AXIAL SKELETON OP THE OSTKICH. 387 



dorso-lumbar vertebra, there being three such with ribs, the first of which is connected 

 by a styliform bone with the side of the sternal rib of the last true dorsal vertebra. 



The next eight vertebrae (twenty-eighth to thirty-fifth) do not support long rib-like 

 processes and are constantly anchylosed into one mass in adults ; they may be called 

 lumbar vertebrae. 



Next follow three vertebrae with long distally expanded rib-like processes abutting 

 against the ilia. It will be at least convenient to call these sacral vertebrce. 



At the distal end of the sacral mass we have in the adult eight vertebrae (thirty-ninth 

 to forty-sixth), which may be termed sacro-caudal veHehros. 



Finally there are ten postsacral vertebrae normally free in the adult, except the last 

 two ; these ten are the true caudal vertelrce (fig. l'^). The number of these vertebrae 

 may sometimes, however, be reduced to eight \ 



There are thus normally fifty-six vertebrae from the atlas to the coccyx inclusively. In 

 some skeletons, however, there may be one or two vertebrae short or a vertebra in ex- 

 cess ; and when such divergences exist, the differential characters of all the various ver- 

 tebrae are correspondingly modified ; and this should be borne in mind when the descrip- 

 tion here given is compared with such skeletons. 



THE PRESACRAL VERTEBRA. 



The Cervical Vertebra. 



The Atlas. — The atlas of the Ostrich presents an extreme contrast to the same bone 

 in aU mammals, even the lowest, in that it is so smaU a bone, being little more than an 

 osseous ring, ventrally thickened with three short postaxial projections, and not being 

 more than a quarter the bulk of the axis. 



Nevertheless, though this vertebra as a whole is relatively so small compared with the 

 atlas of mammals, yet that part of it which is median and ventral {i. e. that hypapo- 

 physial ossification which holds the place of a " centrum ") is relatively much larger 

 than in any mammal. This might perhaps be anticipated from the articulation of the 

 vertebral column with the skull being median in birds, through a single condyle, instead 

 of lateral as in mammals through a pair of condyles. 



The atlas of the Ostrich consists of this g^uasi body and two neural laminae, which 

 meet together dorsally, but do not develop a neural spine. 



The whole vertebra in the adult consists of one bone, no trace remaining of the 

 primitive separation between the neural laminae and the median ventral portion. 



This latter (the quasi-body) apart from its junctions with the neural arch, may be 

 said to have four surfaces — one ventral, one dorsal (or neural), one preaxial, and one 

 postaxial ; and these four surfaces are divided by four corresponding margins. 



The preaxial surface of the centrum, which articulates with the occipital condyle, 



' As in the mounted specimen, No. 1362, in the Museum of the College of Surgeons. 



3l2 



