THE VEETEBKAL COLUMN. 327 



the only segmentation it ever shows is the permanent or ' secon- 

 dary ' one. 



The reason why the permanent or vertebral segmentation 

 alternates with the primary or myotomic segmentation is probably 

 to be found in mechanical considerations. The longitudinal muscle 

 fibres of the myotomes are attached at their ends to the verte- 

 bras or to their processes, and the strain on the axial skeleton, and 

 consequent tendency to lateral bending, caused by alternate con- 

 tractions of the muscles of the two sides of the body, will be great- 

 est, not opposite the attachments of the muscles, but midway be- 

 tween these ; and it is at these midway points that the interver- 

 tebral joints are formed when the axial skeleton becomes too 

 rigid to allow of free bending of the body, without segmentation. 

 The mechanical advantage of the arrangement, by which each 

 vertebra is acted on by two myotomes on each side, one pulling 

 it forwards and one backwards, is sufficiently clear ; and the actual 

 segmentation is probably due, in the first instance, to the direct 

 action of the muscles themselves, causing bending, and subse- 

 quently jointing, of the originally continuous cartilaginous tube 

 at points midway between the attachments of the muscles. 



Up to the sixth or seventh day (cf. Fig. 116, Cfi), the notochord 

 remains of full size and nearly uniform diameter ; but from this 

 time it becomes gradually encroached on by the vertebral centra 

 surrounding it, which grow inwards, constricting it, and causing 

 its gradual absorption and final disappearance. Ossification of the 

 vertebras begins about the twelfth day, in the centrum of the 

 second or third cervical vertebra, and gradually extends back- 

 wards along the column; the neural arches ossify rather later 

 than the centra, and independently of them ; each having two 

 centres of ossification. 



At an early stage, about the seventh day, the t rue centrum 

 of the first, or atlas, vertebra separates from the outer ring, and 

 becomes attached to the second, or axis vertebra, as its odontoid 

 process. The atlas and axis vertebrae have no rib elements, but 

 these are present in the remaining cervical vertebras ; they lie 

 to the outer sides of the vertebral arteries, and they are from 

 the first continuous with the centra of the vertebras to which 

 they belong, except in the case of the hindmost two or three 

 cervical vertebras, in which the rib elements are for a time 

 independent. 



