THE CAKTILAGINOUS COLUMN. 



103 



cartilaginous vertebra, and are so disposed that they alternate in position with the 

 muscle plates which are lying on either 'side. In this way a vertebral body corre- 

 sponds in position to the caudal half of the anterior myotome, and the cephalic 

 half of the posterior myotome, the intermyotomic intervals, which contain the 

 connective tissue plates separating the muscle segments, lie in line laterally 

 with the mid -points of the sides of the cartilaginous vertebrae. It is by chondri- 

 fication of these intersegmental layers that in certain regions the ribs are 

 ultimately developed. Meanwhile, the scleratogenous tissue between the chondri- 

 fying vertebral bodies undergoes little change and persists as the intervertebral 

 fibro-cartilage. Here the embedded notochord undergoes but slight compression and 

 enlarges, so that if a length of the column be examined in longitudinal section 

 the notochord displays a moniliform appearance, the constricted parts correspond- 

 ing to the bodies, the enlarged portions to the fibre-cartilages. The former 

 disappear at a later stage when ossification begins, but the latter persist in 

 the adult as the pulpy core in the centre of the intervertebral fibro-cartilage. 



The portions of the scleratogenous tissue which lie lateral to the notochord 

 have next to be considered ; these extend dorsalwards around the vertebral canal, 

 and ventralwards beneath the notochord. The former is sometimes called the 

 vertebral bow, the latter the hypochordal bow. The vertebral bow begins to 

 chondrify on each side, and forms the lateral portions of the cartilaginous vertebral 



Vertebral canal 



Vertebral bow 



Notochord 



Sheath 

 Hypochordal bow 



Body of vertebra 



Hypochordal 



bow 



Notochord 



^ 



FIG. 117. THE DEVELOPMENT OF THE MEMBRANOUS BASIS OF A VERTEBRA (after Keith). 



A, in transverse section. B, in horizontal section, showing the relation of the vertebrae to the 



primitive segments. 



arch, the extremities of which usually unite dorsally about the fourth month of 

 foetal life ; if from defective development this union should fail to occur a 

 deformity known as spina bifida is the result. 



From the cartilaginous vertebral arch, so formed, arise the chondrified rudiments 

 of the spinous, transverse, and articular processes. 



The chondrification of the vertebral arch is variously described as being in- 

 dependent of the body or an extension from it ; in any case, union between it and 

 the body is rapidly effected. 



The scleratogenous tissue between the cartilaginous vertebral arches which does 

 not undergo chondrification persists as the ligaments uniting the vertebral laminae. 



As regards the so-called hypochordal bow, for the most part it disappears. 

 By some it is regarded as being represented by a fibrous strand in' the inter- 

 vertebral fibro-cartilage on the cephalic side of the vertebra to which it 

 belongs. It is, however, noteworthy that in the case of the atlas vertebra there 

 is an exception to this arrangement; for here the hypochordal bow chondrifies 

 and subsequently by ossification forms the anterior arch of that bone an arch 

 which lies ventral to, and embraces the dens of the epistropheus (q.v. p. 91). 



It is only in the thoracic region that the ribs, developed as stated above by the 

 chondrification of the intersegmental septa, attain their full dimensions. In the 

 cervical, lumbar, and sacral regions they exist only in a rudimentary or modified 

 form, as has been described elsewhere. In the construction of the chest wall the ribs 

 are supported ventrally by the sternum, as to the development of which there is some 

 difference of opinion. Euge has described this bone as formed by the fusion of two 

 cartilaginous bands produced by the coalescence of the expanded ends of the first five 



