VIII 



VERTEBRAL COLUMN 



195 



(haemal arches} enclosing a haemal canal (Fig. 112, B). The apex 

 of each arch is prolonged into a median process or haemal spine. 

 Although not recognisable in the Dog-Fish, paired inter-ventral 

 cartilages, corresponding with the inter-dorsals above, are present 

 in some Elasmobranchs and alternate with the basi-ventrals. In the 

 caudal region of others (e.g. Skates and Rays) ventral counterparts 

 of the supra-interdorsals are present, and are termed infra-ventral 

 cartilages. Much in the same way that their dorsal equivalents 

 enclose a neural canal, so the ventral arcualia partially surround the 

 viscera-containing coelom in the trunk ; and in the tail, but more 

 completely, the 



sd. 



b,d. 



FIG. 112. A, transverse section of a precaudal vertebra; B, 

 similar section of a caudal vertebra, h.a, Haemal arch 

 (basi-ventrals) ; h.c, haemal canal ; h.s, haemal spine ; n.c, 

 neural canal. Other reference letters as in Fig. 111. 



vestigial coelom 

 of that region or 

 the haemal canal. 

 The different 

 vertebral compon- 

 ents are by no 

 means of equal 

 morphological 

 value. The basi- 

 dorsals and basi- 

 ventrals, and the 

 inter-dorsals and 

 inter-ventrals, are 

 the primary elements and the most important. The supra-dorsals 

 are merely cartilages segmented off from the basi-dorsals and inter- 

 dorsals, while the ribs and the infra-ventrals are similarly derived 

 from the basi-ventrals and inter-ventrals respectively. As to the 

 vertebral elements which collectively form a vertebra in the Dog- 

 Fish, it would seem from evidence afforded by the neurorneres, 1 and 

 more especially by the facts of development, that each complete 

 skeletal segment or vertebra consists of a pair of basi-dorsals with 

 the preceding pair of inter- dorsals, and of a pair of basi-ventrals 

 with the next succeeding pair of inter-ventrals. It must be 

 emphasised, however, that, considered as a joint or segment in a 

 flexible back-bone, a vertebra is a physiological unit, the morpho- 

 logical value of which may differ widely in different Fishes. 

 Hence, in other Fishes, the grouping of vertebral components to 



1 Neuromeres are body-segments defined and limited by the exits of the suc- 

 cessive pairs of spinal nerves from the neural canal. 



