44: SPECIAL SENSES. 



needle to the bottom of the eye and irritated the retina, in 

 two persons. The patients experienced no pain, but the im- 

 pression of flashes of light. 1 The insensibility of the optic 

 nerves has also been repeatedly noted in surgical operations 

 in which the nerves have been exposed. 3 If a current of gal- 

 vanism be passed through the optic nerves, a sensation of 

 light is experienced. 3 The same phenomenon is observed 

 when the eyeball is pressed upon or contused, a fact which is 

 sufficiently familiar. 



Physiological Anatomy of the Eyeball. 



The eyeball is a spheroidal body, partially embedded in a 

 cushion of fat in the orbit, protected by the surrounding 

 bony structure and the eyelids, its surface bathed by the se- 

 cretion of the lachrymal gland, and movable in various direc- 

 tions by the action of certain muscles. When the axis of the 

 eye is directed forward, the globe has the form of a sphere 

 in its posterior five-sixths, with the segment of a smaller 

 sphere occupying its anterior sixth. The segment of the 

 smaller sphere, bounded externally by the cornea, is more 

 prominent than the rest of the surface. 



The eyeball is made up of several coats enclosing certain 

 refracting media. The external coat is the sclerotic, covering 

 the posterior five-sixths, which is continuous with the cornea, 

 covering the anterior sixth. This is a dense, opaque, fibrous 

 membrane, for the protection of the inner coats and contents. 

 The cornea is dense, resisting, and perfectly transparent. 

 The muscles that move the globe of the eye are attached to 

 the sclerotic. 



Were it not for the prominence of the cornea, the eyeball 

 would present very nearly the form of a perfect sphere, as 



1 MAGEXDIE, Sur Vinsensibilite de la retine de Vhomme. Journal de physiolo- 

 gie, Paris, 1825, tome v., p. 37, et seq. 



2 LONGET, Physiologic, Paris, 1869, tome Hi., p. 479. 



3 MEYER, Electricity in its Relations to Practical Medicine^ New York, 1 869, 

 p. 69. 



