1026 ORGANS OF SPECIAL SENSE 



The most superficial part of the true cornea appears homogeneous, even when 

 highly magnified (anterior elastic lamina. Bowman's membrane), though there 

 is reason to believe that its structure only differs from that already described in the 

 closeness of its fibrous texture; the two parts are certainly connected by fine fibres. 

 Anteriorly, the cornea is covered by an extension of the ocular conjunctiva, in the 

 form of an epithelium several layers deep. Posteriorly, the cornea is lined by a 

 firm, thin, glass-like layer (posterior elastic lamina, membrane of Descemet), 

 distinct from the corneal tissue both anatomically and chemically. At the peri- 

 phery this membrane breaks up into a number of fibres, which mainly arch over 

 to join the base of the iris (ligamentum peotinatum iridis). The interstices 

 between these fibres constitute spaces (spaces of Fontana) freely communicating 

 with the aqueous chamber on the one hand, and indirectly with the canal of Schlemm 

 on the other. Descemet's membrane is in turn lined by a single layer of flat cells, 

 which are continuous peripherally with cells lining the spaces of Fontana and the 

 anterior surface of the iris. The cornea is richly supplied with nerves, particularly 

 in its most superficial layers. 



2. The dark, middle, or vascular coat of the eye, generally known as the 

 uveal tract, is formed by the iris, ciliary body, and chorioid. It is closely applied 

 to the sclerotic, but actually joins it only at the anterior and posterior limits of their 

 course together, viz., at the scleral sulcus, and around the optic nerve entrance. In 

 front of the sulcus the middle coat no longer lines the outer, being separated from 

 it (i. e., the iris from the cornea) by a considerable space filled with fluid, called the 

 anterior aqueous chamber. The uveal tract has two openings in it ; a larger one in 

 front, the pupil, and a smaller one behind, for the passage of the optic nerve. Its 

 structure is that of a pigmented connective tissue, supporting numerous blood- 

 vessels and containing many nerves and three deposits of smooth muscle-fibres. 



The chorioid forms the posterior part of the uveal tract, and extends, with 

 slowly diminishing thickness, forwards as far as the ora serrata. Its outer and 

 inner surfaces are both formed by non-vascular layers; that covering the outer, the 

 membrana suprachorioidea, is pigmented, arranged in several fine loose lamella?, 

 and has been seen in our dissection; that covering the inner surface is a thin, trans- 

 parent, homogeneous membrane, called the basal or vitreous lamina of the chorioid, 

 or the membrane of Bruch. The intervening chorioidal stroma is very rich in blood- 

 vessels, which are of largest size next its outer surface, and become progressively 

 smaller as we approach the vitreous lamina, next to which we find a layer of closely 

 placed wide capillaries, called the lamina chorio-capillaris. The pigment becomes 

 less in amount as we pass inwards, and finally ceases, being absent entirely from 

 the chorio-capillary and vitreous laminae. 



In front of the ora serrata the uveal coat becomes considerably modified, and 

 the part reaching from here to the iris may conveniently be termed the ciliary region 

 of the tract, or ciliary body. Its superficial aspects have been already briefly 

 described. In front, the ciliary processes, about seventy in number, project towards 

 the interior of the eye, forming the corona ciliaris. Behind this part lies the 

 orbiculus ciliaris, whose inner surface is almost smooth, faint radial folds only 

 being present, three or four of which join each ciliary process. The more minute 

 structure of this ciliary region resembles closely that of the chorioid, except that 

 the chorio-capillaris is no longer present, that the stroma is thicker and richer in 

 blood-vessels, and that a muscular element (ciliary muscle) exists between the 

 vascular layer and the membrana suprachorioidea. On antero-posterior section the 

 ciliary body is triangular; the shortest side looks forwards, and from about its mid- 

 dle the iris arises; the two long sides look respectively inwards and outwards, the 

 inner having the ciliary processes upon it, while the outer is formed by the ciliary 

 muscle. This muscle possesses smooth fibres and consists of an outer and an inner 

 division; in the outer the fibres run longitudinally, inserted into the outer fibrous 

 coat of the eye at the sclero-corneal junction in front, and passing backwards to 

 join the outer layers of the orbiculus and chorioid; the inner contains circularly run- 

 ning fibres situated next to the ciliary processes. The entire muscle is destitute of 

 pigment, and therefore is recognisable in the section by its light colour. The whole 

 thickening of the uveal tract in this region, muscle and folds and processes together, 

 is named the ciliary body. 



The iris projects into the interior of the front half of the eye in the form of a 

 circular disc perforated in the middle. The appearance of its anterior surface has 



