THE BALL OF THE EYE. 415 



tendinous insertions of the recti muscles. Its internal surface 

 is smooth and somewhat shining, being loosely attached to 

 the cellular substance intervening between it and the choroid 

 coat; but the external surface is rough, and more obviously 

 fibrous, and is attached somewhat strongly to all the adjoining 

 parts. 



This membrane is of a white colour, and consists of a single 

 layer, whose structure is essentially fibrous. The fibres are 

 very closely compacted, and compose an intertexture which 

 cannot be unravelled. It is so closely united to the dura mater 

 covering of the optic nerve, that many anatomists, notwith- 

 standing its greater thickness, are disposed to speak of it as a 

 continuation of the same. Its strength and its want of elasti- 

 city suit it remarkably to maintain the form of the eye, and to 

 resist injuries. Out of the many blood vessels that penetrate it, 

 but few ramify in its structure, and the existence of nerves in 

 it is by no means evident. 



The tunica arachnoidea follows the course of the optic nerve, 

 within its coat of dura mater, and forms, in the eye, just around 

 the cribriform plate, or foramen of the sclerotica, a circular pad : 

 it is then reflected on the internal face of the sclerotica, as 

 far as its anterior edge. It is this which causes the internal 

 face of the sclerotica to be smooth and shining, and thereby to 

 correspond with that of the dura mater.* This circumstance 

 is more readily proved in a very young eye, than in the adult 

 one. 



The Cornea, as mentioned, fills up the aperture at the fore 

 part of the sclerotica, and, of course, has the same diamelers, 

 with this opening; measuring thereby more transversely than 

 in any other direction. It is a segment of a smaller sphere than 

 the sclerotica, and is, consequently, more convex than it. Its 

 thickness is uniform, and commonly exceeds that of the sclero- 

 tica at the fore part of the latter. 



Its circumference adheres very closely to the sclerotica, and 

 presents a bevelled or oblique edge, which is inserted into the 

 corresponding bevel of the sclerotica, so that the latter includes 

 the former. The closeness of this junction induced the older 



* J. F. Meckel. Zinn supposed this surface to be derived from the pia ma- 

 ter. 



