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 scle- 

 rotic vein (PL II, Hg. 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 Empying Directly into the Jugular or into the 



Head 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 Encefhalic Veins (Pis. 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, 



