THE ORGANS OF SENSE. 143 



ossicles are controlled to some extent by the tensor tympani, which is 

 inserted near the root of the handle of the malleus, and the stapedius, 

 which is inserted into the neck of the stapes. 



The Eustachian tube connects the cavity of the tympanum with 

 that of the pharynx. ll is generally closed, but is opened each time 

 swallowing occurs. When it opens, the air pressure in the middle ear is 

 adjusted to that of the atmosphere, and in this way the tympanic 

 structures are protected from the injurious effects of too small or too 

 great a pressure on the membrana tympani. 



THE INTERNAL EAR. 



The internal ear consists of a series of cavities in the temporal bone, 

 forming the osseous labyrinth, within which is a corresponding series 

 of membranous structures, the membranous labyrinth. The osseous 

 labyrinth contains a clear fluid, the perilymph ; the membranous 

 labyrinth is filled with a similar fluid, the endolymph. 



The anterior portion of the labyrinth, or cochlea, contains the end- 

 organs of hearing; the posterior part is concerned with the sense of 

 position, and will be described later. 



The Cochlea. The cochlea consists of a tube coiled in a spiral 

 fashion round a central bony modiolus and making altogether two and 

 a half turns round the latter. A bony ridge projects from the modiolus 

 into the tube and is known as the osseous spiral lamina ; attached to 

 this is a membrane, the basilar membrane, which extends to the outer 

 wall of the tube, where it meets a fibrous projection, the spiral ligament. 

 A relatively thick layer of connective tissue, the limbus laminae spiralis, 

 rests on the osseous spiral lamina, and ends abruptly near the basilar 

 membrane by an overhanging border. A delicate membrane, the 

 vestibular membrane or membrane of Reissner, is attached to the 

 upper surface of the limbus and to the wall of the tube in such a way 

 as to cut off a portion, triangular in area, known as the canal of the 

 cochlea (ductus cochlearis). The bony tube is thus divided by the 

 membrane of Reissner and the osseous spiral lamina with the basilar 

 membrane into three divisions, the scala vestibuli above Reissner's 

 membrane, the canal of the cochlea already described, and the scala 

 tympani. The scala vestibuli and scala tympani form part of the 

 bony labyrinth and communicate at the apex of the cochlea by the 

 helicotrema. The scala tympani is closed at its lower end by the 

 membrane in the fenestra rotunda. The scala vestibuli and scala 

 tympani contain perilymph. The canal of the cochlea forms part of 

 the membranous labyrinth and contains endolymph. It communicates 



