum alone. That bone in the Apteryx 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. pi. 51. fig. 4. -' lb. vol. iii. p. 318. pi. iS. fig. 8. 



= lb. vol. iii. p. 316. pi. 4:3. figs. 1, 2 & 3. 



