608 



THE SENSES. 



space, and movement. The organs of touch, taste, and smell, in order 

 to perform their functions, must be placed in actual contact with the 

 foreign substances which excite their activity ; and even that of hearing 

 is affected only by the sonorous vibrations of the atmosphere, or of some 

 other solid or fluid medium. But the eye is equally sensitive to the 

 impressions of light, whether it come from near or remote objects, or 

 even from the immeasurable distances of the fixed stars. It is also 

 superior to the other organs of special sense in the rapidity of its action, 

 and in the delicacy of the distinctions which it is capable of making 

 in the physical qualities of external objects ; and it affords the most 

 continuous and indispensable aid for all the ordinary occupations of 

 life. 



Organ of Vision. The eyeball consists of a spheroidal fibrous sac, 

 the sclerotic coat (Fig. 189, 2), filled with fluid and gelatinous material, 



Fig. 189. 



HORIZONTAL SECTION OF THE RIGHT EYEBALL.!. Optic nerve. 2. Sclerotic 

 coat. 3. Cornea. 4. Canal of Schlemm. 5. Choroid coat. 6. Ciliary muscle. 7. Iris. 8. 

 Crystalline lens. 9. Retina. 10. Hyaloid membrane. 11. Canal of Petit. 12. Vitreous 

 body. 



provided anteriorly with a transparent portion, the cornea (s), and 

 lined at its posterior part with a nervous expansion, the retina (s), 

 which is sensitive to light, and which receives the luminous rays admit- 

 ted through the cornea. The cavity of the eyeball is therefore like that 

 of a room with but one window, where all the light which enters from the 

 front necessarily strikes the back wall of the apartment. There are, in 

 addition to the above-mentioned parts, a transparent refracting body 

 with convex surfaces, the crystalline lens (s), by which the light is 



