18 



VERTEBRAL COLUMN. 



membranous stage in the development of the vertebral column. It may be proper 

 to remark that there is no similar original division in the cephalic portion of the 

 primordial skeleton, but the chorda dorsalis is prolonged for a short distance into the 

 middle of the basilar part. 



The process of breaking up of the primordial vertebral masses above referred to is 

 followed by a reconstruction, as it were, of the vertebras, which consists in this that 

 in the cylinder investing the chorda dorsalis the lines of separation of the primordial 

 vertebral masses fade away, while midway between them new lines appear, marking 

 the limits of the permanent vertebrae ; and thus each permanent vertebra is formed 

 from pai'ts of two of the primordial masses ; the arch, the transverse processes and 

 part of the ribs, together with half the body, being derived from one primitive 

 vertebra, and the remaining half of the body together with the corresponding pair of 

 nerves proceeding from a part of the next primordial mass in succession. 



In the human embryo, the vertebral column begins to become cartilaginous in the 

 sixth or seventh week. The cartilage spreads rapidly over the bodies, but much 



more slowly into the arches, in which the 



Fig. 1 6. union of the cartilages of opposite sides is 



not completed till during the fourth month 



A &!;:' &'A (Kb'lliker). At the period of the first appear- 



ance of the cartilaginous bodies, the chorda 

 dorsalis consists of a solid column of large 

 thin-walled cells, surrounded by a transpa- 

 rent sheath; but it becomes constricted, and 

 gradually dwindles within each vertebral 

 body, while it remains more fully developed 

 in the intervertebral spaces, and enters into 

 the formation of the intervertebral discs. 



Fig. 16, A & B (from Kolliker). SECTIONS 

 OP THE VERTEBRAL COLUMN OP A HUMAN 

 FCETUS OP EIGHT WEEKS. 



A, transverse longitudinal section of 

 several vertebrae. 1, 1, chorda dorsalis, its 

 remains thicker opposite the intervertebral 

 discs ; 2 is placed on one of the bodies of 

 the permanent vertebrae ; 3, on one of the 

 iutervertebral discs. 



B, transverse horizontal section through 

 a part of one dorsal vertebra. 1, remains 

 of the chorda dorsalis in the middle of the 

 body ; 2, arch of the vertebra ; 3, head of 

 a rib. 



OSSIFICATION. The ossification of each typical vertebra proceeds from three 

 principal nuclei. One of these, which is occasionally double at first, appears in the 

 middle of the cartilage, and is afterwards converted into the main part of the body ; 

 the other two, placed one on each side, appear opposite the roots of the transverse 

 processes, usually a little earlier than the nucleus of the body ; and form the arch and 

 processes, together with an angular part on each side of the body, namely, that part 

 which in the dorsal region supports the heads of the ribs. At different periods 

 subsequent to the age of puberty, five epiphyses, or supplementary centres of ossifica- 

 tion, are added. Three of these are small portions of bone, placed on the tips of the 

 spinous and transverse processes : the other two are thin circular plates, one on the 

 upper, the other on the lower surface of the body, chiefly at its circumference. In 

 the lumbar vertebrse two other epiphyses surmount the mammillary processes. The 

 transverse process of the first lumbar vertebra is sometimes observed to be developed 

 altogether from a separate centre. Most of the anterior divisions of the cervical trans- 

 verse processes are ossified by the extension into them of osseous substance from 

 the neighbouring posterior part of the process and from the arch ; but that of the 

 seventh usually presents a separate osseous nucleus, and small nuclei have also been 

 observed by Meckel in those of the second, fifth, and sixth vertebrae. 



