REPTILIA. 



315 



outwards over the eye-ball, while at the same 

 time it rotates the eye-ball inwards beneath the 

 membrane, the muscle being attached to move- 



Fig. 229. 



An external View of the Eye, Eyelids, Muscles, <?. 

 o/ a Crocodile. (After John Hunter.) 



a, the external surface of the upper eyelid ; b, the 

 external surface of the under eyelid ; c, points to the 

 edge of both eyelids ; d, the inner angle or canthus 

 of both eyelids ; e e, the internal surface of the eye- 

 lids covered by the tunica conjunctiva ; f, point, 

 to the two puncta lachrymalia on the inside of 

 the under eyelid; g, the external surface of the 

 third eyelid, or membrana nictitans ; h, the loose 

 or free edge of the same ; k, the opening of the duct 

 of the lachrymal gland (glandula Harderii) upon 

 the inner surface of the nictitating membrane ; this 

 surface has been raised from the cornea, to which 

 it naturally lies contiguous; ?, the muscle which 

 expands the membrana nictitans, and draws it 

 over the ball of the eye. This is the only muscle 

 which is subservient to the movements of the 

 nictitating membrane ; it is analogous to the pyra- 

 midalis of birds, the quadratus muscles and its 

 sheath being wanting in reptiles ; m, the levator 

 muscle of the upper eyelid ; n, the portion of the 

 above muscle lost in the tunica conjunctiva; o, 

 the depressor muscle of the under eyelid ; p, the 

 rectus superior, or attollens oculi ; q, the rectus in- 

 ferior, or deprimens oculi; r, the rectus externus, 

 or abducens oculi ; s, the obliquus inferior : only a 

 small portion of it is here seen ; t, the cornea ; u u, 

 the globe of the eye behind the cornea ; v, the optic 

 nerve ; x, insertion of the choroid muscle, which 

 consists of four distinct portions surrounding the 

 optic nerve. 



able points at both extremities. The quadratus 

 muscle, which in birds forms a loop for the 

 passage of the tendon of the nictitator, does 

 not exist in the reptilia. There is a gland 

 especially appropriated to facilitate the move- 

 ments of the nictitating membrane by its secre- 

 tion, which escapes through a duct opening 

 upon its inner surface. This gland is ana- 

 logous to the Harderian gland of quad- 

 rupeds. 



The lachrymal gland is generally of large 

 size, and consists of a thick broad conglo- 

 merate mass, which surrounds the upper and 

 outer portion of the eye-ball : its duct is 

 short and wide, and terminates just above the 

 external canthus of the eye. 



In the annexed drawing, copied from one 

 in the Hunterian collection, the eye, eyelids, 

 and muscles of the eye-ball of the crocodile 

 are represented. 



Fig. 230. 



Eye-ball of Tortoise : shewing the lachrymal Gland in 

 situ. (After Bojanus.} 



The common Lizards have, for eyelids, a 

 kind of circular veil, extended before the 



Fig. 231. 



Eyelids of Tortoise. (After Bojanus.} 



orbit, and perforated by a horizontal fissure, 

 which is capable of being closed by a sphinc- 

 ter muscle, and opened by a levator and de- 

 pressor : its inferior part has a smooth round 

 cartilaginous disc, as in birds. There is, be- 

 sides, a small internal eyelid, but it has no 

 proper muscle ; it is entirely wanting in the 

 chameleon, in which animal, also, the slit of 

 the eyelids is so small that the pupil can 

 scarcely be observed through it. The Gecko 

 has no moveable eyelid : its eye is protected 

 by a slight margin of the skin, as in serpents. 

 A similar disposition appears in the Scink. 



The horizontal eyelids of the reptilia close 

 exactly; they are generally slightly enlarged 

 at their margins, but are never furnished with 

 ciliae. 



The eye of Serpents is protected by an 

 eyelid of a very remarkable character ; for 

 that it is an eyelid, and not, as is very gene- 

 rally supposed, the cornea, its anatomical 

 relations abundantly prove. It consists of a 

 transparent membranous expansion, which is 

 immoveably fixed in a kind of frame formed 

 for its reception by a circle of scales, usually 

 seven or eight in number, disposed around 

 the margin of the orbit. This eyelid is formed 

 of three superposed layers* : viz., 1st, An 

 epidermic layer, which is elastic and pretty 

 thick where it covers the middle of the eye, 

 but towards the circumference of the eyelid it 

 becomes thinner, and is manifestly continuous 

 with the epidermis that invests the scales in 

 the vicinity of the orbit. This corneous la- 

 mella by its solidity is well adapted to defend 

 the eye, and it is this which becomes de- 

 tached and cast off with the slough of the 

 snake when it moults its skin. 2dly. Be- 

 neath this epidermic layer is situated a second 



* Vide Cloquet (Jules), Memoire sur 1'Existence 

 et la Disposition des Voies lacrymaux dans les 

 Serpens. 4to, Paris, 1821. 



