THE SKELETON OF THE CODFISH. VERTEBRAE. 85 



The space between one centrum and the next is in the fresh 

 skeleton filled up by the gelatinous remains of the notochord. 



The first few vertebrae differ from the others in having 

 very short centra and no ventri-lateral processes. 



The first vertebra comes into very close relation to the 

 posterior part of the skull, articulating with the exoccipitals. 

 In the next few vertebrae the centra gradually lengthen, 

 and at the fourth or fifth vertebra the ventri-lateral pro- 

 cesses appear and gradually increase in size as followed back. 

 They likewise gradually come to arise at a lower level on the 

 centrum, and also become more and more downwardly directed, 

 till at the last trunk vertebra they nearly meet. 



The neural spines of the anterior trunk vertebrae are 

 much longer than those of the posterior ones, that of the first 

 vertebra being the largest and longest of all, and articulating 

 with the skull. The spinal nerves pass out through wide 

 notches or spaces between the successive neural arches. 



Caudal vertebrae. 



The caudal vertebrae are about thirty -five in number, 

 each consists of a centrum with a slender backwardly-directed 

 dorsal or neural arch, similar to those of the posterior trunk 

 vertebrae. The two halves of the ventral or haemal arch 

 however do not form outwardly-directed ventri-lateral pro- 

 cesses, but arise on the ventral surface of the centrum, and 

 passing downwards meet and enclose a space; they thus 

 form a complete canal, and are prolonged into a backwardly- 

 directed ventral or haemal spine. The anterior haemal 

 arches are much larger than the corresponding neural arches, 

 but when followed back they gradually decrease in size, till at 

 about the twenty-fourth caudal vertebra they are nearly as 

 small as the neural arches. The last caudal vertebra is suc- 

 ceeded by a much flattened hypural bone or urostyle, which 

 together with the posterior neural and haemal spines supports 

 the tail-fin. 



