IV. VERTEBRATA 



453 



(Cyclostome) and lower (figs. 511, 512), or neural and Iiamal arches. 

 These consist of paired parts in the skeletogenous layer which abut 

 against the notochord, and which are usually a pair to the somite, although 

 occasionally two or more pairs, the arches proper and the intercalaria, 

 may occur. The neural arches enclose a spinal canal surrounding the 



FIG. 512. Yertebnc of sturgeon. cJi, notochord;/, exit of nerve; /, dorsal and 

 ventral intercalaria; H, neural canal; ob, neural arch; s, chordal sheath; r, ril>; ul>, ha>mal 

 arch. Bone white, cartilage dotted. 



A 



PsPf 



FIG. 513. FIG. 514. 



FIG. 513. Caudal vertebras of a carp, section (.1) and nearly side view (7>). ch, 

 space rilled by notochord; h, haemal arch; n, neural arch; ob, neural spine; ul>, ha-nuil 

 spine. 



FIG. 514.- Thoracic vertebra, ribs, and sternum of a mammal (from \Yiedrrs- 

 heim). Ca, capitular head of rib; Co, neck of rib; ( '/>, bony rib; A'w, cartilaginous lib; 

 Ps, spinous process; Pt, transverse process (diapophysis) ; 5/, sternum; Tb, tulx-nular 

 head of rib; \VK, vertebral centre. 



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

 to form the spinous process (frequently independent). In the caudal 

 region, in the same way, lice mat arches may be formed of hamapophyses 

 and lucrnal spine, the arches surrounding the blood-vessels of tb" tail 

 (fig. 513). In the trunk region the ventral arch behaves diffrruuly. 



