ORGAN OF HEARING. 179 



tra rotunda sen cocJtlearis, which forms a communication between the 

 middle ear and the external scala of the cochlea. This foramen is 

 closed by a membrane, similar to that of the foramen ovale ; not, like 

 it, parallel, or nearly so, to that of the tympanum, but situate ob- 

 liquely. There is no communication by a chain of bones between it and 

 the membrana tympani. 



The small bones or ossicles are four in number, so connected with 

 each other as to form a bent lever ; one extremity of which is attached 

 to the tympanic surface of the membrana tympani, the other to the 

 membrane of the foramen ovale. These bones are usually termed, 

 from their shape beginning with the most external, and following their 

 order malleus, incus, os orbiculare, (by some not considered a distinct 

 bone, but a process of the incus,) and stapes. A small muscular appa- 

 ratus, consisting of three muscles, anterior muscle of the malleus; 

 internal muscle of the same bone; and muscle of the stapes, is attached 

 to the chain, which it can stretch or relax ; and, of course, it produces 

 a similar effect upon the membranes to which the chain is attached. 



Bellingeri 1 thinks, that the fifth pair regulates altogether the invo- 

 luntary motions of the middle ear. 



At the anterior and inferior part of the cavity is the tympanic ex- 

 tremity of a canal, through which the drum receives the air it contains. 

 This canal, called Eustachian tube, is about two inches long, and pro- 

 ceeds obliquely forwards and inwards from the middle ear to the lateral 

 and superior part of the pharynx, into which it opens behind the pos- 

 terior nares. It is partly osseous, partly fibro-cartilaginous and mem- 

 branous; and, towards its pharyngeal extremity, expands, terminating 

 by an oval aperture resembling a cleft. Throughout its course it is 

 lined by a mucous membrane, which appears to be a prolongation of 

 that of the nasal fossse, and is capable of being more or less contracted 

 and expanded by the muscles, which compose and move the velum palati. 

 The cavity of the tympanum communicates, by a short and ragged 

 canal, with numerous cells contained in the mastoid process. These 

 cells open into each other, and vary in number, size, and arrangement 

 in different individuals, and animals. They are called mastoid cells. 

 The cavity of the tympanum is larger in animals whose sense of hear- 

 ing is most acute. In man, it is about a quarter of an inch deep, and 

 half an inch broad, and is lined by a prolongation of the same mem- 

 brane as that which lines the Eustachian tube. This membrane, as we 

 have seen, covers the membrana tympani, and the membranes of the 

 foramen ovale, and foramen rotundum. It likewise lines the mastoid 

 cells, and is reflected over the small bones. 



The middle ear does not exist in every animal endowed with hearing. 

 It does not begin to appear lower in the scale than reptiles ; and is by 

 no means equally complex in all. Frequently, the chain of bones is 

 entirely wanting; and at other times we find one bone only. 



8, The internal ear or labyrinth is the most important part of the 

 apparatus. It consists of several irregular cavities in the pars petrosa 

 of the temporal bone, in which the nerve of audition is distributed. It is, 



1 Edinb. Medical and Surgical Journal, July, 1834, p. 128. 



