VERTEBRAL COLUMN 377 



vertebral substance. The centra are united by ligament, so 

 that the whole chain of discs is very flexible. Connected 

 with the dorsal aspect of the series of centra is a cartila- 

 ginous tunnel, arching over the spinal cord : it is divided 

 into segments, corresponding with, but usually twice as 

 numerous as the centra, and called the neural arches 

 (Fig. 101, n. a; Fig. 99, N. A). 



In the anterior part of the vertebral column the centra 

 give off paired outstanding processes (Fig. 101, A and B, 

 tr. pr) called transverse processes, to the end of each of 

 which is articulated a short cartilaginous rod, the rib. 

 Further back the transverse processes are directed down- 

 wards, instead of outwards, and in the whole caudal or tail 

 region they unite below, forming haemal arches (Fig. 101, c 

 and D, h. a ; Fig. 99, A, H. A), which together constitute a 

 kind of inverted tunnel in which lie the artery and vein of 

 the tail. In the region of the caudal fin the haemal arches are 

 produced into strong median hamal spines (Fig. 99, A, H. A 

 to the right), which act as supports to the fin. A centrum, 

 together with the corresponding neural arch and transverse 

 processes or haemal arch, forms a vertebra or single segment 

 of the vertebral column. 



It should be noticed that in the vertebral column we 

 have another instance of the metameric segmentation of the 

 vertebrate body. The vertebrae do not, however, correspond 

 with the myomeres, but alternate with them. The myo- 

 commas are attached to the middle of the vertebrae, so that 

 each myomere acts upon two vertebrae and thus produces 

 the lateral flexion of the backbone. 



In the embryo, before the development of the vertebral 

 column, an unsegmented gelatinous rod, the notochord, lies 

 beneath the neural cavity in the position occupied in the 

 adult by the line of centra, by the development of which it 



