16 THE VOYAGE OF H.M.S. CHALLENGER. 



column in the Penguins, as compared with that of other birds, are referrible to two 

 points. Firstly, the freely moveable character of the articulation of these with one . 

 another, and secondly, the opisthocoelous character of certain of the vertebrae. 



In the majority of birds the dorsal vertebrae are more or less firmly anchylosed with 

 one another by means of their bodies, spines, and transverse processes, whereas in the 

 Penguins, as first pointed out by Meckel,^ these structures are freely moveable upon one 

 another, and consequently the dorsal segment of the vertebral column is devoid of that 

 rigidity which characterises it in other birds. This arrangement is perhaps explicable, 

 as pointed out by Owen, by the fact that, the Penguins being incapable of flight, there 

 is no necessity for that extreme rigidity of the dorsal region which is met with in those 

 birds in which the powers of flight are largely developed, and in which, therefore, the 

 dorsal region "has almost exclusively to sustain the shock of the violent contractions 

 of the principal muscles of the wings." It should, however, be remembered that as the 

 wings are the principal agents used by the Penguin in propelling itself through the much 

 denser medium of water, there may possibly be need of some further explanation of the 

 arrangement referred to. 



The opisthocoelous character of the vertebrae among birds is, so far as I can ascertain, 

 confined to the Auks and Penguins.' It was first demonstrated by Owen^ in Aptenodytes, 

 and has since been recognised in other genera of Penguins. So far as my own observa- 

 tions extend, it obtains in every species of Penguin. 



Lumho-Sacral Vertebrce 



Under the name of lumbo-sacral, I include all the vertebrae which are interposed 

 between the last dorsal vertebra (characterised by articulation with the last pair of 

 vertebral ribs), and the first moveable coccygeal vertebra. Thus defined the lumbo- 

 sacral vertebras are twelve in number. The bodies of all these vertebrae in the 

 adult are anchylosed with one another as weU as with that of the last dorsal vertebra, 

 to form a single osseous mass, the composition of which is only rendered evident 

 by the presence of the intervertebral foramina. The bodies of these vertebras, seen from 

 below, form a single osseous lozenge-shaped mass, the widest portion of which is situated 

 immediately in front of the acetabulum, from which point the bodies of the vertebrae 

 diminish in breadth both forwards and backwards. In front of the widest part the bodies 

 form a sharp osseous keel, while behind that point the vertebrae diminish in breadth to 

 that of the bodies of the proximal coccygeal vertebrae. The bodies of the lumbo-sacral 

 vertebrae are altogether devoid of hypapophyses. 



The apices of the transverse processes of all the vertebrae of this region, with the ex- 



■■. Traite general d'anatomie comparte, vol. iii., partie 1, p. 38. 



^ CyclopEcdia of Anatomy, Art. " Aves," vol. i. p. 271. 



' Owen, Anatomy of Vertebrates, vol. ii. p. 16. 



* Cyclopaedia of Anatomy, Art. " Aves," vol. i. p. 270 . 



