154 EEPTILIA. 



one, which is very moveable ; in the Chameleon the eyelids form a 

 thick cushion, furnished with a dense muscular layer, and having a 

 small central aperture opposite the pupil. The nictitating membrane 

 of the Frog slides underneath the upper eyelid, and is moved by a 

 mechanism, that reminds us of that already described in Birds. 

 There are found a detractor and an elevator of the nictitating 

 membrane ; the latter muscle arises superiorly and in front from 

 the nictitating membrane, and gives off a tendon, which passes 

 though a pulley to beneath the eyeball, perforates the infundi- 

 buliform muscle, and terminates by a small muscular belly attached 

 to the posterior angle of the eye. In the Ophidia the eye is covered 

 by a transparent lamella of epidermis, which is shed along with 

 what is called the exuvia or cast skin. Beneath it lies the conjunctiva 

 which is reflected over the sclerotic coat, invents the cornea, and so 

 forms a completely closed sac, which receives the excretory ducts of 

 the lacrymal gland, and conducts the secretion from the latter 

 through a duct between the maxillary and palatal bones into the 

 cavity of the mouth. The Lacrymal gland is situated behind the 

 eyeball, and is particularly large in the non-venomous Serpents, as 

 the genus Coluber, but also in the Vipers, e. g. V., berus, where it 

 was formerly falsely taken from the poison-gland. The Sauria and 

 Chelonia have for the most part two lacrymal glands, an external, 

 the largest, and an internal, smaller ; one of these is destined for 

 the nictitating membrane, and corresponds to the Harderian gland. 

 The naked Amphibia do not appear to be furnished with a lacrymal 

 apparatus. 



Organs of Hearing. 



THE naked Reptiles completely approximate the Fishes in the 

 structure of their organ of hearing, while the Squamigerous sub- 

 class exhibit a higher grade of organization, by the appearance of a 

 true cochlea. The tympanic cavity is absent in the Ichthyodea, 

 and Tailed Batrachia, and both integument and muscles are con- 

 tinued over the external ear ; the oval fenestra is closed by a car- 

 tilaginous operculum, into which is inserted a horizontal, elongated, 

 style-shaped ossicle, like that in Birds, and called the columella ; 

 the Eustachian tube is wanting. The tympanic cavity is also ab- 

 sent in the Ophidia, but they have for the most part a columella 

 and operculum. The Anourous Batrachia have in general a mem- 

 branous tympanic cavity, which commences by an infundibuliform 



