BLOOD-VASCULAR SYSTEM OF THE LORICATI 87 
muscle. Then continuing caudad, behind the superior rectus 
muscle and mesad of the truncus supra-orbitalis, it arrives in the 
posterior dorsal corner of the orbit, where it receives the sc/e- 
rottc vern (Pl. Il, fig. 15; Scl.V.). This vessel, which arises 
from the adipose tissue in the region of the anterior part of the 
eyeball, runs obliquely caudad across it, mesad of the corre- 
sponding nerve and artery. After receiving this branch, the 
main orbito-nasal vein crosses above the ophthalmic and iris 
vessels, and following around the eyeball for a short distance, 
finally comes in between and unites with the external and 
internal jugulars to form the great jugular vein. 
The remarks made under the summary of the carotids apply 
with equal force to the external and internal jugular veins. 
These are simply arbitrary names given to the 2 largest veins 
of the head region, which go to make up the common jugular 
trunk. 
2. Vessels Emptying Directly into the Jugular or into the 
flead Kidney. 
a. Veins Emptying into the Kidney. —'These veins include 
the posterior encephalic and the first and second neural veins. 
They do not empty at once into the jugulars, but penetrate the 
dorsal surface of the head kidney, break up into smaller ves- 
sels, which become reunited forming the renal veins, and these 
empty into the jugular vein. 
Posterior Encephalic Veins (Pls. II and III, figs. 16, 23 to 
25; P.Enc.V.).— These veins may be said to concur in part 
with the first neural or vertebral artery. Each of these veins 
arises from a superficial capillary network from the dorsal sur- 
face of the optic lobes; passing caudad over the cerebel- 
lum it receives a superficial branch from it and several from 
the adipose tissue surrounding the brain and the semi-circular 
canals; and usually anastomoses with the cranial cavity vein 
(see fig. 24), which empties into the encephalic vein. After 
passing over the cerebellum the posterior encephalic vein 
bends ventrad, following along behind the vagus portion of the 
ramus lateralis accessorius to its origin from the dorsal root of 
the vagus, and when the level of the oblongata is reached, 
