THE GLOBE OF THE EYE 805 



terior occupies five-sixths and the anterior one-sixth of the entire 

 spheroid. At about the Hne of junction of these segments is placed a 

 partition consisting of the cihary body, iris and lens. In this way, the 

 cavity of the eyeball is subdivided into two, known respectively as the 

 anterior and posterior cavities. The former is filled with aqueous 

 humor and the latter with vitreous humor. It is also to be noted that 

 the wall enclosing the former is in part translucent (cornea), whereas 

 that of the latter is opaque. 



The Minute Structure of the Eyeball. — The shell of the eyeball 

 consists of three layers arranged concentrically as an external, a 

 middle and an internal coat. The outermost or sclera is made up of 

 dense, tough, opaque fibrous tissue which is interwoven with elastic 

 fibers and is distributed longitudinally and transversely around the 

 eyeball. If the eyelids are widely separated, its anterior zone appears 

 as the ''white of the eye." In children it has a bluish color, owing 

 to the fact that it is not sufficiently thick to prevent the dark choroidal 

 pigment from showing through. It is thickest posteriorly (1.0 mm.) 

 at the entrance of the optic nerve, and thinnest (0.4 mm.) about 6 mm. 

 from the cornea. Anterior to this point it is again thickened to give 

 attachment to the tendons of the recti muscles. The optic nerve and 

 the retinal blood-vessels pierce the sclera about 2.5 to 3 mm. internal 

 to the posterior pole of the eyeball and about 1 mm. below the hori- 

 zontal line uniting its anterior and posterior poles. By virtue of its 

 firmness, the sclera serves to retain the shape of the eyeball and to 

 protect its soft internal structures. In this it is aided by the fact that 

 the hum.ors of the eye are held under a certain pressure which is desig- 

 nated as the intraocular pressure. 



The cornea whicli is really the modified anterior segment of the sclerotic coat, 

 is transparent and allows the rays of light to enter the interior of the eye. Looked 

 at from in front, it possesses a nearly circular outline, measuring about 12 mm. in 

 its transverse and 11 mm. in its vertical diameter. In infants, its central zone 

 is generally somewhat thicker than its marginal, while in the adult it is somewhat 

 thinner^ (0.45 to 1.37 mm.). Its curvature is less than that of the sclerotic, but 

 varies in different persons as well as at different periods of their life; moreover, 

 its curvature is generally greater in its vertical than in its horizontal meridian. 



The substance of the cornea is made up of modified connective tissue which is 

 continuous with that forming the sclera. Its anterior surface is enveloped by 

 stratified epithelium which is supported by a structureless membrane, known as 

 the anterior homogeneous lamella. Its posterior aspect is covered by a simple 

 layer of endothelial cells situated upon the posterior homogeneous lamella. The 

 latter is a very resistant membrane, as may be gathered from the fact that it serves 

 as a barrier to inflammatory processes. In addition, it prevents the absorption 

 of the aqueous humor through the corneal lymphatic spaces. Close tO the margin 

 of the cornea, this membrane breaks up into a number of interconnected lamellae 

 which either serve as attachments to the ciliary muscle or are prolonged backward 

 into the substance of the iris and sclera. The fissures in between these lamellae 

 are known as the spaces of Fontana. They communicate freely with the anterior 

 cavity of the eye as well as with the canal of Schlemm, a circular tube traversing 

 the substance of the sclera, close to its junction with the cornea. The latter is 



1 BUx, Monatsblatt fiir Augenheilkunde, 1872. 



