VASCULAR SYSTEM 



rudiments (Mayer [297a]), 

 runs forward below the 

 branchial skeleton (Fig. 71). It 

 provides five afferent branchial 

 arteries, the first of which 

 passes up the hyoid arch 

 to supply its posterior hemi- 

 branch. These afferent vessels 

 and the efferent vessels are 

 partially derived from, and 

 replace, the embryonic com- 

 plete aortic arches, six in 

 number, running up the man- 

 dibular, hyoid, and succeeding 

 four branchial arches. The 

 paired rudiments of the dorsal 

 aorta unite to form a single 

 median vessel behind ; but in 

 front they diverge and join 

 again below the brain to form 

 in the embryo a complete 

 circle, the circulus cephalicus 

 (Fig. 72). It is this region 

 which receives the efferent or 

 epibranchial vessels. The con- 

 tinuity of the circulus is usu- 

 ally interrupted behind the 

 hyoid arch in later develop- 

 ment in Selachians, and indeed 

 in all Gnathostomes except 

 the Teleostei (Fig. 72). In 

 Selachians the circulus dis- 

 appears, the aorta becoming- 

 single and median to below 

 the base of the skull (Fig. 71). 

 The mandibular arch also is 

 broken off below, with the 



-dcv. 



pvcv. 



Fii 



TO. 



Diagram of the venous system of Mustelus 

 untarcticus (after T. J. Parker), ventral view, a, 

 atrium ; c, eonus ; co.s, orbital sinus ; c.v, caudal 

 vein; d.c, ductus Cuvieri ; d.c.v, hinder portion 



development of the jaws, and of dorsal cutaneous vein ;f.v, vein from pelvic fin.; 



,i i , • ; j • i h.ii.v, hepatic portal vein passing up into liver; 



the reduction of the spiracular 7 ,/ Si n yoid sinus; i.j, inferior jugV 



slit, so that the vessels in this 

 region of 



ular ; il.v, iliac 

 vein; int.v, intestinal vein ; j.v, jugular vein; k, 

 kidney ; l.e.v, right lateral cutaneous vein ; l.p.c, left 

 the head become posterior cardinal; l.v, lateral vein ;p.».c.t>,posterior 

 , .... , T ventral cutaneous vein ; r.p.e, right posterior car- 



mUCll modified. In most dinal; r.p.v, right renal portal vein; s, hepatic 



CktpiprirVivp* thp hvm'rl nrrh is sinus; ^' s P ermatic v, ' in : st > stomach ; /.testis; 

 v^ujiciiLiijLb tue uyuiu cUCii lb v> ventricle ; vn, vein joining portal to posterior 



likewise interrupted below, cardinal. 



and the hyoidean hemibranch 



then receives blood only from the efferent vessel of the next 



