MAMMALIA. 6'99 



ance of course being made for the varying form of the jaws, and 

 for the proportionate size of the different organs connected with 

 mastication. 



(818.) The seventh, or facial nerve, as also the glosso-pharyn- 

 geal, the pneumogastric, and the lingual, have the same origin 

 and general distribution throughout the whole class. 



(819.) The eighth pair of nerves are here, as in all the Verte- 

 brata, devoted to the sense of hearing, which in the Mammifera 

 attains its highest developement and perfection. The sensitive por- 

 tion of the auditory apparatus, or the internal ear, is now enclosed in 

 the petrous portion of the temporal bone, and imbedded in osseous 

 substance of such stony hardness, that, except in very young sub- 

 jects, it is by no means easy to display its different parts. 



As in Fishes and Reptiles, it consists of several membranous 

 chambers or canals, filled with a limpid fluid, over which the 

 filaments of the auditory nerve spread out. The whole apparatus, 

 indeed, except in its proportionate size, very accurately resembles 

 the auditory organ of the lower Vertebrata : the semicircular canals 

 exhibit nearly the same arrangement, and in like manner commu- 

 nicate with the vestibule by five orifices. The vestibule itself is 

 small, and no longer contains any chalky concretions : it commu- 

 nicates on the one hand with the cavity of the tympanum, by 

 means of the foramen ovale ; and on the other sends off a canal 

 (scala) to form the cochlea, an organ which in the Mammifer 

 assumes its full developement and perfection. 



In the Reptilia and Birds, as the reader will remember, the 

 cochlea was a simple canal bent upon itself (Jig> 281, e), one end 

 of which (scala vestibuli) opened into the vestibule, while the 

 other (scala tympani) terminated at the tympanic cavity, from 

 which it was separated by the membrane of the fenestra rotunda ; 

 but in the Mammalia the two scalse of the cochlea are consi- 

 derably elongated, and wind in a spiral direction around a central 

 axis (modiolus), so as very accurately to resemble the whorls in 

 the shell of a snail, whence the name of the organ is derived.* 



It is in the increased complexity of the cochlea, therefore, 

 that the chief character of the labyrinth of the Mammal consists ; 

 but in the tympanic cavity the differences between the Mammi- 



* In Man, and by far the greater number of Mammals, the scalae of the cochlea 

 make two turns and a-half around the modiolus; but in a few Rodent quadrupeds, as, 

 for example, in the Guinea-pig, the Cavy, and the Porcupine, there are as many as 

 three turns and a-half. 



