20 THE SKELETON 



distinct niuk-i. Sometimes the nucleus is double, and remains separate throughout 

 life, tlie vertebra l»eing divided by a vertical fissure (fig. 21). The bifid character 

 of the nucleus of the vertebral l)ody is further emphasised by the occasional occur- 

 rence of half-vertcbn¥. The lateral centres are deposited near the bases of the 

 superior articular processes, and give rise to the pedicles, lamina?, articular pro- 

 cesses, and a large part of the transverse and spinous processes. 



At h\T\\\ a vertebra consists of three parts — a body and two lateral masses 

 connected by hyaline cartilage. The line of union of the lateral masses with the 

 V)odies is known as the aearo-centml suture (fig. 22), and this is not obliterated for 

 several years after l)irth. An examination of a thoracic vertebra at the fifth year 

 will show tliat a portion of the body of each vertebra is derived from the lateral 



Fig. 22. — A Vektebra at Birth. 



UTERAL MASS 



NEURO-CENTRAL SUTURE 

 CENTRUM OR BODY 



masses, and that the demi-facets for the rib-heads are situated behind the neuro- 

 central suture, and therefore belong to the pedicles. 



During the early years growth progresses rapidly, and at puberty the secondary 

 centres make their appearance in the cartilaginous tips of the transverse and 

 spinous processes. During the seventeenth year a meniscus of bone forms around 

 the margins of the superior and inferior surfaces of the centra. These are the 

 epiphysial discs; they are thickest at the periphery, and gradually become thin 

 towards the central perforation. By the twenty-fifth year the various secondary 

 nuclei have coalesced wdth the main bone, and the vertebra is then complete. 



In several verteVjrse the mode of ossification deviates from the account given 

 above, and requires separate consideration. 



The atlas. — This bone has three primary centres — one for each lateral mass 



Fig. 23.— Lumbar Vertebra at the Eighteenth Year with Secondary Centres. 



epiphysial plate or disc 



— mammillary tubercle 

 -transverse process 



-SPINOUS PROCESS 

 EPIPHYSIAL PLATE OR DISC 



(neural arch) ap])earing in tlie sixth week of embryonic life. The third appears 

 a few montlis after Ijirth for the anterior arch. The lateral portions coalesce 

 ]»osteri(>iiy al)OUt the fourth year; the union with the anterior nucleus is delayed 

 until the sixth year. An additional centre occasionally appears for the posterior 

 segment. 



The axis. — This is the most exceptional of all the vertebrtc. It has the usual 

 three primary nuclei — one for the body, and one on each side for the neural arch. 

 The centre for the body appears in the embryo about the fifth month, and a feAV 

 weeks later, two laterally disposed nuclei are seen for the base of the odontoid 

 process; these fuse together in the middle line, and by the third year ankylose 

 peripherally to the centrum of the axis. The line of iinion between the body of 

 the axis and the odontoid ])rocess is indicated even in advanced life by a persistent 



