68 



SKELETON 



elements lying above the ventral longitudinal blood-vessel. All 

 these structures become greatly modified in the higher and terres- 

 trial vertebrates. 



While the notochord itself decreases in importance as a skeletal 

 axis in the adult, the vertebral column becomes more and more 

 developed. Vertebral centra of some sort, either within or outside 

 the notochordal sheaths (p. 99), are almost always formed in con- 

 nection with the bases of the arches. The centra are therefore 

 primarily intersegmental, like the neural arches (Fig. 45). In the 

 caudal region, behind the abdominal coelom, corresponding haemal or 

 ventral arches meet below, enclosing a space in which run a caudal 

 artery and vein. Further forward the haemal arches often tend to 



fun 



md/. 

 Fig. 44. 



Skeleton and nerves of the head of Mustelus laevis, liisso (the nerves partly from Allis). be, 

 buccal nerve; d.g, dorsal branch of glossopharyngeal; d.r, dorsal root of spinal nerve; d.v, 

 dorsal branch of vagus ; e, labial cartilage; ij, glossopharyngeal; km, hyomandibular, the 

 spiracle is indicated by a dotted line in front, and the prespiracular cartilage is shown in front 

 of the spiracle ; h.n, hyomandibular nerve; hy, hypoglossal nerve; l.l, lateral-line branch of 

 vagus ; vi. r, Meckel's cartilage ; md, mandibular nerve ; md.f, mandibular branch of facial nerve ; 

 n.c, nasal capsule; opt, optic nerve; ot.f, otic branch of facial; p.q, palato-quadrate ; prb, 

 pretrematic nerve ; pro, profundus nerve ; pt.b, post-trematic nerve ; r, dorsal rostral cartilage ; 

 sop, superior ophthalmic branch of trigeminal and facial ; v, ventral rostral cartilage ; r.r, 

 ventral root; v.v, visceral branch of vagus. 



disappear, being represented by mere stumps ; or they may be 

 prolonged as separate ribs embracing the abdominal cavity, the 

 so-called pleural or ventral ribs, lying outside the peritoneum on 

 the inner surface of the myotomes (Fig. 40), (Bruch [66], Dollo 

 [119], Goppert [179], Claus [79]). True or dorsal ribs, on the 

 contrary, are situated above in the horizontal longitudinal septum, 

 which in all Gnathostomes separates the myotomes into dorsal 

 and ventral portions (Fig. 40). It is to A. Goette that we are 

 indebted for the clear exposition of this important distinction 

 [167]. 



The neural arch generally meets above the neural canal a 

 median neural spine situated in the longitudinal median septum. 

 The homology of the ' neural spines ' throughout the Gnathostomes 



