ORGANS OF THE BODY. 



629 



distinct system of vessels. Two twigs from the posterior ciliary arteries 

 form a ring around the nerve, sending off one set of small branches in 

 between the fibres of the latter, and anothe.r externally to the choroid. 

 In this way provision is made for indirect communication between the 

 two circulations. In addition to this there is also a direct communication, 

 however, consisting in arterial and line venous twigs and capillaries, which 

 pass directly from the choroid into the optic nerve. 



The conjunctiva of the sclerotic is supplied by the vessels of the lids 

 and lachrymal gland ; it is, therefore, also independently provided. Its 

 arteries are represented at (o) and (p). Near the border of the cornea 

 only are they connected with those of the sclerotic. 



Here, namely, the end branches of the scleral vessel:? communicate 

 with one another in arches. From the latter, then, in the first place, 

 recurrent loops are seen to spring, which course back through the 

 conjunctiva, anastomosing with the proper vessels of that membrane. 

 Further, there arise partly from these loops, and partly also from the 

 terminal twigs of the anterior ciliary arteries themselves, the branches for 

 that capillary network already mentioned ( 309) situated round the 

 border of the cornea. This is drained by the anterior ciliary veins to 

 which we now turn. 



These (5) receive their blood from four different sources. 



(1.) The radicles springing from the border network of the cornea, 

 which form a polygonal network encircling the latter, in the form of a 

 ring from 4 to 7 mm. in breadth, lying upon the sclera. This is known 

 as the episcleral venous network, and from it twigs of the ciliary veins 

 take their rise (7). 



(2.) In its whole extent the venous network just mentioned is rein- 

 forced from the capillaries of the sclerotic itself (6). 



(3.) Into the ciliary veins, also, the efferent vessels of the canal of 

 Schlemm (11), as well as those of the 

 ciliary muscle (12), empty themselves. 



(4.) Finally, we have venous twigs spring- 

 ing up in the neighbouring portions of 

 the conjunctiva and corresponding to the 

 arterial connecting arches, which unite with 

 them. 



313. 



The refracting media of the eye, situated 

 behind the cornea, consist of the aqueous 

 humour, the lens, and the vitreous humour. 



Of these the lens (fig. 585), with its 

 capsule, have been already described in 

 speaking of the tissue of which it is com- 

 posed (p. 276). The vitreous humour was 

 also alluded to in connection with gelatin- 

 ous tissue (p. 190). 



There remains then for our considera- 

 tion, in the first place, the aqueous humour. 

 With it both chambers of the eye are 

 filled. According to His it passes easily through the tissue of the cornea, 

 and we know that when evacuated it is very rapidly replaced. The 

 aqueous humour is an alkaline fiuid of a sp. gr. of 1 '003-1 "009. It 



Fig. 585. Diagram of the structure of 

 the lens, a, capsule ; 6, epithelium ; 

 c, lens fibres; d, anterior; and e, pos- 

 terior end ; /, nuclear zone. 



