KEPTILIA. 



In all reptiles there is but one ossiculum 

 auditus (the columnella\ which is generally of 

 a trumpet shape. Its external extremity is in 

 the Saurians connected to the membrana tym- 

 pani by means of a cartilaginous process; but 

 in tortoises it is implanted directly into the 

 membrana tympani, which has a cartilaginous 

 texture. The inner extremity of the auditory 

 ossicle enlarges into an oval or triangular 

 disc (Patina), which is applied to the fenestra 

 rotunda, in the same way as the disc of the 

 stapes is in the ears of mammalia. 



It does not appear that there are any mus- 

 cles implanted into the auditory ossicle of the 

 rep til ia. In most reptiles that possess a 

 membrana tympani, it is situated on a level 

 with the general integument. In the crocodile, 

 however, something like an external meatus 

 exists. 



Organ of Vision. In all the reptilia the 

 eye resembles, in its general structure, that 

 of birds and quadrupeds : there are, however, 

 certain modifications rendered necessary by 

 the habits of these animals, many of which 

 are semi-aquatic in their habits, which it will 

 be important to notice. 



The sclerotic coat of the eye is very similar 



Fig. 227. 



a, lachrymal gland of a tortoise ; b, eye-ball of a 

 tortoise ; c, circle of osseous sclerotic plates. {After 

 Bojanus.} 



in its composition to the sclerotic of a bird ; 

 and in like manner, in many reptiles, contains 

 in its anterior portion a circle of horny plates, 

 which are enclosed between its lamina? with- 

 out being continuous with the substance of 

 the membrane, from which they are conse- 

 quently easily detached. These plates are 

 generally ten or twelve in number, and are 

 constantly met with in tortoises, and also 

 in the crocodiles, chameleons, and many other 

 lizards. In many genera the sclerotic is di- 

 vided into two layers, the external being 

 fibrous and of equal thickness throughout its 

 whole extent ; whilst the internal layer is of 

 a cartilaginous texture, and is thicker at the 

 posterior part of the eye than it is in front. 

 This layer in the vicinity of the entrance of 

 the optic nerve is perforated by numerous 

 foramina for the passage of bloodvessels. 



The structure of the cornea presents no- 

 thing remarkable, but its convexity varies 

 considerably in different genera. 



The choroid coat of the eye exhibits the 

 usual structure. In lizards and serpents the 

 ciliary processes are scarcely distinguishable ; 

 and in the Chelonians their existence would 

 be doubtful were it not for the elegant im- 

 pression left by them upon the vitreous hu- 

 mour. In the crocodile, however, these 

 processes are well developed, and very beauti- 

 ful. 



The iris, to some extent, resembles that of 

 fishes, having frequently the same metallic 

 splendour. The shape of the pupil varies : 

 in the crocodile it is a vertical slit, like that 

 of the cat ; in the tortoise it is round, as 

 likewise in the chameleon and the generality 

 of lizards. 



The optic nerve enters the eye externally 



Fig. 228. 



Section of the Eye-ball of a Tortoise. (After 

 Bojanus.} 



to the axis of vision, piercing the membranes 

 of the eye, as in the mammiferous classes : ar- 

 rived in the interior of the organ, it forms a 

 small tubercle, from the circumference of 

 which the retina takes its origin. 



In many reptiles the falciform ligament 

 usually met with in the eyes of fishes is still 

 perceptible ; and in some genera, such as the 

 lizards, the iguana, and the monitor, there 

 is a cylindrical membranous process covered 

 with black pigment, which passes from the 

 insertion of the optic nerve to the capsule of 

 the crystalline lens, and which is evidently 

 the representative of the pecten common to 

 the eyes of birds. 



The aqueous and vitreous humours offer 

 no peculiarity worthy of notice ; and the same 

 may be said of the crystalline lens, which, 

 however, differs in the convexity of its facets 

 in different genera. 



Appendages to the Eye. The eye of rep- 

 tiles is moved by the six ordinary muscles, 

 which are disposed as in fishes ; but besides 

 these, there are four smaller muscles repre- 

 senting the suspensory or choroid muscle of 

 quadrupeds: these latter closely embrace the 

 optic nerve, and spread over the convex por- 

 tion of the sclerotic. 



In the Chelonian and Saurian reptiles the 

 upper and lower eyelids are completely de- 

 veloped, and accurately close the conjunc- 

 tival cavity. There is also a well-developed 

 nictitating membrane, or third eyelid, which is 

 situated vertically at the inner canthus of the 

 eye, and has a horizontal motion over the 

 cornea. In the crocodiles the nictitating 

 membrane is moved by a special muscular ap- 

 paratus ; its muscle, the nictitator, arises from 

 the inner and upper part of the eye-ball, and 

 running outwards and downwards winds round 

 the optic nerve, and the suspensory muscles 

 of the eye (which latter serve to protect the 

 nerve from the pressure of the nictitator mus- 

 cle), and is inserted into the inferior angle of 

 the nictitating membrane, which it thus draws 



