IV. VERTEBRATA 



453 



(Cyclostome) and lower (figs. 511, 512), or neural and Immal 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. — ^\'ertebrrc of sturgeon, cli, notochord;/, exit of nerve; ;', dorsal and 

 ventral intercalaria; n, neural canal; ob, neural arch; 5, chordal sheath; r, rib; tih, ha:mal 

 arch. Bone white, cartilage dotted. 



rsFt 



S'i 



FiG. 513. Fig. 514. 



Fig. 513. — Caudal vertebrre of a carp, section (.1) and nearl)' side view (i?). rft, 

 space tilled by notochord; /?, htenial arch; n^ neural arch; oh^ neural spine; %ih^ hsnial 

 spine. 



Fig. 514. — Thoracic vertebra, ribs, and sternum of a mammal (from AVieders- 

 heim). Co, capitular head of rib; Co, neck of rib; Cj>, bony rib; Kn, cartilaginous rib; 

 Ps, spinous process; P<, transverse process (diapophysis) ; 5', sternum; Th, tubercular 

 head of rib; WK, vertebral centre. 



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

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

 region, in the same way, hmmal arches may be formed of htsmapophyses 

 and Jmnial spine, the arches surrounding the blood-vessels of the tail 

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



