NEEVOUS SYSTEM. 707 



(1992.) 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. 



(1993.) The nerves belonging to the fifth pair likewise correspond 

 both in their distribution and office with the trifacial nerves of mammi- 

 ferous Vertebrata. 



(1994.) The facial nerve, or portio dura of the seventh pair, is small, 

 in proportion to the limited development of the soft parts of the face, 

 but it is constantly present. 



(1995.) 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, Keptiles present very important 

 and interesting advances towards a higher form of the acoustic appa- 

 ratus, which we must proceed to notice. 



(1996.) The ear of Fishes, being only adapted to hear sounds con- 

 veyed 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 con- 

 trary, living in air, must be enabled to appreciate the sonorous vibra- 

 tions 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. 



(1997.) 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 or internal ear 

 through the intervention of an ossicle that represents the stapes of 

 Mammalia. 



(1998.) The drum of the ear is situated immediately beneath the skin, 

 the parts composing the external ear of quadrupeds being as yet entirely 

 deficient. The membrana tympani, that now for the first time makes 

 its appearance in the series of animals, is tensely stretched across the 

 tympanic aperture, being covered externally by the integument of the 

 head. In the Turtle (fig. 349) the tympanic membrane is represented 

 by a cartilaginous plate (a). The ossicle, or columnella as it is here 

 called, is single and trumpet-shaped ; it passes quite across the tympanic 

 cavity (6), its external extremity being inserted into the drum, while at 

 its opposite end it expands into a disk (c), which closes an aperture 

 (foramen ovale) that communicates with the membranous vestibule of 

 the internal ear. It is obvious, therefore, that every tremor impressed 

 upon the membrana tympani will be conveyed by the columnella to the 

 foramen ovale, and thus communicated to the fluid contained in the laby- 

 rinth, upon which, as in fishes, the auditory nerve is distributed. 



(1999.) The cavity of the tympanum communicates with the interior 

 of the mouth by a wide opening, that represents the Eustachian tube, 



2z2 



