952 THE SENSORY APPARATUSES. 



ear constitutes an irregular cavity, which we may consider as composed of two 

 walls and a circumference. 



The external wall is principally constituted by the membrane of the tympanum. 

 The internal wall, formed by the petrous bone, offers two openings — the fenestra 

 ovalis and fenestra rotunda — the one situated behind the other, and separated by 

 a small eminence named the promontory. The circumference is occupied for 

 nearly the whole of its extent by the mastoid cells — large open cavities in the 

 tympanum. 



Internally, the tympanum contains a chain of small bones named the malleus, 

 incus, OS orhiculare, and stapes ; these bones form the medium of communication 

 between the tympanum and the fenestra ovalis— from one wall to the other of 

 the cavity of the tympanum. 



This cavity is lined by a fine mucous membrane, which is continuous with 

 that lining the pharynx, by means of a cartilaginous canal — the Eustachian tube, 

 that conveys the external air to the middle ear. 



"We will glance briefly at the anatomical characters of the parts enumerated, 

 and which enter into the formation of the middle ear. 



1. The Membrana Tympani (Fig. 515, B). 



Situated on the external wall of the middle ear, which it separates from the 

 bottom of the auditory canal, this membrane is oval in shape ; its greater axis 

 measures 11 mm. It is thin and capable of vibrating. Its inner face, inclining 

 inwards and slightly convex, is adherent to the handle of the malleus. Its 

 external fcue — forming the bottom of the auditory canal — is slightly concave 

 (towards the meatus). The circumference is fixed in a bony frame named the 

 tympanal circle, which is sharply defined, but incomplete at its upper part, and 

 enveloped by the mastoid cells, the cavities of which radiate around this circle. 



Although very thin, this membrane is composed of three layers — a middle, 

 of a fibrous (and muscular) character (fibres radiating towards the centre, and 

 also circular) ; an external — epidermic ; and an internal — the mucous membrane 

 of the middle ear. It has vessels and nerves, but not in the external and middle 

 layers. The nerves are numerous, and extend to the epithelium. (This 

 membrane receives those vibrations of the air which set in movement the chain 

 of bones in the ear, and thus propagates them to the fenestra ovalis and labyrinth.) 



2. The Promontory, Fenestra Ovalis, and Fenestra Rotunda (Fig. 515). 



Placed in the upper part of the tympanic wall, the promontory is only a very 

 small eminence separating the fenestra rotunda from the fenestra ovalis. (It is 

 marked by grooves in which lie the branches of the tympanic nerves.) 



The fenestra ovcdis {fenestra vestibuJi), situated in front of the promontory, 

 is an opening the form of which is sufficiently indicated by its name. It is the 

 opening between the tympanum and osseous vestibule, and is closed by the base 

 of the stapes. In the Horse, its average diameters are '004 by "002 mm. 



The fenestra rotunda {fenestra cochlear) is separated from the preceding by 

 the promontory, and, behind this small projection, it is closed in the fresh state 

 by a thin membrane {m. tympani secundaria), that forms a kind of diaphragm 

 between the middle ear and the tympanic scala of the cochlea. Its dimensions 

 are about the same as those of the fenestra ovalis. (The aqueduct of Fallopius 

 is a canal commencing at the internal ear, passing above the fenestraa and 



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