VASCULAR SYSTEM 



in 



rpc- 



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 rpv. *"' 

 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). 



nrliVmlnY' arnli ilen ie Diagram of the venous system of Mustelnx 



s antarcticus (after T. J. Parker), ventral view, o, 



off hplow with t,hp atrium ; c, conus ; co.s, orbital sinus ; c.v, caudal 



)W, \ Dne ve . n; ^ -c ; dnctus ' Cuv ' ieri ; d.c. V , hinder portion 



development Of the jaWS, and of dorsal cutaneous vein if.v, vein from pelvic tin ; 



,1 p , r i h.p.v, hepatic portal vein passing up into liver ; 



L>ne reduction OI t/ne SpiraCUiar j^ t g hyoid sinus * i.? inferior juirular * i/.v iliac 



slit, so that the vessels in this ^{f^^^^^i^^iefe 



region Of the head become posterior cardinal ;l.v, lateral vein; p. v.c.v, posterior 



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



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



(~\ 4- ' U4-1, 4.1. l U l ,' sinus; sp, spermatic vein; st, stomach ; t, testis; 



OsteichthyCS the hyoid arch IS r> ven ' tri cie; ; , vein joining portal to posterior 



likewise interrupted below, cardinal. 



and the hyoidean hemibranch 



then receives blood only from the efferent vessel of the next 



rpc." 



lev.-"' 

 fv.-" 



-dcv. 



cv. 



FIG. 70. 



