270 THE ANATOMY OF THE HORSE. 



inserted into the choroid and ciliary processes ; and (2) of an inner 

 circular set, which surround the rim of the iris. When the radiating 

 fibres contract they pull forward the choroid coat and ciliary pro- 

 cesses, and allow the lens to bulge forwards by slackening its tense 

 suspensory ligament. This is the mechanism by which the eye is 

 accommodated for near objects. 



The Choroid Coat. This is a bell-shaped dark membrane which 

 lines the sclerotic. Behind it is pierced by the optic nerve; and in 

 front it is continued as the ciliary processes, which form, as it were, 

 the rim of the bell. Its outer surface, when exposed by the removal 

 of the sclerotic, has a shaggy appearance due to the tunica fusca which 

 unites the two coats. Between the two the ciliary vessels and nerves 

 pass forwards. The inner surface of the choroid is lined by the layer 

 of pigmented hexagonal cells belonging to the retina, and these adhere 

 to this surface when the retina is detached from it. The whole of the 

 lower half, and the peripheral part of the upper half, of the inner surface 

 of the choroid is black from the presence of melanin in the membrane, 

 but a large somewhat triangular area in the inner half, termed the 

 tapetum, has a remarkable metallic lustre, and presents shades of colour 

 varying from bluish - green to azure -blue. The lower limit of the 

 tapetum is a nearly straight line extending horizontally across the 

 fundus of the eye, a little above the point of entrance of the optic nerve. 



Directions. — In the eye prepared to expose the iris and choroid, a 

 segment of the former and of the ciliary muscle should be carefully and 

 delicately removed with scissors, so as to lay bare a number of the 

 ciliary processes. This is to be done while the eye remains immersed 

 in water. 



The Ciliary Processes. These form a fringe around the slightly 

 inverted rim of the choroid. They number upwards of a hundred, 

 and each projects on the inner side of the rim, as a small swelling- 

 separated by depressions from the adjacent processes. The outer 

 surface of each is covered by the ciliary muscle ; the inner surface 

 rests in a depression on the suspensory ligament of the lens ; behind 

 each is continuous with the texture of the choroid ; and in front it 

 terminates in a rounded end which bounds in part the so - called 

 posterior chamber of the aqueous humour, behind the peripheral 

 part of the iris. 



Structure. — The choroid possesses a stroma of connective-tissue with 

 ramifying corpuscles containing brown or black pigment — melanin. 

 This stroma is lined internally by a structureless layer — the lamina 

 vitrea, and it supports the vessels of the choroid. The arteries — which 

 are derived from the ciliary branches of the ophthalmic — and the veins 

 lie together in the outer part of the stroma, while the capillaries lie in 

 its deeper part and form there the tunica Ruyschiana. The smaller 



