EPISTROPHEUS 



47 



Epistropheus. — The arch, and the processes associated with it, are formed from two lateral 

 centres which appear, like those in the other vertebrae, about the seventh week. The common 

 piece of cartilage which precedes the body and dens is ossified from four (or five) centres, one 

 (or two) for the body of the axis, in the fourth month, two, laterally disposed, for the dens, a 



Fig. 55. — Immature Atlas. (Third year.) 



few weeks later, and one, for the apex of the dens, in the second year. The two collateral centres 

 for the main part of the dens soon coalesce, so that at birth the axis consists of four osseous pieces 

 — two lateral portions which constitute the arch, the body, and the dens, surmounted bj' a 

 piece of cartilage. During the third or fourth year the dens joins with the body, the line of 



Fig. 56. — Developmext of the Epistropheus. 



Suspensory ligament 

 Nucleus for tip of odontoid denS' 



Lateral centres for odontoid dens 



( 



Epiphysial plate or disc 

 Pedicle 



Centrum or body 



Epiphysial plate 



union being indicated even in advanced life by a small disc of cartilage, and the arch unites in 

 front and behind about the same time or a Uttle later. The apical nucleus of the dens, which 

 represents an epiphysis, joins the main part about the tweHth year and in the seventeenth year 



Fig. 



-The Epistropheus at Four Years of Age, showixg the Size and Extent of 

 THE Dens. (Natural size.) 



an epiphysial plate appears for the lower surface of the body. There are also rudiments, 

 adjoining the cartilaginous disc, of the upper epiphysial plate of the body. 



Cervical vertebrae. — In the cervical vertebrae the lateral centres form a larger share of the 

 body than in the vertebrae of other regions, and the neuro-central suture runs almost in a sag- 



FiG. 58. — The Epistropheus (from an Adult) in Sagittal Section. 



Dens (odontoid process) 



Cartilage representing the inter- 

 vertebral disc between the dens 

 and the body of the epistropheus 



Body of epistropheus - 



ittal direction. The sixth, seventh, and even the fifth have additional centres which appear 

 before birth for the anterior or costal divisions of the transverse processes. In the other cer- 

 vical vertebra; the costal processes are ossified by extension of the lateral nuclei. Ihe costal 

 processes of the seventh cervical sometimes remain separate, constitutmg cervical ribs. 



