ORGAN OF SIGHT OF MAMMALIA. 



255 



within a short distance of the pupil ; this is immediately en- 

 circled by a band, ^th inch in breadth, which is the orbicular or 

 sphincter muscle. The radiating lines, by analogy with the eye 

 of the Whale and Giraffe, indicate the ' dilator fibres ' of the 

 pupil. The peculiar contractile office or muscular character of 

 the iris calls for the large supply of nerves ; it is also highly 

 vascular. The two long ciliary arteries which penetrate the 

 sclerotic posteriorly, advance horizontally, about the middle of 

 the eyeball, between that membrane and the choroid, to the iris, 

 where each divides into two branches, which proceed round the 

 circumference and inosculate with each other, thus forming an 

 arterial circle, from which numberless branches converge to the 

 pupil. The nerves are derived from the third and fifth pairs, 

 with communications from the sympathetic, and consequently 

 having connections with the sixth. They penetrate the sclerotic 

 posteriorly, and advance towards the iris between the sclerotic 

 and choroid, about fifteen or twenty in number : arrived at the 

 ciliary ligament, they divide at acute angles, as in fig. 197, and 

 may be traced through 

 this structure until they 

 are finally lost in the 

 iris. The optic nerve, 

 on entering the orbit, 

 bends a little forward 

 and enters the eye about 

 an eighth of an inch be- 

 low and internal to the 

 axis of the globe : it un- 

 dergoes a constriction, 

 as in fig. 200, a, just 

 before piercing the scle- 

 rotic : on entering the cavity of the eyeball the neurine forms a 

 slight prominence, before expanding into the sheet called ' retina/ 

 The branch of the ophthalmic artery which penetrates the optic 

 nerve before it reaches the eye, emerges from the centre of the ter- 

 minal prominence by the ' porus opticus,' and ramifies, as ' arteria 

 centralis retinae,' upon the vascular layer adherent to the hyaloid 

 membrane of the vitreous humour. The microscopic character of 

 the retina itself is given in vol. i. p. 332. It is covered externallv 

 by a delicate transparent membrane, by which the retina is 

 connected with the Ruyschian layer of the choroid. In the 

 Horse, Ox, and Sheep, this membrane is more easily demonstrated 

 than in Man, where it is obscured by the black pigment : the 



Optic nerve and retina. 



