VERTEBRAL COLUMN'. 



125 



wedges into the centrum. These calcified wedges are formed in a fibrous 

 cartilage or fibrous tissue and contain blood vessels. Calcifications are 

 sometimes found on the surface of the arches or even in their tissue. 



It is often difficult to determine in any given vertebra whether the arches 

 grow round and reinforce the centrum unless the elastica externa persists, 

 and there seems to be considerable variation in this respect in different 

 animals and even in different parts of the vertebral column of the same 

 animal. In Hexanchus, Heptanchus (anterior), Myliobates (posterior part), 

 Rhinobatus (posterior part), Trygon and Squatina the arches remain separate 

 and do not grow round the chordal sheath. In Heptanchus (posterior), 

 Centrophorus, some Spinacidae and Scymnidae, Squatina, many Rajidae 

 the arches grow round and reinforce the centrum. In the Carchariidae 

 and Lamnidae the centrum is strongly reinforced by perichondrial calcifi- 

 cation and the cruciform appearance in section is very marked, but it is 

 not clear whether the arches contribute to the centrum. 



The neural and haemal arches are always segmented. The 

 neural arch is 

 generally incom- 

 plete, being sup- 

 plemented by the 

 intercalated pieces 

 (Fig. 67), and there 

 are often some 

 special supra- dorsal 

 pieces which form 

 the actual roof. In 

 some cases (Alo- 

 pecias] both neural 

 arches and /""intercalated pieces extend across the roof . 

 of the spinal canal (Fig. 70). ( The anterior root of a spinal 

 nerve frequently perforates the neural arch, the posterior 

 root the intercalated piece, or the nerves may pass out 

 between the arch and the intercalated piece^ In the trunk 

 region the vertebrae carry short dorsal spinous processes, but 

 in the caudal region these may attain some length. There is a 

 longitudinal elastic ligament extending along the ridge of the 

 upper arches ; it may be embedded in the cartilage. 



Sometimes (Fig. 71) more than one pair of intercalated pieces may occur 

 in each vertebra, and the number may vary in different parts of the verte- 

 bral column of the same animal. In Zygaena there are two. It frequently 

 happens that in the caudal region there are two complete vertebrae (cen- 

 trum, arch, intercalated pieces, etc.) for every spinal nerve (diplospondyly).* 



* W. Ridewood, " Caudal diplospondyly of Sharks," Journ. Linn. 

 Soc., 27. 



FIG. 69. Diagrammatic transverse sections of vertebrae, to 

 illustrate A the cyclospondylous ; B the tectospondylo'us ; 

 and C the asterospondylous condition. C notochord; 

 I) central calcareous ring ; elastic externa ; S neural, 

 H haemal arch (from Hasse). 



