730 DEVELOPMENT OF THE VERTEBKAL COLUMX. 



tliro'wn into connection w4th the anterior half of the one next following, and 

 tlius each permanent body is formed from imrts of two protovertebral masses ; 

 ■while in respect to the arches, each one proceeds from the hinder segment of the 

 anterior of the two protovertebra3 concerned, the spinal gangiion and root being 

 thrown into connection with the hinder part of the permanent vertebra imme- 

 diately in front of the protovertebra of which they originally formed a part. 



Formation of Vertelbral Matrices. — While the material for the 

 vertebral bodies is laid dowu round, tlie notochord, a further extension 

 of mesoblastic substance from the primordial vertebral plates takes place 

 at the sides and round the medullary cavity for the matrix of the 

 vertebral arches, and in due course, by differentiation of the formative 

 cells, chondrification of the substance occurs in the form of the strips 

 which constitute the first rudiments of the vertebral arches, and the 

 accompanying transverse and other processes. The first ossification of 

 these bones is from cartilage, but doubtless in them, as in other bones, 

 much of the subsequent growth and extension of the bone substance 

 proceeds from sub-periosteal deposit. It is also to be remarked that 

 in some bones originating in membrane, cartilage may subsequently 

 contribute to the growth and extension of the bone, as ajspears to occur 

 in the lower jaw and clavicle. 



The chondrification of the formative matrix of the bones in the 

 human embryo takes place chiefly during the fifth and sixth weeks of 

 foetal life, and in the seventh and eighth, ossification has begun in 

 several of the long bones. But even before this time an ossific deposit 

 shows itself in the fibrous matrix of the clavicle and lower jaw. By 

 the ninth week the greater number of the bones have begun to ossify. 



The formation of cartilages for the arches of the vertebrae begins first 

 in those of the dorsal region, and spreads from these forwards into the 

 cervical vertebrae and basis of the skull, and backwards * into the 

 lumbar and sacral vertebrae : but the extension of the matrix upwards 

 ceases in the lower sacral and coccygeal region where the arches are 

 deficient. 



A small cartilaginous band forms the matrix of the subcentral por- 

 tion or anterior arch of the atlas vertebra, quite distinct from that of 

 the body of the axis, and out of the line of prolongation of the noto- 

 chord. 



In the lateral plates the cartilaginous matrices of the ribs are formed 

 in connection with those of the transverse processes, and in the verte- 

 bral part of the ribs themselves ossification is comparatively early ; but 

 a considerable part remains unossified in the sternal portion, or costal 

 cartilages, in connection with their special use in the mechanism of the 

 respiratory movements. 



Certain portions of the transverse parts of the cervical and lumbar 

 vertebrae are undoubtedly homologous with ribs ; but we give the name 

 only to those costal bars which are separately articulated to the verte- 

 bra, and the first of the vertebras with which a rib reaching the sternum 

 is articulated is reckoned as dorsal. Among the thoracic ribs a certain 

 number, as elsewhere stated, of the cartilaginous matrices behind the 

 first, are in the commencement united together at their ventral extre- 



Hci'fi aud elsewhere, unless otherwise explained, the terms used to indicate position 

 npply to the primitive proue position of the embryo as it lies in the blastoderm, the 

 dorsal aspect upwards aud the veutral dowuwiu-ds. 



