UTRICLE ANT) SACCULE 267 



Further, if both the slopes and round nuMnl)r<uu' wci-c free to 

 move, hearing' in the ease of a diseased middle ear would not be 

 so good as when t>nly one ol" the pair were free. This is because 

 part of the displacement of the cochlear fluid caused by the 

 vibrations of the surrounding bone is dissipated by moving the 

 stapes outwards. Peo{)le may hear fairly well after the stapes has 

 become inmiovably fixed in the fenestra ovalis. In cases where the 

 drum of the ear has been punctured, hearing may be improved by 

 fixation of the stapes, e.g. by application of a plug of cotton wool. 



When sounds are conducted to the inner car by means of the 

 bones of the skull, in people with normal hearing, the intensity 

 of the sound is markedly increased if the movement of the stapes 

 is hindered. For example, if a vibrating tuning-fork is placed on 

 the region of the interparietal suture, when both ears are un- 

 obstructed and normal, sound is heard equally by both. If the 

 drum of one ear and appended ossicles are hindered from taking a 

 full excursion by blocking the meatus with a finger, the sound 

 appears most distinctly at this ear. When both ears are treated 

 in this way, localisation is again median. A conuuon entotic 

 phenomenon is the audibility of the pulse in an obstructed ear. 

 It may be due to the transmission of the pulse-wave oscillation 

 to the air of the middle ear — which acts as a resonator — reinforcing 

 the vibrations and then transmitting them to the internal ear. 

 It is more probable, however, that the beat of the carotid artery 

 is transmitted through the parietal bone direct to the fluid of the 

 cochlea. 



Balance. 



The ear is a double organ. We have just dealt with its function 

 as the receiver and analyser of certain vibrations in matter trans- 

 mitted to it generally through the air, but capable of transmission 

 through much denser media. The other and more ancient function 

 is that of giving information as to (i.) the position of our head in 

 space, and (ii.) the acceleration of the head in space. 



Structure (Fig. 63). The vestibule (Fig. 63 (9) ) and the semi- 

 circular canals (10), like the cochlea (6), are double organs. The 

 outer or osseous part is hollowed out of the substance of the bone, 

 lined by periosteum and filled with perilymph in which the mem- 

 branous part is placed. The various parts of the membranous 

 labyrinth, viz. {a) utricle and saccule, contained in the vestibule, 

 (6) three semicircular ducts, with their ampullae, in the osseous 

 semicircular canals, and (c) the scala media in the cochlea are filled 

 with endolymph and are in fluid connection with one another. 

 The semicircular ducts open into the utricle, the utricle into the 



