ORGANS OF SIGHT IX REPTILES. 



341 



226 



a blacker pigment than in other Reptiles, and the ciliary plica? are 

 longer and more distinct, extending beyond the origin of the iris. 

 This is anteriorly of a pale yellow colour ; the pupil is vertical. 



The eye of the Crocodile is chiefly peculiar for the massive and 

 complex character of its appendages, tig. 226, to which the eye- 

 ball itself, x, u, t, bears but a small 

 proportion. No other or higher 

 animal offers such a structure : it 

 was one of the discoveries of 

 Hunter, who left a drawing of it, 

 which was engraved, and* with his 

 preparation, no. 1770, described in 

 xx. vol. iii. In the copy of this 

 drawing, fig. 226, the upper, e, and 

 lower, b, eyelids are severed at the 

 outer canthus, and drawn apart to 

 show the third or nictitant eyelid, h, 

 and the extent of the conjunctiva. Of 

 this membrane e is the free surface of 

 the part which lines the ordinary 

 eyelids, whence it is reflected over 

 the nictitant lid at [/, h, k ; and then 

 upon the cornea at the line marked e , upon the part of the 

 circumference next the outer canthus. The free margins of 

 the upper and lower lids are marked c ; they are devoid of cilia, as 

 in all Hcematocrya : h is the free margin of the third lid. The 

 glands sending their secretion to the conjunctival space are the 

 proper lacrymal and the harderian ; the duct of the latter termi- 

 nates on the inner surface of the base of the nictitant lid, at k. 

 From the conjunctival chamber the secretion of both glands is 

 conveyed by the two puncta lacrymalia, /, to the duct termin- 

 ating in the nasal cavity. The muscles are divisible into those of 

 the eyelids and those of the eyeball. The nictitator, fig. 226, z, 

 arises from the inner and upper part of the ball, proceeds outward 

 and downward, winding round the optic nerve and choanoid muscle 

 (which protects the nerve from the pressure of the nictitator in 

 action), and is inserted into the inferior angle of the third lid. 

 Whilst the muscle draws this outward over the eyeball, it at the 

 same time rotates the ball inward beneath the third lid, beina - 

 attached to movable points at both extremities. The upper eyelid 

 has a levator muscle, m, chiefly inserted into the palpebral ossicle, 

 but also sending a few fibres, n, to be attached to the palpebral 

 conjunctiva near its angle of reflection. The under lid has a 



An externa] view of the eye, eyelids, muscles, 

 &c. of a Crocodile, xx. 



