580 REPTILIA. 



animals possess an additional eyelid or nictitating membrane, si- 

 milar to that of Birds, which can be drawn at pleasure over the 

 front of the eye, so as entirely to conceal it. This is effected 

 by a special muscle provided for the purpose, which arises from the 

 posterior part of the globe of the eye, and, after winding round the 

 optic nerve, passes beneath the eye-ball, to be inserted into the 

 free margin of the membrana nictitans. In Frogs and Toads 

 the upper and lower eyelids are nearly motionless ; but the third 

 is largely developed, and moved in the same way as that of the 

 Crocodile. 



In the higher Reptilia a distinct lacrymal gland and puncta 

 lacrymalia are met with, occupying the same positions as those of 

 the human subject. 



(635.) The third, fourth, and sixth pairs of the cerebral nerves, 

 have the same distribution in all the Vertebrata; and represent 

 respectively the oculo-muscular, the pathetici, and the abducentes 

 of man. 



(636.) The nerves belonging to the fifth pair likewise corre- 

 spond both in their distribution and office with the trifacial nerves 

 of mammiferous Vertebrata. 



(637.) The facial nerve, or portio dura of the seventh pair, 

 is small in proportion to the limited developement of the soft 

 parts of the face ; but it is constantly present. 



(638.) The auditory nerve of course is destined to the ear, 

 and its distribution is almost the same as in Fishes ; nevertheless, 

 in the general construction of the organ of hearing, Reptiles present 

 very important and interesting advances towards a higher form of 

 the acoustic apparatus, which we must proceed to notice. 



The ear of Fishes, being only adapted to hear sounds conveyed 

 through a watery medium, was found to consist only of the mem- 

 branous labyrinth, enclosed in the cavity of the skull, and without 

 any communication with the exterior of the body. Reptiles, on 

 the contrary, living in air, must be enabled to appreciate the sono- 

 rous vibrations of the atmosphere, and are consequently provided 

 with an auditory apparatus, capable of responding to pulsations of 

 sound of far greater delicacy than those transmitted through the 

 denser element. 



The first great improvement therefore which the anatomist 

 notices in the composition of the ear of a Reptile, is the addition 

 of a tympanic cavity, and of a tense and delicate membranous 

 drum, the vibrations of which are communicated to the labyrinth 



