716 



THE EYE. 



when shreds are removed they curl up always with the attached surface 

 innermost. Its transparency is not impaired by acids, by boiliug in water, 

 or by maceration in alkalies. In thickness it varies between 3-oVo*k an( l 

 "2oVo~*k of an inch. At its circumference the membrane breaks up into 

 bundles of fine threads, which are partly continued into the front of the iris, 

 forming the " pillars of the iris," and partly into the fore part of the 

 choroid and sclerotic coats. It is lined with an epithelial covering, which 

 resembles that on serous membranes, consisting of a single layer of flat poly- 

 gonal transparent cells with distinct nuclei. 



Blood-vessels and nerves. In a state of health the cornea is not provided with 

 blood-vessels, except at the circumference, where they form very fine capillary loops 

 and accompany the nerves. The existence of lymphatics has not been satisfactorily 

 ascertained. The nerves of the cornea are very numerous, according to Schlemm.* 

 Derived from the ciliary nerves they enter the fore part of the sclerotic, and are from 

 twenty-four to thirty-six in number. Continued into the fibrous part of the cornea, 

 they retain their dark outline for ith to ith of an inch, and then becoming trans- 

 parent, ramify and form a network through the laminated structure. 



MIDDLE TUNIC OF THE EYEBALL. 



This coat consists of two parts, one a large posterior segment the choroid, 

 reaching as far as the cornea, and formed chiefly of blood-vessels and pig- 

 mentary material ; the other, a small anterior muscular part the irh. 

 Between these and connected with both is situated the white ring of the 

 ciliary muscle. 



Fig. 462. Fig. 462. CHOROID MEM- 



BRANE AND IRIS EXPOSED 



BY THE REMOVAL OP THE 

 SCLEROTIC AND CORNEA 

 (after Zinn). f 



a, one of the segments of 

 the sclerotic thrown back ; 

 &, ciliary muscle and liga- 

 ment ; c, iris ; e, one of the 

 ciliary nerves ; /, one of the 

 vasa vorticosa or choroidal 

 veins. 



THE CHOROID COAT. 



The choroid coat of 

 the eye (tunica choro- 

 idea s. vasculosa) is a 

 dark brown membrane 

 lying between the scle- 

 rotic and the retina. It 

 reaches forwards to the 

 ciliary ligament, or 

 nearly to the cornea, 



where it ends by a series of plaits or folds named ciliary processes, disposed 

 in a circle projecting inwards at the back of the circumferential portion of 

 the iris. Afc the hinder part, where the tunic is thickest, the optic nerve is 

 transmitted through a circular opening. The outer surface is rough, and 

 is connected to the sclerotic by loose connective tissue (lamina fusca of 



* Berl. Encycl. Wort. art. Augapfel, Vol. iv. p. 22. 



