682 ORGANS OF SENSE. 



orifices in this layer serve for the transmission of the nervous filaments, and 

 the fibrous septa dividing them from one another are continuous with the mem- 

 branous processes which separate the bundles of nerve-fibres. One of these 

 openings, larger than the rest, occupies the centre of the lamella ; it is called 

 theporus opticus, and transmits the arteria centralis retinas to the interior of the 

 eyeball. Around the cribriform lamella are numerous smaller apertures for the 

 transmission of the ciliary vessels and nerves. In front, the sclerotic is con- 

 tinuous with the cornea by direct continuity of tissue ; but the opaque sclerotic 

 overlaps the cornea rather more on its outer than upon its inner surface. 



Structure. The sclerotic is formed of white fibrous tissue intermixed with 

 fine elastic fibres, and fusiform nucleated cells. These are aggregated into 

 bundles, which are arranged chiefly in a longitudinal direction. It yields 

 gelatin on boiling. Its vessels are not numerous, the capillaries being of small 

 size, uniting at long and wide intervals. The existence of nerves in it is 

 doubtful. 



The Cornea is the projecting transparent part of the external tunic of the 

 eyeball, and forms the anterior sixth of the globe. It is not quite circular, 

 being a little broader in the transverse than in the vertical direction, in conse- 

 quence of the sclerotic overlapping the margin above and below. It is convex 

 anteriorly, and projects forwards from the sclerotic in the same manner that a 

 watch-glass does from its case. Its degree of curvature varies in different indi- 

 viduals, and in the same individual at different periods of life, it being more 

 prominent in youth than in advanced life, when it becomes flattened. The 

 cornea is dense and of uniform thickness throughout; its posterior surface is 

 perfectly circular in outline, and exceeds the anterior surface slightly in extent, 

 from the latter being overlapped by the sclerotic. 



Structure. The cornea consists of five layers: namely, of a thick central 

 fibrous structure, the cornea proper; in front of this is the anterior elastic 

 lamina, covered by the conjunctiva; behind, the posterior elastic lamina, 

 covered by the lining membrane of the anterior chamber of the eyeball. 



The proper substance of the cornea is fibrous, tough, unyielding, perfectly 

 transparent, and continuous with the sclerotic, with which it is in structure 

 identical. The anastomosing fusiform cells of which it is composed are arranged 

 in superimposed flattened lamina?, at least sixty in number, all of which have 

 the same direction, the contiguous lamina becoming united at frequent inter- 

 vals. If the relative position of the component parts of this tissue is in any 

 way altered, either by pressure or by an increase of its natural tension, it im- 

 mediately presents an opaque milky appearance. The interstices between the 

 laminae are tubular, and usually contain a small amount of transparent fluid. 



The anterior and posterior elastic laminae, which cover the proper structure of 

 the cornea behind and in front, present an analogous structure. They consist 

 of a hard, elastic, and perfectly transparent homogeneous membrane, of extreme 

 thinness, which is not rendered opaque by either water, alcohol, or acids. This 

 membrane is intimately connected by means of a fine cellular web to the proper 

 substance of the cornea both in front and behind. Its most remarkable pro- 

 perty is its extreme elasticity, and the tendency which it presents to curl up, 

 or roll upon itself, with the attached surface innermost, when separated from 

 the proper substance of the cornea. Its use appears to be (as suggested by Dr. 

 Jacob), "to preserve the requisite permanent correct curvature of the flaccid 

 cornea proper." 



The conjunctival epithelium, which covers the front of the anterior elastic 

 lamina, consists of two or three layers of transparent nucleated cells, the deepest 

 being of an oblong form and placed perpendicular to the surface, the superficial 

 ones more flattened. 



The epithelial lining of the aqueous chamber covers the posterior surface of 

 the posterior elastic lamina. It consists of a single layer of polygonal transpa- 

 rent nucleated cells, similar to those found lining other serous cavities. 



