PHYSIOLOGICAL ANATOMY OF THE INTERNAL EAR. T55 



selves would be infinitely rapid as compared with the highest tones which can 

 be appreciated by the ear, if it were possible to induce in these bones regular 

 vibrations. Practically, then, the ossicles have no independent vibrations 

 that can be appreciated. This being the fact, the ossicles simply conduct to 

 the labyrinth the vibrations induced in the membrana tympani by sound- 

 waves ; and their arrangement is such that these vibrations lose very little in 

 intensity. While it has been shown experimentally that the amplitude of 

 vibration in the membrana tympani and the ossicles diminishes as the tension 

 of the membrane is increased, it would seem that when the tensor tympani 

 contracts, it must render the conduction of sound-waves to the labyrinth 

 more delicate than when the auditory apparatus is in a relaxed condition, 

 which may be compared with the " indolent " condition of accommodation of 

 the eye. When the membrana tympani is relaxed and the cog-like articula- 

 tion between the malleus and the incus is loosened, the vibrations of the 

 membrane and of the malleus may have a greater amplitude ; but when the 

 malleo-incudal joint is tightened and the stapes is pressed against the fenestra 

 ovalis, the loss of intensity of vibration, in conduction through the bones to 

 the labyrinth, must be reduced to the minimum. With this view, the tensor 

 tympani muscle, while it contracts to secure for the membrana 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 conduc- 

 tion of the vibrations of the membrane to the labyrinth, with the smallest 

 possible loss of intensity. 



PHYSIOLOGICAL ANATOMY OF THE INTERNAL EAR. 



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

 bule, 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 

 upon 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 communicates, also, with the semicircular 

 canals and with the cochlea. 



General Arrangement of the Membranous Labyrinth The bony labyrinth 

 is lined by a moderately thick periosteum, consisting of connective tissue, a 

 few delicate elastic fibres, nuclei and blood-vessels, with spots of calcareous 

 concretions. This membrane adheres closely to the bone and extends 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 by a 

 membrane formed by an extension of the periosteum lining the cochlea, 



