33-2 ANATOMY OF VERTEBRATES. 



are directed towards the heavens : in the Hammer-headed Sharks 

 they are supported on long outward-projecting pedicles. 



The optic nerve, fig. 216, a, usually perforates the eyeball 

 oljlifiuely out of its axis, but sometimes directly in its axis. In 

 Osseous Fishes it is compressed where it 

 216 passes through the sclerotic and choroid, 



and then forms the retina by unfolding 

 itself, like a fan spread out and bent into 

 the form of a cone, leaving a fissure, 5, where 

 tlie free lateral borders meet after lining 

 about two-thirds of the hollow globe. This 

 fissure extends from the entry of the nerve 

 to the anterior margin of the retina, and 

 through it a fold of the innermost layer of 

 Eye or s»ord-nsh ; ouc-uiird tlic clioroid passcs iuto tlic vltrcous humour, 

 sometimes accompanied by the dark pig- 

 mental Ruyschian layer.' The fold of the vascular choroid, 

 whetlicr accompanied by the pigmental layer or not, is called the 

 ' lalciform process,' c ; it carries before it a fold of the proper 

 tunic of the vitreous humour ('membrana hyaloidea'), and usually 

 extends to the capstde of the lens, d, to which it is attached by 

 means of a clear but firm substance, called the 'campanula 

 Ilallcri.' 



The posterior or outer layer of the retina consists of the cel- 

 lular basis, supporting the stratimi of cylindricules, standing 

 vertically upon its concave surface, with the interblended twin- 

 fusifiirm corpuscles, both of which microscopic structures are more 

 easily demonstrated in the present than in the higher classes of 

 Vertclirata. Each twin-cor])uscle is surromuled by a circle of 

 cyli)Klricules. TIic ])rimitive nerve-fibres radiate over the cylin- 

 dricules, without anastomosing, and terminate in free ends, not by 

 loops, at tlie Ijasis of the ciliary zone. A delicate but well- 

 defined raised rim or ' bead' runs alone; both the anterior margins 

 of the retina, and along those which form the falciform slit. 



The crystalline lens {d) is spherical, or nearly so, large, firm, 

 with a dense nucleus : it is almost luiried in the \ itrcous hinuour, 

 where it is steadied liy the attachment of the falcifiirm ligament 

 to its tliin ca]isule: the fore part projects through the pupil 

 against Ihe fiat cornea, and so nearly fills the anterior chamlier, 

 that but a very small space is left for ' aqueous humour.' In the 

 cod and other Ciidiiliv the fibres of the lens converge, like the 



' .\,x. vol. iii. p. lU; cju uf the Bouito, ])rcii. no. 1G51. 



