THE INTERNAL EAR. 369 



The muscles of the Eustachian tube are the levator and the tensor 

 palati, which lie beneath and to the inner side of the tube and to its outer 

 side respectively. By reason of the intimate attachment which both muscles 

 have to the cartilage of the tube, since both take partial origin from this 

 structure, contraction of their fibres, particularly those of the tensor palati. 

 tends to draw apart the walls of the canal; they thus serve as dilators. 



The Mastoid Cells. The tympanic cavity communicates posteriorly, 

 through the antrum, with a variable number of irregular pneumatic cavities, 

 the mastoid cells, so called because the majority of these spaces occupy the 

 mastoid process. Unlike the antrum, these cells are not developed at birth. 

 As the mastoid process develops, the original diploetic structure is usually 

 more or less replaced by larger cavities forming the pneumatic type. These 

 spaces are filled with air and lined by a very thin mucous membrane, which 

 is continuous with that of the antrum and of the tympanic cavity. It is 

 closely united with the periosteum and possesses a layer of low nonciliated 

 squamous epithelium. 



THE INTERNAL EAR. 



The internal ear consists essentially of a highly complex membranous 

 sac, connected with the peripheral ramifications of the auditory nerve, and 

 a bony capsule, which encloses all parts of the membranous structure and is 

 embedded within the substance of the petrous portion of the temporal bone. 

 These two parts, known respectively as the membranous and the bony laby- 

 rinth^ are not everywhere in close apposition, but in most places are sepa- 

 rated by an intervening space filled with a fluid, the perilymph, the inner 

 sac lying within the osseous capsule like a shrunken cast within a mould. 

 The membranous labyrinth is hollow and everywhere filled with a fluid, 

 called the endolymph, which nowhere gains access to the cavity occupied by 

 the perilymph. The internal ear is closely related with the bottom of the 

 internal auditory canal, which its inner wall contributes, on the one side, and 

 with the inner wall of the tympanic cavity on the other. Its entire length 

 is about 20 mm., and its long axis corresponds closely with that of the 

 pyramidal or petrous portion of the temporal bone. The irregular cavity of 

 the bony labyrinth comprises three subdivisions : a middle one, the vestibule; 

 an anterior one, the cochlea; and a posterior one, the semicircular canals. 

 Both the front and hind divisions communicate freely with the vestibule, but 

 neither communicates with the membranous labyrinth nor, in the recent con- 

 dition, with the tympanic cavity. Although corresponding in its general 

 form with the bony compartments of the cochlea and semicircular canals, the 

 membranous labyrinth less accurately agrees in its contour with the bony 

 vestibule, since, instead of presenting a single cavity, it is subdivided into 

 two unequal compartments, known as the saccule and the utricle, which are 

 lodged within the bony vestibule. The divisions of the membranous laby- 

 rinth are, therefore, four, which from before backwards are: the membranous 

 cochlea, the saccule, the utricle, and the membranous semicircular canals. 



THE OSSEOUS LABYRINTH. 



The vestibule, the middle division of the bony labyrinth, lies between 

 the cochlea in front and the semicircular canals behind and communicates 

 freely with both. It is an irregularly elliptical cavity, measuring about 5 

 mm. from before backwards, the same from above downwards, and from 3-4 

 mm. from without inwards. The lateral (outer) wall separates it from the 



