ORGANS OP SENSE. 65 



substance. Then comes the vitreous humor, transparent 

 but much firmer than the aqueous humor, and behind it, 

 in the form of a delicate film, the retina which is the 

 termination of the optic nerve. The retina is at the 



FIG. 36. 



VERTICAL SECTION ANTERO-POSTERIORLY OP THE EYEBALL. 1, optic nerve; 

 2, sclerotic; 3, its posterior thicker portion; 4, sheath of the optic nerve 

 continuous with the sclerotic ; 5, the nerve within the sheath ; 6, insertion 

 of the recti muscles into the sclerotic ; 8, 8, superior and inferior recti 

 muscles; 9, cornea ; 10, its conjunctival surface ; 11, aqueous humor; 12, 13, 

 bevelled edge of the cornea fitting into the sclerotic ; 14, circular sinus of 

 the iris; 15, choroid; 16, the anterior portion, constituting the ciliary 

 body ; 17, the ciliary muscle ; 18, the ciliary processes ; 19, retina ; 20, its 

 origin ; 21, the ora; 22, central retinal artery; 23, vitreous humor; 24, 25, 26, 

 hyaloid tunic; 27, suspensory ligament of the crystalline lens, 28; 29, iris ; 

 30, pupil ; 31, posterior chamber, and 32, anterior chamber, occupied by the 

 aqueous humor. 



back of the eye, resting on a deep black membrane 

 called the choroid, which absorbs the indirect rays of 

 light, and prevents them from disturbing by reflec- 

 tion the sharpness of the luminous image on the retina. 

 The iris and the frame holding the crystalline lens, 

 e 6* 



