440 THE DOGFISH CHAP, 



calcified bars also extend longitudinally through each 

 centrum from face to face, so as to show a radial arrange- 

 ment in transverse section. The biconcave intervals 

 between the centra are filled with a soft intervertebral 

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

 first vertebra and the skull and which represents 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. 113 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 intercalary 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 



