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. 
(a2) External Jugular Veins (Pls. I and II, figs. 1, 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 faczalis-mandibularis vein 
(Pls) and) Ut} fios. 35 and) 22);'H.Man.V.).| This vessel’ has 
its source in the anterior part of the lower jaw from the genzo- 
hyowdeus vein (Pls. I and II, figs. 1 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 mandibule 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 mandibule muscle, receiving several 
rather large branches from each. At the level of the levator 
arcus palatini muscle it receives, from the rear, the hyoidean 
vein. 
Hyoidean Veins (Pls. I and II, figs. 1 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 artery, 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 
