64 



A COURSE ON ZOOLOGY. 



FlG. 35. 



and lashes arrest dust that might enter in front. The 



tears with whffch its 

 surface is continu- 

 ally moistened pre- 

 vent the drying up 

 of its membranes, 

 and during sleep 

 the eyelids cover it 

 as with a sheath. 



The eyeball is 

 covered exteriorly 

 with a hard, white, 

 opaque membrane, 

 called the sclerotic 

 coat, in the centre 

 of which is inserted 

 a thick, perfectly 

 transparent disk, 

 much like the crys- 

 tal of a watch. 

 Within this mem- 

 brane, which is 

 named the cornea, 

 is seen a variously- 

 colored circle, called 



the iris, in whose centre is a round opening, the pupil. 

 The pupil is the window by which the light enters ; it en- 

 larges or contracts by the play of the muscular fibres com- 

 posing the iris, thus allowing the entrance of a larger or 

 smaller amount of light, as may be required for vision. A 

 transparent liquid, called aqueous humor, fills the space 

 in front of the iris, and behind it to the crystalline lens, 

 a sort of kernel composed of a gelatinous, transparent 



MUSCLES OF THE EYE. 1, the palpebral elevator ; 

 2, the trochlear muscle ; 3, the pulley through 

 which the tendon of insertion plays ; 4, supe- 

 rior rectus muscle; 5, inferior rectus muscle; 

 6, external rectus muscle; 7,8, its two. points 

 of origin ; 9, interval through which pass the 

 oculo-motor and abducent nerves ; 10, inferior 

 oblique muscle ; 11, optic nerve ; 12, cut surface 

 of the malar process of the superior maxillary 

 bone; 13, the nasal notch; A, the eyeball. 



