OSSIFICATION OF VERTEBRA 



45 



and in the thoracic region, by the laminae and transverse processes. The transverse processes 

 project laterally for a considerable distance in the atlas, first thoracic, and the middle of 

 the lumbar series; they are shortest in the third cervical and the twelfth thoracic. 



In the lateral view, the intervertebral foramina appear oval in shape, and are small in the 

 cervical, larger in the thoracic, and largest in the lumbar region. 



Structure ol a vertebra. — The bodies of the vertebrae are largely composed of cancellous 

 tissue, with a thin^outer covering of compact tissue. In a vertical section through the centrum 



Fig. 49. — A Divided Thoracic Vertebra. (After Turner.) 



( 



the fibres of the cancellous tissue are seen to be arranged vertically and horizontally, the vertical 

 fibres being curved with their concavities directed toward the centre of the bone. The hori- 

 zontal fibres are sUghtly curved parallel with the upper and lower surfaces, and have their con- 

 vexities toward the centre of the bone. They are not so well defined as the vertical set. 

 (Wagstaffe.) 



Ossification. — The vertebrae in general. — The ossification of each vertebra takes place 

 in cartilage from three primary and five secondary centres. The three primary centres 



Fig. 50. — A Vertebral Centrum in Section to Show the Pressure Curves. 



appear, one in the '.body and two in the arch, about the seventh week of intra-uterine Ufe. 

 In the thoracic region the nucleus for the body appears first, but in the cervical region it is pre- 

 ceded by the centres for the arch. The nucleus for the body soon becomes bilobed, and this 

 condition is sometimes so pronounced as to give rise to the appearance of two distinct nuclei . 

 Indeed, the nucleus is very rarelv double and the two parts of the body may remain separate 

 throughout Ufe (fig. 49). The bilateral character of the nucleus is further emphasised by the 

 occasional formation of half- vertebrae. The lateral centres are deposited near the bases of the 



Fig. 



A Vertebra at Birth. 



Lateral mass 



Neuro-central suture 

 Centrum or body 



superior articular processes and give rise to the roots, laminae, articular, and the greater 

 parts of the transverse and spinous processes. 



At birth a typical vertebra consists of three osseous pieces — a body and two lateral masses, 

 which constitute the arch, the parts being joined together by hyahne cartilage. The Ime of 

 union of the lateral portion with the body is known as the neuro-central suture, and is not actuaUy 

 obliterated for several years after birth. In the thoracic region the central ossification does not 

 pass beyond the point with which the head of the rib articulates, and leaves a portion of the body 

 on each side formed from the lateral ossification. A thoracic vertebra at the fifth year shows 



