102 



OSTEOLOGY. 



level of the third sacral segment is reached, at which point the transverse diameter 

 is somewhat abruptly diminished, a reduction in width which is further suddenly 

 accentuated opposite the fifth sacral segment. 



As viewed from the side, the bodies display a gradual 

 increase in their antero-posterior extent until the second 

 lumbar vertebra is reached, below which, that diameter is 

 slightly reduced. In the sacral region the reduction in 

 the antero-posterior diameter is great in the first and 

 second sacral segments, more gradual and less marked in 

 the last three segments. The facets for the heads of the 

 ribs in the upper thoracic region lie on the sides of the 

 bodies ; those for the tenth, eleventh, and twelfth are placed 

 farther back on the roots of the vertebral arches. 



The intervertebral foramina increase in size from above 

 downwards in the movable part of the column, being largest 

 in the lumbar region. In the sacral region they decrease 

 in size from above downwards. In the cervical region the 

 highest two cervical nerves pass out behind the articular 

 processes of the atlas and epistropheus, and lie, therefore, 

 behind the corresponding transverse processes of those 

 vertebrae. The succeeding cervical nerves pass out through 

 the intervertebral foramina, which are placed between the 

 transverse processes and anterior to the articular processes. 

 In the thoracic and lumbar vertebras the intervertebral 

 foramina lie anterior to both the articular and transverse 

 processes. The arrangement of the intervertebral foramina 

 in the sacrum has been already sufficiently explained. 



The vertebral canal for the lodgment of the spinal medulla 

 and its meninges is largest in the cervical and lumbar 

 regions, in both of which it assumes a triangular form ; 

 whilst it is narrow and circular in the thoracic region. 

 These facts are correlated with the movements of the 

 column which are most free in those regions where the 

 canal is largest, i.e. the neck and loins. 



The average length of the vertebral column is from 70 

 to 73 centimetres, or from 27 to 28J inches. Of this the 

 cervical part measures from 13 to 14 cm. ; the thoracic, 27 to 

 29 cm.; lumbar, 17 to 18 cm.; and the sacro-coccygeal, 12 

 to 15 cm. The individual differences in the length of the 

 column are less than one might expect, the variation in height 

 of different individuals being often largely dependent on the 

 length of the lower limbs. In the female the average length 

 of the column is about 60 centimetres, or 23 J inches, and the 

 curve in the lumbar region is usually more pronounced. 



DEVELOPMENT OF THE VERTEBRAL 

 COLUMN. 



The Cartilaginous Column. 



As has been already stated (p. 37), the neural tube 

 and the notochord are enveloped by a continuous sheath of 

 mesodermal tissue which forms the membranous vertebral 

 column. It is by the chondrification of this that the car- 

 tilaginous column is developed. This process commences 

 about the end of the first or the beginning of the second 

 month of foetal life. In correspondence with each vertebral segment, two 

 symmetrical nodules of cartilage appear on either side of the notochord; these 

 rapidly surround and constrict it. By their fusion they constitute the body of a 



