12 NATURAL HISTORY OF VERTEBRATES. 



cartilage forming its key, is merely membranous. Below the notochord is the princi- 

 pal artery, the aorta, with the trunk veins and the kidneys on either side. These lie 

 above and outside the coelom, a space which contains the viscera (in this region chiefly 

 the air bladder and the oesophagus), and a part of which is partitioned off as the peri- 

 cardial cavity to accommodate the heart. The lateral walls of the cceloni are foi-med by 

 muscles which attach themselves to the supporting cartilage of the fore fin, the 

 ' pectoral arch.' Fig. 12 shows how the chief parts of the permanent vertebra arise. 



Two cartilaginous haemal arches (called so on ac- 

 count of their relationship to the chief blood ves- 

 sel) jut outwards and backwards, and two neural 

 arches of the same tissue project upwards so as 

 partly to wall in the spinal cord. But bone is 

 beginning to appear chiefly in the membrane sur- 

 rounding the cartilage (perichondrium) and is ex- 

 tending also in the rest of the membranous wall 

 round the spinal cord. In this way are formed the 

 essential elements of a vertebra, viz. a central 

 part surrounding the notochord, the body or cen- 

 trum; two lateral parts, the neural arches, which 

 serve to protect the spinal cord, and are aided in 

 „ , , , this by a dorsal neural spine: and, finally, two 



Fig. 12.— Section through vertebra of young •' . . 



gar-pike ; ha, cartilaginous haemal and (no) haemal arches continuous with the ribs, which, in 



neural arches; the surrounding perichon- 

 drium is giving rise to bone continuous with their turn, help to strengthen the wall of the 

 the osseous neural arch (na) developed in the -.,.,« . . 



membranous -wall of the spinal canal («p) ,• coelom, and may, in higher lorms, unite in a 



el, elastic ligament ; nc, notochord. ■, f <• 



breast-bone or sternum on the ventral suriace or 

 that cavity. Further elements of the vertebrae are present in higher forms, serving 

 for the purpose of securing union between the arches of contiguous vertebrae 

 (articular processes), or as additional points of support for the ribs (transverse 

 processes), or for the attachment of muscles. The edges of the centra likewise occa- 

 sionally develop articular processes, but as a rule the connection between contiguous 

 centra (except where they are fused together) is effected by ligaments. 



The extent to which the notochord persists varies greatly in different groups. 

 Teleosts present little advance upon the stage of the gar-pike described above, the 

 individual vertebrae being deeply hollowed on either face (amphicoelous), and retaining 

 in the hollows a very considerable amount of notochord, while in the gar-pike itself 

 a ball-and-socket joint forms between contiguous vertebrEe, the sockets occupying 

 the hinder faces (opisthocoelous vertebra). Little or none of the notochord is left 

 in the interior of each vertebra, and this is likewise frequently the case in reptiles, 

 where the socket often occupies the anterior face of the vertebra, which is then styled 

 proccelous. In most Mammalia a special provision is made for the slow attainment 

 of the adult length of the vertebral column, in that the faces of the vertebrae are fur- 

 nished with ' epiphyses,' separate plates of bone, between which and the centra proper 

 a zone of cartilage allows for additions to the length, until, when the adult size is 

 attained, the epiphyses fuse with the centra and then form their faces. In the 

 mammals also well-formed intervertebral discs of fibro-cartilage unite the faces of 

 contiguous vertebrse, and contain some trace of the notochord, even in the adult 

 condition. 



An important difference between the vertebral column of higher and lower forms 



