826 



THE ORGANS OF SPECIAL SENSE. 



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

 eyeball, and forms the anterior sixth of the globe. It is almost circular in shape, 

 occasionally a little broader in the transverse than in the vertical direction. It 

 is convex anteriorly, and projects forward from the sclerotic^in the same mariner 

 that a watch-glass does from its case. Its degree of curvature varies in diiferent 

 individuals, 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 arid 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 four layers namely, (1) several strata 

 of epithelial cells, continuous with those of the conjunctiva ; (2) a thick central 



Canal of Schlemm. 



Posterior 

 chamber. 



Ciliary 



body. 



Ciliary 

 processes. 



Cavity occupied 

 by vitreous humor. 



Canal of 

 Petit. 



NTERNAL 

 RECTUS 

 MUSCLE. 



Retina.- 



Choroid coat. 



Canal for 

 "central artery. 



-Optic nerve. 

 FIG. 440. A horizontal section of the eyeball. (Allen.) 



fibrous structure, the substantia propria ; (3) a homogeneous elastic lamina ; and 

 (4) a single layer of endothelial cells forming part of the lining membrane of the 

 anterior chamber of the eyeball. 



The conjunctival epithelium, which covers the front of the cornea proper, con- 

 sists of several strata of epithelial cells. The deepest layers are columnar : then 

 follow two or three layers of polyhedral cells, the majority of which present finger- 

 like processes (i. e., prickle-cells), similar to those found in the cuticle. Lastly, 

 there are three or four layers of scaly epithelium with flattened nuclei. 



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

 transparent, and continuous with the sclerotic. It is composed of about sixty 

 flattened lamellae, superimposed one on another. These lamellae are made up of 

 bundles of modified connective tissue, the fibres of which are directly continuous 

 with the fibres of the sclerotic. The fibres of each lamella are for the most part 

 parallel with each other ; those of alternating lamellae at right angles to each other. 

 Fibres, however, frequently pass from one lamella to the next. 



The lamellae are connected with each other by an interstitial cement-substance, 



