742 SPECIAL SENSES 



the tensor-tympani muscle, while it contracts to secure for the mem- 

 brana tympani the degree of tension most favorable for vibration under 

 the influence of certain sounds, puts the chain of bones in the condition 

 best adapted to the conduction of the vibrations of the membrane to the 

 labyrinth, with the smallest loss of intensity. 



PHYSIOLOGICAL ANATOMY OF THE INTERNAL EAR 



The internal ear consists of the labyrinth, which is divided into the 

 vestibule, semicircular canals and cochlea. The general arrangement 

 of these parts has already been described ; and it remains only to study 

 the structures contained within the bony labyrinth, in so far as their 

 anatomy bears directly on the physiology of audition. Passing inward 

 from the tympanum, the first division of the internal ear is the vestibule. 

 This cavity communicates with the tympanum by the fenestra ovalis, 

 which is closed in the natural state by the base of the stapes. It com- 

 municates, also, with the semicircular canals and with the cochlea. 



General Arrangement of the Membranous Labyrinth. The bony laby- 

 rinth is lined with a moderately-thick periosteum, consisting of connective 

 tissue, a few delicate elastic fibres, nuclei and bloodvessels and spots of cal- 

 careous concretions. This membrane adheres closely to the bone and ex- 

 tends over the fenestra ovalis and the fenestra rotunda. Its inner surface 

 is smooth and is covered with a single layer of cells of endothelium, which 

 in some parts is segmented and in others forms a continuous nucleated 

 sheet. In certain portions of the vestibule and semicircular canals, the 

 periosteum is united to the membranous labyrinth more or less closely 

 by fibrous bands, which have been called ligaments of the labyrinth. 

 The fenestra rotunda, which lies between the cavity of the tympanum 

 and the cochlea, is closed with a membrane formed by an extension of 

 the periosteum lining the cochlea, on one side, and the mucous mem- 

 brane lining the tympanic cavity, on the other. 



In the bony vestibule, occupying about two-thirds of its cavity, are 

 two distinct sacs : a large ovoid sac, the utricle, situated in the upper 

 and posterior portion of the cavity, and a smaller rounded sac, the saccule, 

 situated in its lower and anterior portion. These two sacs communicate 

 with each other through a small canal in the form of the letter Y, which 

 is represented in the upper diagram in Fig. 191. The utricle communi- 

 cates with the three semicircular canals, and the saccule is connected 

 with the true membranous cochlea by the canalis reuniens. At a point 

 in the utricle corresponding to the entrance of a branch of the auditory 

 nerve, is a round whitish spot, called the acoustic spot (macula acustica), 

 containing otoliths, or otoconia, which are attached to the inner surface 



