44 INTRODUCTION. 



views of the internal ear, by which the foregoing description may be more 

 clearly understood. The curious cavities of the labyrinth are all lined 

 with a most delicate membrane, on which the minute filaments of the 

 auditory nerve (portio mollis of the seventh pair) are ramified, and are 

 also filled with a peculiar fluid, termed the water of Cotunnius, the aqua 

 labyrinthi, or perilymph. Within the vestibule and the semicircular 

 canals, but not extending into the cochlea, is a continuous sac of deli- 

 cate membrane, floating in the perilymph, and assuming nearly the form 

 of the parts through which it is extended : it is united to the vestibule 

 by nervous filaments, which ramify upon it, and it contains a fluid, 

 termed by Blainville, " la vitrine auditive," from its resemblance to the 

 vitreous humour of the eye. The vestibular portion of this curious 

 sac, or series of membraneous inner tubes, is composed of two cavities 

 communicating with each other (one termed utriculus, the other sacculus), 

 each containing a small mass of white calcareous matter, resembling chalk, 

 suspended in the vitreous fluid, by the intermedium of minute nervous 

 filaments. The use of these bodies is unknown : they are, however, 

 always present, and are larger and harder in aquatic than in terrestrial 

 Mammalia ; and it cannot be doubted but that they serve some important 

 office. 



It yet remains to describe the series of small bones alluded to, the 

 first of which is connected with the membrana tympani. These bones 

 serve as the vibratory conductor between that membrane and the more 

 interior parts of the labyrinth (fig. 23) : they are four in number, and are 

 termed, from their shape, malleus, or hammer 

 (A) ; incus, or anvil (B) ; os orbiculare, or 

 tQ spherical bone (c) ; and stapes, or stirrup (D). 

 Each is susceptible of motion, and the tension 

 of the chain they form is regulated by the mem- 

 brana tympani, and other muscular fibres. The malleus rests with its 

 handle on the membrana tympani, and is united by its head to the incus ; 

 of which latter a long process joins the os obiculare, a minute bone situ- 

 ated between this branch of the incus and the stapes ; the base of the 

 stapes rests upon the membrane spread over the orifice termed fenestra 

 ovalis. Of all these bones the stapes seems to be the most important : 

 it is that which immediately imparts the vibrations of the air to the peri- 

 lymph contained in the winding channels of the labyrinth : hence it is 

 indispensable, and it has been ascertained that, though the other bones be 

 lost by disease, this, alone remaining, preserves the sense of hearing from 

 being utterly destroyed. 



Such, then, is a sketch of the auditory apparatus in Man. In the 

 lower Mammalia it is essentially the same ; but the mastoid process very 

 often ceases to fulfil the same office as in Man, and is merely a simple 



