DEVELOPMENT OF THE VERTEBRAE. 125 



puberty, no other changes occur, excepting a gradual increase in the growth 

 of these primary centres, the upper and under surface of the bodies, and the 

 ends of the transverse and spinous processes, being tipped with cartilage, in 

 which ossific granules are not as yet deposited. At sixteen years (Fig. 76), 

 four secondary centres appear, one for the tip of each transverse process, and 

 two (sometimes united into one) for the end of the spinous process. At twenty- 

 one years (Fig. 77), a thin circular plate of bone is formed in the layer of car- 

 tilage 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. 78) is developed by two primary centres, and by one or more 

 epiphyses. The two primary centres are destined for the two lateral or neural 

 masses, the ossification of which commences before birth, near the articular 

 processes, and extends backwards : these portions of bone 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 altogether cartilaginous, and this 

 portion of the atlas is completed by the gradual extension forwards and ulti- 

 mate junction of the two neural processes. Occasionally, a separate nucleus is 

 developed in the anterior arch, which, extending laterally, joins the neural pro- 

 cesses in front of the pedicles; or, there are two nuclei developed in the anterior 

 arch, one on either side of the median line, which join to form a single mass, 

 afterwards united to the lateral portions in front of the articulating processes. 



The Axis (Fig. 79) 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 consists originally of an extension upwards of the cartila- 

 ginous 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 

 bi-lobed 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 Dr. Hum- 

 phry has 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 

 transverse process between the fifth and sixth years. Sometimes this process 

 continues as a separate piece, and, becoming lengthened outwards, constitutes 

 what is known as a cervical rib. 



The Lumbar Vertebrae (Fig. 80) have two additional centres (besides those pecu- 

 liar to the vertebrae 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 perma- 

 nently unconnected with the remaining portion of the bone; thus forming a 

 lumbar rib, a peculiarity which is rarely met with. 



Progress of Ossification in the Spine generally. Ossification of the laminae of 

 the vertebrae 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 tile spinal column (about the ninth or tenth dorsal 

 vertebrae), and extends both upwards and downwards. Although, however, the 

 ossific nuclei make their first appearance in the lower dorsal vertebrae, the 

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



