THE SENSE OF HEARING. 



751 



stantly moving his ears to determine the direction of sounds, but in 

 man this function is greatly subordinated. The twisting of the 

 mouth of the meatus, and the hairs and wax in the external meatus, 

 also keep out dust and insects. 



Auditory Field. Like the visual field, we have an auditory field. 

 It is all the points in space from which sound-waves can be collected 

 by the auricle and transmitted by the auditory canal. Its extent 

 and form depend upon the conformation of the auricle. 



MIDDLE EAK, OR TYMPANUM. The tympanum is a space situ- 

 ated within the substance of the petrous portion of the temporal 

 bone. It is composed of two bony and four soft parts. 



Fig. 308. Tympanic Membrane and Auditory Ossicles, seen from the 

 Tympanic Cavity. (LANDOIS.) 



M, Manubrium, or handle of the malleus. T, Insertion of the tensor tym- 

 panl. h, Head. IF, Long process of the malleus, or incus-tooth. The short 

 (K) and the long (I) process. 8, Plate of the stapes. Ax is the common axis 

 of rotation of auditory ossicles. 8, The pinion-wheel arrangement between the 

 malleus and incus. 



The two bony parts comprise the walls of the cavity, with the 

 mastoid cells and Eustachian tube; also the ossicles or bones of 

 the ear. 



The soft structures are: (1) the ligaments and muscles of the 

 little ossicles, (2) the mucous membrane of the tympanic cavity, (3) 

 the lining of the Eustachian tube, and (4) the membrana tympani 

 and membrane of the round window. 



In otitis media pus may cause a disintegration of the mastoid 

 colls, from which it frequently extends to the membranes of the 

 brain. 



