BLOOD-VASCULAR SYSTEM OF THE LORICATI 55 



or efferent ^seiidohranchial artery (Pis. I, II and III, figs. 1,5, 

 15, 19 and 20; Oph.A.), which supplies onl}- the choroid coat 

 of the eye.- This vessel pursues a dorso-cephalic course, pas- 

 sing along the outer posterior edge of the levator arcus palatini 

 muscle to the parasphenoid bone ; it then runs parallel to the 

 parasphenoid for a short distance, and when the anterior surface 

 of the dorsal parasphenoid process is reached, sends off a 

 branch inward in front of this process to anastomose with the 

 corresponding artery from the opposite side. Here the main 

 stem bends dorsad nearly encircling the orbito-nasal arter}'^, and 

 passing between the inferior and internal rectus muscles in 

 company with the ramus ciliaris brevis and the ophthalmic vein 

 it follows along the posterior surface of the optic nerve, but be- 

 fore penetrating the sclerotic coat the artery makes a dorsal 

 curve around the ciliaris brevis and the ophthalmic vein, and 

 pierces the eyeball a little dorso-caudad of the optic nerve. 

 After passing through the silver layer of the choroid it bifur- 

 cates in the vascular layer of the choroid into an anterior 

 choroid artery (PI. Ill, figs. 20 and 21 ; Chor.A.^j)) and a 

 ^\ior\.itx posterior choroid artery (PL III, figs. 20 and 21 : Chor. 

 A. ^2))' These 2 vessels have somewhat the shape of a horse- 

 shoe, having its curved end dorsad and its open end ventrad. 

 Radiating from the outer surface of this horseshoe-shaped vessel 

 are numerous short vessels, which soon break up into smaller 

 vessels, and these in turn break up into minute parallel capil- 

 laries, forming the arterial retia mirabilia (PI. Ill, figs. 19, 

 20 and 21 ; A.Ret.M.) of the so-called choroid gland or vaso- 

 ganglion, which has already been accurately described by Jones 

 (41), Miiller (50), Stannius (74) and Emery (24). Distally these 

 capillaries reunite, forming the choroid arteries proper (PI. Ill, 

 figs. 20 and 21 ; Chor. A.), which supply the choroid with 

 arterial blood. A section through the choroid and retina (PI. 

 Ill, fig. 21) shows us that the choroid vessels are arranged in 



suppose that the arterial blood which passes through these filaments receives 

 additional oxygen from the water. 



2 1 have made several separate injections of the ophthalmic artery, cephalad, 

 after it leaves the pseudobranchia to see if it had any connection with the other 

 arteries, especially the orbitonasal artery with which it comes in such close con- 

 tact ; but no connection whatever was found. 



