SIGHT. 493 



The filaments of the optic nerve, after running forward and pene- 

 trating the posterior part of the eyeball, spread out into the sub- 

 stance of the retina (a), thus forming a delicate and vascular nerv- 



Vertical Section of the EYE BALI, .1. Sclerotic. 2 Choroid. 3. Ketina. 4. Lens. 5. Hyaloid 

 membraue. 6. Cornea. 7. Iris. 8. Ciliary muscle and processes. 



ous expansion, in the form of a spheroidal bag or sac, with a wide 

 opening in front, where the retina terminates at the posterior mar- 

 gin of the ciliary body. This expansion of the retina is the essen- 

 tial nervous apparatus of the eye. It is endowed with the special 

 sensibility which renders it capable of receiving luminous impres- 

 sions ; and, so far as we have been able to ascertain, it is incapable of 

 perceiving any other. On the outside, the retina is covered by the 

 choroid coat (2), a vascular membrane, which is rendered opaque by 

 the presence of an abundant layer of blackish-brown pigment-cells, 

 and which thus absorbs the light which has once passed through 

 the retina, and prevents its being reflected in such a way as to 

 confuse and dazzle the sight. Inside the retina is the vitreous body, 

 a transparent spheroidal mass of a gelatinous consistency, which is 

 surrounded and retained in- position by a thin, structureless mem- 

 brane, called the hyaloid membrance (5), lying immediately in 

 contact with the internal surface of the retina. The lens (4) is 

 placed in front of the vitreous body, in the central axis of the eye- 

 ball, enveloped in its capsule, which is continuous with the hyaloid 

 membrane. Just at the edge of the lens, the hyaloid membrane 

 divides into two laminae, which separate from each other, leaving 

 between them a triangular canal, the canal of Petit, which can be 

 seen in the above figure. In front of the lens is the iris (7), a nearly 



