210 COMPARATIVE ANATOMY 



cylinder of cartilage, deeply excavated at both ends so that it 

 is biconcave, or amphicoelous (see vol. i. p. 22). The con- 

 cavities are connected by a small canal running through the 

 middle of the centrum, and this canal and the concavities are 

 occupied by the remains of the notochord, which is therefore 

 continuous from end to end of the vertebral column. As the 

 biconcave centra are united end to end by ligaments, it is 

 evident that within each centrum the notochord is constricted 

 to a mere thread, but between two centra it swells up to 

 occupy the whole space formed by two apposed concavities. 

 In technical language the notochord is constricted intra- 

 vertebrally and enlarged intervertebrally. 



From the upper sides of each centrum, at the middle of its 

 length, two short neural processes project upwards. To the 

 upper edge of each neural process a pentagonal plate of 

 cartilage, the vertebral neural plate, is fused, and the spaces 

 between successive vertebral neural plates are filled in by 

 hexagonal intervertebral neural plates, the two sets of plates 

 forming the sides of the neural arch, while the centra form the 

 floor. The arch is completed above by a number of wedge- 

 shaped pieces of cartilage, the neural spines, which nil in the 

 spaces between the upper angles of the vertebral and inter- 

 vertebral neural plates. The lower posterior margins of the 

 vertebral plates are notched to admit the passage of the 

 ventral roots of the spinal nerves, and the upper posterior 

 margins of the intervertebral plates are similarly notched for 

 the exit of the dorsal roots. 



In the most anterior vertebrae the haemal arches are mere 

 ridges projecting horizontally from the lower sides of the 

 centra. Further back they become more prominent and form 

 distinct transverse processes, having small cartilaginous ribs, 

 about half-an-inch in length, articulated to their distal ends. 

 At about the 3oth vertebra the transverse processes begin 

 to bend downwards so as to enclose a groove on the ventral 

 surface of the centra, and at the 42nd vertebra the groove 

 is converted into a canal by the processes meeting below and 

 forming a complete haemal arch, which in life contains the 

 caudal artery and vein. The haemal arches of the last 48 

 or 50 vertebrae are produced ventrally into laterally com- 

 pressed haemal spines, which afford support to the lower lobes 

 of the caudal fin. 



