CHAP. XVII.] STRUCTURE OF THE CORNEA. 17 



dioptric media. This nerve contains a considerable quantity of fibrous 

 tissue separating and supporting its fasciculi, and as it traverses the 

 sclerotic this tissue becomes continuous with the borders of the aper- 

 ture, so that the aperture itself may be said to be cribriform ; the 

 nerve passing through a number of distinct canals of fibrous tissue, 

 before it reaches the inner surface of the sclerotic. It is indifferent 

 whether this cribriform tissue be called neurilemma or sclerotic. 

 One thing, however, may be always observed. The nerve, as it 

 pierces the sclerotic, contracts, and lies in a smaller compass, so 

 that the entire aperture is somewhat funnel-shaped, and wider 

 behind than in Front; and, though the nerve is moveable on 

 its entrance into the sclerotic aperture, it is always fixed firmly 

 at the inner surface of that aperture, where the retina com- 

 mences. 



The aperture in the sclerotic in front, for the cornea, is circular, 

 and usually about ^ ths of an inch in transverse diameter, and rather 

 less vertically, though in some individuals altogether smaller. Be- 

 tween the point of entrance of the optic nerve, and the attachments 

 of the recti muscles, there are several minute apertures for the trans- 

 mission of vessels and nerves to the interior. The nutrition of this 

 tunic itself is provided for by small vessels ramifying on its surface, 

 and sparingly continued into its substance. Its own proper vas- 

 cularity does not seem to be greater than that of other fibrous 

 structures. 



Of the Cornea. The size and shape of this transparent part of 

 the outer case of the eye have been already indicated. It is spheri- 

 cal rather than spheroidal; and its posterior surface is of parallel 

 curvature with the anterior ; so that it does not appear to be a me- 

 niscus lens, thicker in the middle, as some authors have described : 

 this at least is the result of our careful examination. The cornea, 

 when its concavity is filled up with the aqueous humour, is of course 

 a powerful converger of the rays of light towards the iris, and through 

 the pupil to the lens. Viewed from within, its circumference is ex- 

 actly circular ; but on the outside it generally appears wider trans- 

 versely, from the sclerotic, which overlaps it on all sides, encroach- 

 ing upon it rather more above and below. The cornea and sclerotic 

 are firmly connected by continuity of texture, and cannot be dis- 

 united even by maceration. The cornea is possessed of great 

 toughness, and will even resist a force capable of rupturing the 

 sclerotic. 



The cornea, though a beautifully transparent substance, and ap- 

 pearing at first sight as homogeneous as glass, is nevertheless full of 



VOL. II. C 



