DISSECTION or THE EYEBALL. 263 



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

 lines the sclerotic. Its outer surface^ when exposed by the removal of 

 the sclerotic, has a shaggy appearance due to the timica fusca which 

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

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

 of pigmented hexagonal cells belonging to the retina. 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. 



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 continuovis 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 

 veins converge in whorls — the vasa vorticosa — to join four or five 

 principal trunks. The ciliary processes have the same sti-ucture as the 

 choroid. Each contains a rich plexus of tortuous vessels, The branched 

 cells at the anterior end of each process are without pigment. Over a 

 considerable area on the inner surface of the choroid the pigment is 

 absent ; and there the choroid shines with a peculiar iridescent, metallic 

 appearance termed the tapetum lucidum. In the eyes of albinos the 

 choroid is entirely free from pigment. 



The Ciliary Nerves are efferent branches of the lenticular ganglion. 

 They'J^erforate the sclerotic in company with the ciliary arteries, and 

 run forwards between the sclerotic and cornea. They give branches to 

 the cornea and ciliary muscle, and terminate in the iris. They contain 

 sensory fibres, which are derived from the ophthalmic division of the 

 5th nerve j motor branches to the ciliary muscle and sphincter muscle 

 of the pupil, which come from the third nerve ; and motor fibres to the 



