440 THE DOGFISH CHAP. 



faces densely calcified ; calcified bars also extend longitudin- 

 ally through each centrum from face to face, so as to show 

 a radial arrangement in transverse section. The biconcave 

 intervals between the centra are filled with a soft inter- 

 vertebral substance (Fig. 113, ntc), which is also present 

 between the first vertebra and the skull and which re- 

 presents part of the embryonic notochord (see pp. 203 

 and 418). The centra are united by ligament, so that the 

 whole vertebral chain is very flexible. 



In the frog it will be remembered that the completely ossified centra 

 are proccelous, and are articulated with one another (pp. 36 and 57). 



Connected with the dorsal aspect of the series of centra 

 is a cartilaginous tunnel, consisting of the neural arches . 

 (Figs. 1 13 and 117, n. a] enclosing the spinal cord: it is 

 divided into segments, corresponding with, but usually twice 

 as numerous as the centra, owing to the presence of inter- 

 calary pieces. Arising from each centrum on either side is 

 a neural process which is fused with a neural plate 

 (Fig. 113, n.p) and perforated posteriorly for the exit of 

 the ventral root of a spinal nerve (for) ; and fitting between 

 two consecutive neural plates is an intercalary piece, the 

 interneural plate (i. n. p), perforated by an aperture for the 

 dorsal root of a spinal nerve (for). The arch is completed 

 above by the neural spines (n. sp\ which fit in between the 

 neural and interneural plates respectively, and are thus, like 

 the lateral elements of the arch, twice as numerous as the 

 centra. The first vertebra has facets for articulation 

 with the condyles of the skull. 



In the anterior part of the vertebral column the centra 

 give off paired, outstanding transverse processes (Fig. 113 

 B, tr. pr\ to the end of each of which is articulated a 

 short, cartilaginous rod, the rib (r). Further back ribs 

 are wanting, the transverse processes are directed down- 



