616 ORGANS OF THE SENSES. 



lamina (lamina cribrosa) ; the minute orifices in this part serve for the trans- 

 mission of the nervous filaments, and the fibrous septa dividing them from one 

 another are continuous with the membranous processes which separate the bundles 

 of nerve fibres. One of these openings, larger than the rest, occupies the centre 

 of this lamellee ; it is called the porus opticus, and transmits the arteria centralis 

 retinae 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, this membrane is continuous with the cornea by direct continuity of tissue ; 

 but the opaque sclerotic overlaps it rather more on its outer than upon its inner 

 surface. 



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

 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 eye- 

 ball, and forms the anterior sixth of the globe. Its form 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 concavo- 

 convex, 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 individuals, 

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

 youth than in advanced life, when it becomes flattened. This difference in the 

 greater or smaller convexity of the cornea influences considerably the refractive 

 power of the eye, and is the chief cause of the long or short sight peculiar to 

 different individuals. It is dense and of uniform thickness throughout, its pos- 

 terior 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 : a thick central fibrous structure, 

 the cornea proper ; in front of this the anterior elastic lamina, covered by the con- 

 junctiva ; 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 laminae, at least sixty in number, all of which have the 

 same direction, the contiguous laminge becoming united at frequent intervals. 

 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 immediately presents 

 an opaque milky appearance. The interstices between the Iamina3 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 property 

 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 correc^ 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 



