IV. VERTEBRATA. 



517 



of human anatomy) enclose a spinal canal surrounding the spinal 

 cord, the parts of the arch, neurapophyses, uniting above the cord 

 to form the spinous process (frequently an independent part of the 

 skeletal axis). In the caudal region, in the same way, hcemal 

 arches may be formed of licemapopliyses and hcemal spine, the arches 

 surrounding the blood-vessels of the tail (fig. 557). In the trunk 

 region the ventral arch behaves differently. Since the large body 



n 



FIG. 556. Vertebrae of sturgeon, ch, notochord; /, exit of nerve ; j, dorsal and ventral 

 intercalaria ; n, neural canal: o7>, neural arch; s, chordal sheath; r, rib; ub f 

 haemal arch. Bone white, cartilage dotted. 



A. 



FIG. 557. FIG. 558. 



FIG. 557. Caudal vertebrae of a carp, section (.4) and nearly side view (B). cli, space 

 filled by notochord; /t, haemal arch; ?i, neural arch; 06, neural spine ; ub, haemal 



FIG. 558. Thoracic vertebra, ribs, and sternum of a mammal. (From Wiedersheim.)' 

 Co, capitular head of rib ; Co, neck of rib; Cp, bony rib; Kn, cartilaginous rib : Ps, 

 spinous process ; Pt. transverse process (diapophysis) ; St, sternum ; T6, tuber- 

 cular head of rib; WK, vertebral centre. 



cavity with its viscera, varying in size (digestive and reproductive 

 organs), is here, the haemapophyses extend outwards and down- 

 wards and are divided into two parts, a basal apopliysis and a, 

 lower movable portion, the rib (fig. 556). Also the lower union 

 of haemapophyses with haemal spine does not occur; the ribs ar& 

 either free (fishes) or are (at least in part) connected ventrally by 



