50 



THE SKELETON 



The Serial Morphology of the Vertehrce 



Although at first sight many of the vertebrjB exhibit peculiarities, nevertheless a study 

 of the mode by which they develop, and their variations, indicates the serial homology off the 

 constituent parts of the vertebrae in each region of the column. 



The body (centrum) of the vertebra is that part which immediately surrounds the noto- 

 chord. This part is present in all the vertebra; of man, but the centrum of the atlas is disso- 

 ciated from its arch, and ankylosed to the body of the epistropheus. The reasons for regard- 



FiG. 64. — Morphology of the Transverse and Articular Processes. 



Cervical vertebra 



Transverse process 

 Costo-transverse foramen 



Neuro-central suture 

 Cervical rib 



Costal process 



Thoracic 

 vertebra 



Transverse process 

 Costo-transverse foramen 



Neuro-central suture 

 Rib 



Transverse process 

 Lumbar rib 



Sacral vertebra 



Neuro-central suture 

 Costal process 



ing the dens as the body of the atlas are these: In the embryo the notochord passes through 

 it on its way to the base of the cranium. Between the dens and the body of the axis there is 

 a swelling of the notochord in the early embryo as in other intervertebral regions. This swell- 

 ing is later indicated by a small intervertebral disc hidden in the bone, but persistent even in 

 old age. Moreover, tlie dens o.ssifies from primary centres, and in chelonians it remains a.s a 

 separate ossicle throughout life; in Ornithorhynchus it remains distinct for a long time, and it 

 has been found separate even in an adult man. Lastly, in man and many mammals, an epi- 



