Cornea; Arcus Senilis 83 



The strands of the optic nerve riddle the sclerotic ^ in. into the nasal 

 side of the axis at the lamina cribrosa, at the centre of which is a 

 conspicuous opening, \heporus opticus,\>y which the arteria centralis 

 reaches the retina. 



Although the sclerotic is extremely strong, nevertheless, as the 

 result of long-continued inflammation and intra-ocular pressure, the 

 choroid stretches and bulges through it under the name of staphyloma, 

 from the resemblance of the protrusion to a bunch of grapes (o-ra^uA??). 



The cornea, the transparent part of the fibrous envelope of the 

 eye-ball, stands out like the convex glass in the front of a watch-case. 

 It is continuous peripherally with the sclerotic, by which it is some- 

 what overlapped. It is on account of this overlapping that in the 

 operation for cataract the surgeon makes his incision through the 

 sclerotic just beyond the cornea. As a matter of fact he divides the 

 bevelled edges of both sclerotic and cornea. Some operators, however, 

 make the incision entirely through the uncovered part of the cornea. 



Sometimes a degenerative (fatty ?) change occurs at the periphery 

 of the cornea, rendering it white and opaque. As it is chiefly met 

 with in old persons, it is termed arcus senilis. 



Structure. The cornea consists of a central, thick fibrous layer, 

 which is covered in front by several layers of the epithelium of the 

 conjunctiva, and behind by a homogeneous elastic lamella, at the back 

 of which is the epithelium of the anterior chamber. 



The fibrous foundation consists of about sixty lamellae connected 

 by a transparent cement ; in this cement delicate lymph channels 

 run, by which the nutrition of the cornea is carried on. 



The nerves, represented only by the axis cylinders, come from the 

 ciliary branches, and pass between the lamellae. The cornea receives 

 no blood-vessels, but just beyond its periphery is an important circle 

 of conjunctival vessels which are engorged in corneitis or keratitis 

 (cornu, Kepas, horn, lantern ; and tils}. But, when keratitis advances 

 to ulceration, a branch of a ciliary or conjunctival artery may pass 

 boldly over the cornea to it. If the ulcer extend into the substance of 

 the cornea it may ultimately traverse the elastic and the endothelial 

 layer, and involve the escape of the aqueous humour. The pupillary 

 border of the iris, escaping with the stream, may protrude upon the 

 surface and there become glued by plastic effusion synechia anterior 

 (a-wexysi a keeping together}. If the perforation be extensive, even 

 the lens and the vitreous may escape. 



The cicatricial patch which eventually makes the site of an ulcer 

 may look like a little puff of smoke (nebula} upon the clear cornea ; 

 but if it be pearly white (Aeu*os) it is called leucoma. A central 

 leucoma blinds a most important area of the retina, and compels the 

 surgeon to let light through an artificial pupil (p. 88). After this 

 operation the white patch may be tattooed with a fine needle and 

 Indian ink, and so rendered unnoticeable. 



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