DEVELOPMENT OF THE VERTEBRAL COLUMN. 



953 



tebrre, so that already iu the eighth week, a complete cartilaginous column 

 with the membranous intervertebral ligament is formed ; the chorda dorsalis 

 now becomes attenuated and begins to disappear in the bodies of the verte- 

 bra) (Fig. 354), whereas it still remains distinct in the Ligamenta iuterver- 

 tebralia, and consequently presents a moniliform appearance. The formation 



FIG. 354. 



FIG. 355. 



FIG. 354. Diagram showing the attenuation of the Chorda Dorsalis in the middle of the bodies of 

 the vertebra; whilst preserving its original diameter in the iutervertebral spaces : ch, chorda dorsalis; 

 v, body of vertebra ; li, intervertebral substance. 



FIG. 355. Diagram showing the position of the chorda dorsalis in the body of the vertebra and the 

 formation of the neural arches; eh, chorda dorsalis; cv, body of the vertebra; a, neural arch or neur- 

 apophysis ; c, rib ; pr, transverse process. 



of cartilage, which, according to Robin, 1 is everywhere developed before the 

 substance of the brain and spinal cord, takes place much more slowly in the 

 vertebral arches, not being more advanced at the eighth week than is shown 

 in Fig. 355 ; so that at this period the medulla spiualis and its ganglia are 

 only covered by the membrana reuniens superior. The arches first meet and 

 inclose the medulla in the dorsal region ; in the fourth mouth this is com- 

 pleted throughout the column, and, except in the coccygeal region, where no 

 arches are developed, the form of the future vertebrae, in which ossification 

 has already commenced, is perfect. The development of the atlas is irreg- 

 ular, its body, which is originally traversed by the chorda dorsalis, as shown 

 by Rathke and Robin, becoming connected, not with its proper arch, but 

 Avith the body of the axis forming the odontoid process of this vertebra. 

 The ossification of the vertebrae begins at the end of the second or the begin- 

 ning of the third month, with three centres for each vertebra, one for the 

 body and one for the arch on each side. These parts, however, do not unite 

 till the second year after birth. Accessory centres of ossification are subse- 

 quently formed at the tips of the spiuous and transverse processes, and in 

 the form of thin leaves at the upper and lower surfaces of the bodies, which 

 resemble the epiphyses of the long bones. The cartilages of the ribs do not 

 develop at once in their whole length, but gradually from the spine towards 

 the sternum. Their ossification proceeds contemporaneously with those of 

 the vertebrae from one centre for each rib, the anterior extremities of the 

 upper seven uniting together to form a broad cartilaginous band, which con- 

 stitutes one lateral half of the sternum, the two halves subsequently uniting 

 from above downwards. An arrest of this mode of development explains 

 the monstrosity called " Fissura Sterni," a case of which, iu the person of 

 M. Groux, excited so much interest in this country. The morphological 

 relations of the several parts constituting the shoulder girdle in man appear, 



1 Eobin's Journal de 1'Anatomie, vol. i, 1864, pp. 274-299. 



61 



