THE EYE AND ITS APPENDAGES. 365 



canal extends from the optic papilla to the vicinity of the posterior 

 lens-capsule ; during foetal life it transmits the hyaloid artery, and 

 afterwards contains the remains of the supporting connective tissue, 

 and, rarely, the atrophic artery itself. The canal is defined by a thin 

 membranous wall, the continuation of the hyaloid membrane. 

 The existence of other additional small lymphatic spaces has 

 been demonstrated within the periphery of the vitreous body. 



The minute arrangement and ultimate distribution of the blood- 

 vessels in the various parts of the eye have already been described 

 in connection with the individual structures ; it here remains to out- 

 line briefly the general relations of the larger trunks. 



The blood-vessels of the eyeball belong to two distinct systems, 

 the retinal and the ciliary, which are connected by meagre anasto- 

 moses only around the optic nerve entrance, otherwise they 

 remain entirely separate. 



The retinal system is formed by the ramifications of the reti- 

 nal artery and vein, which constitute the permanent circulation 

 within the nervous layer. During fcetal life an additional transient 

 supply, represented by the hyaloid artery, is distributed to embryo- 

 nal structures which disappear before birth. 



The ciliary system consists of the ramifications of the short, the 

 long, and the anterior ciliary arteries and their complementary veins, 

 and furnishes the blood-supply to the bulbar conjunctiva, the sclera, 

 the choroid, the ciliary body, and the iris, and indirectly aids in 

 maintaining the nutrition of the cornea, the lens, and the epithelial 

 division of the retina. 



The short ciliary arteries supply principally the choroid, and 

 form the choriocapillaris, at the same time giving off twigs, before 

 piercing the sclerotic coat, to the posterior segment of the sclera and 

 to the dural sheath of the optic nerve. The long ciliary arteries 

 pierce the sclera and pass in the horizontal meridian between the 

 scleral and choroid coats as far forward as the ciliary body, in which 

 they form the larger arterial circle of the iris ; additional recurrent 

 twigs are given off to the choroid and the ciliary muscle. The larger 

 arterial circle sends branches to the ciliary processes and to the iris, 

 as well as a few twigs to the choroid. 



The anterior ciliary arteries pass to the anterior segment of the 

 ball, and pierce the sclera near the corneal margin to gain access to 

 the ciliary muscle behind the canal of Schlemm. Before entering 

 the eyeball they send branches to the anterior segment of the sclera, 

 to the scleral conjunctiva, and to the corneal limbus. From the 

 branches which pierce the eyeball twigs communicate with the larger 

 arterial circle of the iris, and supply the ciliary muscle and the fore 

 part of the choroid. 



