THE EYE. 



615 



eyeballs 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. Vitreous. 



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

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

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

 transparent. 



TUNICS or THE EYE. 



SCLEEOTIC AXD CORNEA. 



The SCLEROTIC (oxirjos, hard} has received its name from its extreme density 

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

 peculiar form of the globe (fig. 303). It is much thicker behind than in front. 



Fig. 303. A Vertical Section of the Eyeball. (Enlarwl.) 



r ef>don of RECTUS 



Sclf.rotic 

 Choroid 

 Retina 



Hyaloid Membrane 



Cilinr-y Muscle 

 If- Z.iyament 



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

 where the Recti and Obliqui muscles are inserted into it, and covered, for part of 

 its extent, by the conjunctiva! 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 exceedingly 

 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 its fibrous sheath, which is derived from the dura mater. At the point where 

 the optic nerve passes through the sclerotic, this membrane forms a thin cribriform 



