SPECIAL A-NxVrOMY. 165 



^smallest hone in the body.'' The incus resembles in shape a 

 human molar, one of the fangs of which articulates with os 

 orbiculare ; the other becomes attached to the bony wall of 

 the cavity. The crown has a synovial articulation with the 

 upper portion of head of the malleus, the handle of which 

 is attached to the inner surface of membrana tympani, 

 vibrations of which are thus communicated to the bony 

 chain, which, by the mechanical arrangement of its com- 

 ponent ossicles, communicates them in an increased degree 

 of intensity to the liquid contents of the vestibule, 

 which produces impressions upon the auditory cells, 

 made more powerful by the presence of the solid particles, 

 otoconites, in the liquid. The bones of the middle ear are 

 acted upon by muscles, of which we will only particularise 

 the stapedius, running from the wall of the cavity to the 

 stapes at its attachment to os orbiculare and tensor tympanij 

 running from the margin of the bony ring surrounding the 

 tympanum to the handle of the malleus at its attachment 

 to the membrana. The middle ear is lined by transparent 

 and thin mucous membrane, which covers all the structures 

 contained in it, and is continued into the mastoid cells. 

 It closes the foramen auditorium externum in forming the 

 inner lining of membrana tympani, and also fenestrae 

 rotunda and ovalis. It is continuous through the Eusta- 

 <;hian tube with that lining the guttural pouch and the 

 pharynx. The internal and middle ear are supplied with 

 blood by the styloid division of the posterior auricular 

 branch of the external carotid ; they receive sympathetic 

 fibres from the otic ganglion, a small collection of nerve- 

 <iells situate'l on the inner surface of the inferior maxillary 

 division of the fifth nerve, just after it has emerged from 

 sthe cranium throup^h foramen lacerum basis cranii. 



