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 



obliquely 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, b, where 

 the 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 



Bye of sword-fish ; one-third the choroid passes into the vitreous humour, 



natural *izL>. . x . . 



sometimes accompanied by the dark pig- 

 mental Ruyschian layer. 1 The fold of the vascular choroid, 

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

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

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

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

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

 Halleri.' 



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

 lular basis, supporting the stratum of cylindricules, standing 

 vertically upon its concave surface, with the interblended twin- 

 fusiform corpuscles, both of which microscopic structures are more 

 easily demonstrated in the present than in the higher classes of 

 Vertebrata. Each twin-corpuscle is surrounded by a circle of 

 cylindricules. The primitive nerve-fibres radiate over the cylin- 

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

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

 defined raised rim or ' bead' runs along 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 buried in the vitreous humour, 

 where it is steadied by the attaclnnent of the falciform ligament 

 to its thin capsule : the fore part projects through the pupil 

 against the flat cornea, and so nearly fills the anterior chamber, 

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

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



1 xx vol. iii. p. 144; eye of the Bouito, prep. no. 1651. 



