I09 2 THE ORGANS OF SPECIAL SENSE 



The Sclera (ay/ty/wic, hard). The sclera has received its name from its extreme 

 Density and hardness; it is a firm, unyielding, opaque, fibrous membrane serving 

 to maintain the form of the globe. It is much thicker behind than in front. 

 Its external surface is of a white color, and is in contact with the inner surface of 

 the capsule of Tenon, a lymph space intervening; it is quite smooth, except one- 

 quarter of an inch back of the sclerocorneal junction, at the points where the 

 Recti and Obliqui muscles are attached to it, and its anterior part is covered by 

 the conjunctival membrane (Fig. 820); hence the whiteness and brilliancy of the 

 front of the eyeball. Its inner surface is stained a brown color, marked by grooves, 

 in which are lodged the ciliary nerves and vessels (Figs. 806 and 807); the inner 

 surface of the sclera is loosely connected with the outer surface of the choroid by a 

 layer of exceedingly fine cellular pigmented tissue (lamina fusca), which traverses an 

 extensive lymph space, the perichoroidal space (spatium perichoroideale) (Figs. 806 

 and 820) intervening between the sclera and choroid. Behind, the sclera is pierced 

 by, the optic nerve and is continuous with the fibrous sheath of the nerve, which is 

 derived from the dura (Fig. 801). At the point where the optic nerve passes 

 through the sclera, the lamina fusca is represented by an arrangement of the fibrous 

 tissue which forms a thin network, the cribriform lamina (lamina cribrosa sclerae) 

 (Fig. 811); the minute orifices in this lamina serve for the transmission of nerve 

 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 (porus opticus), larger than the rest, occupies the centre of the 

 lamella; it transmits the arteria centrah's retinae to the interior of the eyeball (Fig 

 811). Around the cribriform lamella are numerous small apertures for the trans- 

 mission of the ciliary nerves and the short ciliary arteries, and about midway between 

 the margin of the cornea and the entrance of the optic nerve are four or five large 

 apertures, for the transmission of veins (venae vorticosae} (Fig. &20J. In front, 

 the fibrous tissue of the sclera is continuous with the substantia propria of the 

 cornea (Fig. 820), but the opaque sclera slightly overlaps the outer surface of the 

 transparent cornea. 



Structure. The sclera is formed of white fibrous tissue intermixed with fine elastic fibres, 

 and of flattened connective-tissue cells, some of which are pigmented, contained in cell spaces 

 between the fibres (Fig. 811). These fibres are aggregated into bundles, some of which are 

 arranged in layers having an equatorial direction, but most of which are arranged in layers 

 lying in meridian lines. Its vessels (Figs. 801 and 802) are not numerous, the capillaries being 

 of small size and uniting at long and wide intervals. It obtains arterial blood from the short 

 posterior ciliary and the anterior ciliary arteries. The venous blood is removed by the venae 

 vorticosae and the anterior ciliary veins. There are lymph spaces between the cells which empty 

 into the periscleral (Fig. 794 and p. 1088) and perichoroidal lymph spaces (Fig. 806). Its nerves 

 are derived from the ciliary nerves (Fig. 800). They lose their myelin sheaths and enter among 

 the bundles of fibrous tissue, but it is not known how they terminate. 



The Cornea (Figs. 796 and 801). The cornea is the projecting transparent 

 part of the external tunic of the eyeball, and forms the anterior sixth of the surface 

 of the globe. It is almost, but not quite, circular in shape, occasionally a little 

 broader in the transverse than in the vertical direction. It is convex anteriorly, 

 and projects forward from the sclera in the same manner that a watch-glass does 

 from the case. Its degree of curvature varies in different individuals, and in the 

 same individual at different periods of life, it being more prominent in youth 

 than in advanced life. Usually the curvature is slightly greater in the vertical 

 plane than in the horizontal plane; at its centre than at its periphery, and at its 

 temporal than at its nasal side. 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 sclera. 

 The anterior surface is covered with conjunctival epithelium (Fig. 806). 



