THE EYE. 333 



extent consequently readies from the optic nerve to the cir- 

 cumference of the cornea. It is a pearly white, dense, very 

 strong, inelastic fibrous membrane, designed to preserve 

 the shape of the eye, and protect the delicate structures 

 within. 



The external surface is perforated round the optic nerve, 

 which enters at the posterior part, with many small fora- 

 mina for the passage of the ciliary vessels and nerves. It is 

 covered by the tunica conjunctiva, with which it is loosely 

 connected by cellular tissue, and gives insertion to several 

 muscles. Divide this membrane circularly from its centre 

 and reflect it forward towards the cornea, and backward 

 to the optic nerve, carefully raising it from the choroid, so 

 that its internal surface and density can be examined. 



The internal surface of the sclerotic, presents a brown 

 color, and is attached by delicate filamentous tissue, and 

 by the ciliary vessels and nerves to the choroid, which 

 gives this surface a rough appearance, but when these con- 

 nections are removed, which can be readily done, it pre- 

 sents a smooth and glossy surface, from which it is said a 

 serous layer can be dissected. This surface also exhibits 

 the openings for the ciliary vessels and nerves which enter 

 externally. 



The density of this coat is greatest behind, and becomes 

 gradually thinner as it approaches the centre ; which, 

 however, in front of this is again increased in thickness by 

 the tendinous addition of the recti muscles. 



It is perforated behind by the optic nerve, about one line 

 and a half internal to the antero posterior axis. The por- 

 tion of the sclerotic where the optic nerve enters, presents a 

 cribriform appearance, and is called lamina-cribrosa, having 

 its edges beveled, and containing a groove for the recep- 

 tion of the cornea. The outer layer of the sclerotic over- 

 laps the cornea, and the union of the two is most intimate. 



Structure. The sclerotic is regarded as one of the strong- 

 est fibrous membranes of the body, having its fibres inter- 

 lacing in every direction, and not capable of being separated 

 into any true laminae. It is continuous with the sheath of 



