BLOOD-VASCULAR SYSTEM OF THE LORICATI 79 



artery, but upon emerging from this foramen rapidly increases 

 in caliber. Then continuing caudad it passes over the efferent 

 branchial arteries, and when the head kidney is reached follows 

 along its ventral surface and terminates by anastomosing with 

 the corresponding cardinal vein to form the great precaval 

 trunk. Throughout its short course it receives numerous 

 branches from the dorsal branchial muscles and the head kidney, 

 which will be described in detail after considering the 3 princi- 

 pal trunks which go to make up the jugular vein. 



{a) External Jugular Veins {^\^. I and II, figs, i, 5 and 15 ; 

 Ex.J.V.). — Of the 3 vessels which unite to form the jugular 

 vein this is the largest. It also arises from 3 rather large 

 trunks, the largest of which is the facialis-mandilmlaris vein 

 (Pis. I and II, figs, i and 12; F.Man.V.). This vessel has 

 its source in the anterior part of the lower jaw^ from the genio- 

 hyoideus vein (Pis. I and II, figs, i and 12; Ghs.V.), which 

 runs along the ventral surface of that muscle just outside of the 

 corresponding artery, which is a branch of the left hyoidean 

 artery. The facialis-mandibularis vein at first passes along 

 the inner side of the dentary bone, receiving numerous branches 

 from the mandibular portion of the adductor mandibulse muscle. 

 Shortly before leaving the articular bone it receives a large 

 secondary mandibular vein, coming from the ventral side of 

 the muscle, and a posterior branch coming from the inner side 

 of the quadrate bone. The facialis-mandibularis vein then 

 makes a dorsal bend ; leaving the corresponding artery it fol- 

 lows up behind the ramus mandibularis V, or ramus maxillaris 

 inferior of other authors, between the superficial and profundus 

 portions of the adductor mandibular muscle, receiving several 

 rather large branches from each. At the level of the levator 

 arcus palatini muscle it receives, from the rear, the h3'oidean 

 vein. 



Hyoidean Veins (Pis. I and II, figs, i and 12; Hyo.V.) — 

 These vessels have their origin in the hyohyoideus superior 

 muscles. Each vein runs along in that muscle some little dis- 

 tance ventrad of the hyoidean arter}', which follows along on 

 the surface of the arch. The vein receives a branch from the 

 region of each branchiostegal ray and when the end of the arch 



