THE EYE. 681 



sphere, which forms about five-sixths of the globe, is opaque, and formed by 

 the sclerotic, the tunic of protection to the eyeball; the smaller sphere, which 

 forms the remaining sixth, is transparent, and formed by the cornea. The axes 

 of the ej^eballs are nearly parallel, and do not correspond to the axes of the 

 orbits, which are directed outwards. The optic nerves follow the direction of 

 the axes of the orbits, and enter the eyeball a little to their inner or nasal side. 

 The eyeball is composed of several investing tunics, and of fluid and solid 

 refracting media, called humors. 



The tunics are three in number: 



1. Sclerotic and Cornea. 



2. Choroid, Iris, and Ciliary Processes. 



3. Eetina. 



The refracting media, or humors, are also three : 



Aqueous. Crystalline (lens) and Capsule. Yitreous. 



The sclerotic and cornea form the external tunic of the eyeball ; they are 

 essentially fibrous in structure, the sclerotic being opaque, and forming the 

 posterior five-sixths of the globe; the cornea, which forms the remaining sixth, 

 being transparent. 



The Sclerotic (trxMfpdj, hard) (Fig. 368) has received its name from its extreme 

 density and hardness ; it is a firm, unyielding, fibrous membrane, serving to 



Fig. 368. A Vertical Section of the Eyeball. (Enlarged.) 



Sclerotic ^-""^ p^^- f^Tunrfnti of RECTUS 



Choroid - 

 Retina 



Hyaloid Me mi rant 



Ciliary Mnnclf 

 te Liyament 



'irft/far Stntt* 

 aal ff Petit 



maintain the form of the globe. It is much thicker behind than in front. Its 

 external surface is of a white color, quite smooth, except at the points where 

 the Kecti and Obliqui muscles are inserted into it, and covered, for part 6f its 

 extent, by the conjunctival membrane; hence the whiteness and brilliancy of 

 the front of the eyeball. Its inner surface is stained of a brown color, marked 

 by grooves, in which are lodged the ciliary nerves, and connected by an exceed- 

 ingly fine cellular tissue (lamina fusca with the outer surface of the choroid. 

 Behind, it is pierced by the optic nerve a little to its inner or nasal side, and is 

 continuous with the fibrous sheath of the nerve, which is derived from the dura 

 mater. At the point where the optic nerve passes through the sclerotic, this 

 membrane forms a thin cribriform lamina (the lamina cribrosa)* the minute 



