DEVELOPMENT OF THE VERTEBRAE. 49 



one years (fig. 10), a thin circular plate of bone is formed in the thin layer of carti- 

 Jage situated on the upper and under surface of the body, the former being the 

 thicker of the two. All these become joined ; and the bone is completely formed 

 about the thirtieth year of life. 



Exceptions to this mode of development occur in the first, second, and seventh 

 cervical, and in the vertebrae of the lumbar region. 



The Atlas (fig. 11) is developed by two primary centres, and by one or more 

 epiphyses. The two primary centres consist of the two lateral of neural masses, 

 ossification of which commences before birth, near the articular processes, and 

 extending backwards, they are separated from one another behind, at birth, by a 

 narrow interval filled in with cartilage. Between the second and third years, 

 they unite either directly or through the medium of an epiphysal centre, developed 

 in the cartilage near their point of junction. The anterior arch, at birth, is alto- 

 gether cartilaginous, and this portion of the atlas is completed by the gradual 

 extension forwards and ultimate junction of the two neural processes. Occasion- 

 ally a separate nucleus is developed in the anterior arch, which, extending laterally, 

 joins the neural processes in front of the pedicles ; or, there are two nuclei deve- 

 loped in the anterior arch, one on either side of the median line; they join to form 

 a single mass, which is afterwards united to the lateral portions in front of the 

 articulating processes. 



The Axis (fig. 12) is developed by six centres. The body and arch of this bone 

 are formed in the same manner as the corresponding parts in the other vertebra : 

 one centre for the lower part of the body, and one for each lamina. The odontoid 

 process, which is really the centrum or body of the axis, consists originally of an 

 extension upwards of the cartilaginous mass, in which the lower part of the body 

 is formed. At about the sixth month of foetal life, two osseous nuclei make their 

 appearance in the base of this process : they are placed laterally, and join before 

 birth to form a conical-shaped bilobed mass, deeply cleft above; the interval 

 between the cleft and the summit of the process is formed by a wedge-shaped 

 piece of cartilage ; the base of the process being separated from the body by a 

 cartilaginous interval, which gradually becomes ossified, sometimes by a separate 

 epiphysal nucleus. Finally, as Mr. Humphry has lately demonstrated, the apex 

 of the odontoid process has a separate nucleus. 



The Seventh Cervical. The anterior or costal part of the transverse process of 

 the seventh cervical is developed from a separate osseous centre at about the 

 sixth month of foetal life, and joins the body and posterior division of the trans- 

 verse process between the fifth and sixth years. Sometimes this process continues 

 as a separate piece, and becoming lengthened outwards constitutes Avhat is known 

 as a cervical rib. 



The Lumbar Vertebrae (fig. 13) have two additional centres (besides those peculiar 

 to the vertebras generally), for the tubercles, which project from the back part of 

 the superior articular processes. The transverse process of the first lumbar is 

 sometimes developed as a separate piece, which may remain permanently uncon- 

 nected with the remaining portion of the bone ; thus forming a lumbar rib, a 

 peculiarity which is sometimes, though rarely, met with. 



PROGRESS OF OSSIFICATION IN THE SPINE GENERALLY. Ossification of the laminse 

 of the vertebra commences at the upper part of the spine, and proceeds gradually 

 downwards ; hence the frequent occurrence of spina bifida in the lower part of 

 the spinal column. Ossification of the bodies, on the other hand, commences a 

 little below the centre of the spinal column, about the ninth or tenth dorsal 

 vertebra, and extends both upwards and downwards. Although, however, the 

 ossific nuclei make their first appearance in the lower dorsal vertebras, the lumbar 

 and first sacral are those in which these nuclei are largest at birth. 



Attachment of Muscles. To the Atlas are attached the Longus colli, Eectus 

 anticus minor, Eectus lateralis, Eectus posticus minor, Obliquus superior and 

 inferior, Splenius colli, Levator anguli scapula^ Interspinous, and Intertransverse. 



To the Axis are attached the Longus colli, Obliquus inferior, Eectus posticus 

 4 



