

THE ORGANS OF SENSE 141 



SECTION IV. 

 THE SENSE OF HEARING. 



The ear consists of three parts, the outer, middle, and inner ear. 

 The outer ear consists of the pinna and the external auditory meatus. 

 The pinna is f unctionless in man, but in some of the lower animals it 

 serves to collect the sound waves and conduct them to the meatus, 

 along which they are transmitted to the membrana tympani. The 

 meatus is about 2'5 cm. in length, and is .directed inwards and forwards. 

 It is slightly curved in its course, the convexity of the curve being 

 upwards ; in consequence of the curve it? is difficult for foreign bodies 

 to reach the membrana tympani, which stretches across the inner end 

 of the meatus. 



THE MIDDLE EAR. 



The middle ear or tympanic cavity consists of a chamber in the 

 temporal bone containing a chain of ossicles by which the sound waves 

 are transmitted to the mternaL-eyr (fig. 45). The cavity is bounded 

 laterally by the membrana tympani^ its medial, superior, inferior, and 

 posterior walls are bony, and anteriorly it exhibits two openings, that 

 of the Eustachian tube (auditory tube) below, and the canal for the 

 tensor tympani muscle above. 



The membrana tympani, which separates the external from the 

 middle ear, lies obliquely, and is shaped like a shallow funnel with the 

 concavity outwards, the central depression being called the umbo. The 

 membrane is semi-transparent and. is composed of three layers, an outer 

 cutaneousTayer continuous with the skin lining the meatus, an inner 

 mucous layer formed of the mucous membrane lining the tympanic 

 cavity, and .a middle fibrous layer composed of radial and circular 

 fibres. 



Two openings are present in the medial wall of the tympanic cavity, 

 both bridged across by membrane in the fresh condition. One, the 

 fenestra ovalis (fenestra vestibuli), is oval in shape ; the other, the 

 fenestra rotunda (fenestra cochleae), lies below and behind the fenestra 

 ovalis. 



The ossicles of the middle ear are three in number, the malleus, incus, 

 and stapes. The malleus, or hammer bone, consists of a head and two 

 processes, the longer of which (the handle) is attached to the tympanic 

 membrane, its tip reaching to the umbo, while the shorter process, the 

 processus gracilis, projects anteriorly. The posterior surface of the head 

 of the malleus articulates with the body of the incus by a saddle- 



