VASCULAR SYSTEM OF THE RETINA. 317 



V. ophthalmica inferior.* The arteria and vena centralis retinae 

 (fig. 367, e e) run close to one another in the axis of the optic nerve, 

 surrounded by some connective tissue as far as to the intra- 

 ocular extremity of the nerve. In their course they give off 

 small branches to the trunk of the optic nerve, which run 

 in the plexiform trabeculae of connective tissue that invest the 

 nerve fasciculi. 



In addition to the branches of the central vessels, the 

 optic nerve also receives many twigs from the vessels supplying 

 the internal sheath, /, (or proper neurilemma of the nerve,) 

 and, though in smaller number, from those of the external 

 sheath, g. These vessels are branches of the ophthalmic artery, 

 and of its primary divisions. The intra-cranial portion of the 

 optic nerves, the chiasma and the tractus opticus, are supplied 

 by the vessels running in the adjoining parts of the pia mater 

 and brain, the branches of which communicate with those of 

 the intra-orbital part of the nerve. 



At its point of entrance into the eye, the optic nerve receives 

 also branches from a few (two or three) of the posterior short 

 ciliary arteries, 7c. These form a complete vascular circle, named 

 the circle of Zinn or Haller* which gives off numerous fine 

 branches into the optic nerves, that anastomose with the 

 branches of the central artery. 



Veins corresponding to these branches of the ciliary arteries, 

 there are none ; on the other hand, the small arteries, veins, and 

 capillaries of the choroid communicate directly at the margin 

 of the optic disk with the corresponding vessels of the papilla 

 and of the external sheath of the optic nerve, so that a toler- 

 ably ultimate connection is here established between the retinal 

 and ciliary vascular systems, I. No other communication exists 

 between the two. At the ora serrata the vascular system of 

 the retina ceases with the formation of capillary loops, without 

 communicating at any point with that of the choroid. 



* Waller, De venis oculi, Berol, 1778. Sesemann, Die Orbitalvenen des 

 Menschen und ihr Zusammenhang mit den oberflachlichen Venen des Kopfes 

 (The orbital veins of Man and their communications with the superficial 

 veins of the head), in Reichert and Dubois-Reymond's Archiv, 1869, p. 2. 



* Illustrations of which will be found in Jayer ueber die Einstellungen 

 des dioptric. Apparats, Taf iii. , figs. 34 36 (Wien, 1861) ; and Th. Leber, 

 Denkschrift. d. Wien. Akad., Band xxiv., Taf. iv. 



