843 SPECIAL SENSES. 



ossicles and the action of the tensor tympani muscle, both as regards the chain of bones 

 and the membrana tyrapani, direct observations are wanting to show the exact relations 

 of these different conditions of the ossicles and of the membrane to the physiology of 

 audition. One very important physical point, however, which has been the subject of 

 much discussion, is settled. The chain of bones acts as a single solid body in conducting 

 vibrations to the labyrinth. It is a matter of physical demonstration that vibrations of 

 the bones themselves 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 vibra- 

 tions. Practically/then, the ossicles have no independent vibrations that we can appre- 

 ciate. 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 

 with the tension of the membrane, it would seem that, when the tensor tympani con- 

 tracts, it must render the conduction of sound-waves to the labyrinth more delicate than 

 when the auditory apparatus is in a relaxed condition, which we may compare with the 

 "indolent" condition of the apparatus of accommodation of the eye. When the mem- 

 brana tympani is relaxed and the cog-like articulation between the malleus and the incus 

 is loosened, the vibrations of the membrane and of the malleus may have a greater ampli- 

 tude ; 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 tones, 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 possible loss of intensity. 



Physiological Anatomy of the Internal Ear. 



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

 circular canals, and cochlea. The general arrangement of these parts has already been 

 described ; and it remains for us only to study the structures contained within the bony 

 labyrinth, in so far as their anatomy bears upon the physiology of audition. The most 

 delicate and complicated points, by far, in the anatomy of the auditory apparatus are 

 connected with the histology of the internal ear, which, since the researches of Corti, has 

 been studied very closely, particularly in Germany. We shall avoid, however, the dis- 

 cussion of histological questions of purely anatomical interest and confine ourselves to 

 those points which have a direct bearing upon physiology. 



Passing inward from the tympanum, the first division of the internal ear is the ves- 

 tibule. 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, numerous nuclei, and blood-vessels, with spots of calcareous concretions. This 

 membrane adheres closely to the bone and extends over the fenestra ovalis and the fenes- 

 tra rotunda. Its inner surface is smooth and covered with a single layer of cells of pave- 

 ment-epithelium, which in some parts is segmented and in others forms a continuous 

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

 teum 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 



