NERVES OP THE EYE-BALL. 347 



receiving a filament from tlie nasal branch, of the ophthal- 

 mic, and the posterior inferior a motor branch from the 

 third pair; while from the anterior angles proceed the 

 ciliary nerves. These are from ten to twenty in number, 

 accompany the ciliary arteries through the sclerotic coat, 

 surround the optic nerve and proceed forward to the ciliary 

 ligament between the sclerotic and choroid, and terminat- 

 ing, as before described, upon the iris. Filaments of the 

 sympathetic also go to this ganglion. The ophthalmic gang- 

 lion is thus seen to embody nerves both of motion and sen- 

 sation, the former coming from the third, the latter from 

 the fifth, besides having sympathetic branches. 



The eye proper, or the ball, is thus seen to be a very 

 complex organ consisting of a great variety of parts, at 

 once most delicate and beautiful in their structure, as well 

 as most surprising and admirable in their nice adaptations 

 to each other and to light. 



It may now be proper to make a remark or two on the 

 whole collectively, in their conjoint action, and harmonious 

 co-operation in enabling us to see. 



Light, in coming from any object, falls first upon the 

 cornea, which being a convex lens on its anterior surface, 

 and more dense than the air, refracts the light and causes 

 it to converge towards a focus ; after leaving the cornea it 

 passes through the aqueous humor, a medium less dense, 

 consequently having less refraction, and producing rather 

 a divergence or bending of the rays from the perpendicular; 

 on leaving the aqueous fluid the rays pass next in order 

 through the crystalline lens, and here, from the double con- 

 vexity and density of this body, they undergo a powerful 

 convergence or bending to a focus to obviate which, before 

 reaching the retina they are made to pass through another 

 medium, the vitreous humor, which is neither so convex 

 nor so dense, and are thus brought to the proper focal dis- 

 tance by the time they reach the retina or seeing membrane 

 of the eye. 



The rays of light in passing through the lenses of the 

 telescope would be liable to confusion, and the production 



