VERTEBRAL COLUMN 49 



calcified rings arc also developed in the sheath : these are con- 

 siderably more numerous than the arches. The latter remain 

 cartilaginous in the Cartilaginous Ganoids and Holocephali, but 

 become densely ossified in the Dipnoi (Fig. 37). The upper arches 

 may be completed above by neural spines. In the caudal region 

 the hcemal arches usually enclose the caudal aorta and vein com- 

 pletely ; further forwards the cartilages do not meet in the middle 

 line below, thus only surrounding the coelome to a slight extent, and 

 consequently the lower arches end on either side in a laterally- 

 directed cartilaginous projection, the transverse process or " basal 

 stump." 



The relations of the arches in Plagiostomes, Bony Ganoids, and 

 Tcleosts are similar to those described above, but their structure 

 is more complicated in all Fishes than is there indicated. 



The upper arches, which in many Fishes are not closed in 

 dorsal ly, 1 consist on either side of several distinct elements, which 



06 Je 



asg^spsppigsiEg^^^'ltSJSIS^'" 1 



. , 



Jr 



FK;. 38. PORTION OF THE VERTEBRAL COLUMN OF Scymnm. 



Ic, intercalary pieces (interdorsals) ; Ob, neural arches (basidorsals) ; WK, 

 centra. The apertures for the roots of spinal nerves are shown. 



are most plainly distinguishable in Cartilaginous Fishes. One of 

 these (directly above the centrum where such is developed) may 

 be described as the basidorsal or neural plate, and is usually 

 perforated by the foramen for the motor root of a spinal nerve. 

 Intercalated between successive basidorsals is another cartilage, 

 the interdorsal (intercalary piece, or interneural plate}, through 

 which the sensory root of a spinal nerve, situated anteriorly to the 

 corresponding dorsal root, usually passes : both these cartilages 

 may meet above, so as to complete the arch (Fig. 38). In some 

 cases, more than one intercalary piece is present on either side, 

 and frequently another series of cartilages becomes segmented off 

 from the basidorsals and interdorsals respectively to form the 

 keystone of the arch : these give rise in the median line to more 

 or less marked neural spines. The bases of this series of upper 

 intercalary pieces or supradorsals fit in alternately between the 



1 A longitudinal elastic ligament is constantly present in this region (Figs. 

 36 and 42) and also in relation with the ventral arches. 



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