zygapophyses 



intercentrum 



groove of segmental artery 



ligament canal 



neural canal 



neural arch 



notochord cona 



haemal canal 



intercentrum 



B 



Figure 6-25. Vertebral structure of Eusfhenopteron. A, lateral view of two thoracic vertebrae; B, 

 anterior view of a thoracic; C, anterior view of a caudal vertebra showing neural and hemal arches. 

 (After Jorvik, 1952) 



sclerotome material, and this is mainly in the region of what 

 is to become the neural arch. The notochord has a distinct 

 sheath with fibrous and external layers. The first evidence 

 of the skeleton is the appearance of the neural arches fol- 

 lowed by the hemal arches. The cartilaginous dorsal and 

 the ventral arches, or arcualia, lie in contact with the noto- 

 chord. and a perichordal tube encloses their bases (Figure 

 6-27). These arches lie mainly in front of the myoseptum, 

 but extend partly into the area behind the myoseptum. In 

 some cases, a secondary procartilaginous center is observed 

 behind the ventral arch center. This interventral center 

 unites with the anterior arch. The vertebral body ossifies 

 without a cartilaginous precursor in the perichordal con- 

 nective tissue enclosing the bases of the arches. The dorsal 



arch, ossifying perichondrally, generally fuses to this body. 

 The ventral arch unit forms the parapophysis of the hemal 

 arch, ossifies perichondrally. and fuses to, or remains sepa- 

 rate from, the body. In the process of development, the noto- 

 chord is constricted within the body and eventually is 

 reduced to a strand of tissue. There is a large ovoid inter- 

 vertebral mass of connective tissue and notochord in the 

 adjacent ends of the amphicoelous vertebrae. 



In teleosts, the notochord may be much constricted, or 

 only slightly so. In some teleosts (Clupea and Lebistes), the 

 outer part of the notochordal sheath ossifies, but this is, 

 probably, not usually the case. The notochord is, as a rule, 

 modified in terms of its internal structure; there are large 

 internal spaces, or strands and patches of denser tissue. 



suprodorsol 



Figure 6-26. Vertebral structure of a salmon (Blue-Backed Salmon). A, lateral view of anterior trunk 

 vertebrae; B, anterior view of a trunk vertebra; C, cross section through the middle of the centrum 

 of a trunk vertebra showing the roots of the orches; D, caudal vertebrae as seen from the side. 



THE VERTEBRAL COLUMN 



159 



