THE SENSES. 561 



Physiological Action of the Membranous Labyrinth. The sacculus 

 and utricle are membranous formations suspended in the fluid of the 

 vestibule and supplied by fibres of the auditory nerve. They are the 

 essential parts, in the auditory apparatus, for the reception of sonorous 

 impressions. The vibrations of the atmosphere, communicated to the 

 membrana tympani, are thence transmitted through the malleus, incus, 

 and stapes. From the base of the stapes they pass to the perilymph 

 of the vestibular cavity ; thence, through the wall of the membranous 

 sac, to the endolymph or fluid in its interior ; and the vibration of this 

 internal fluid acts upon the nervous terminations at the auditory spot. 

 It is thus, through a series of intermediate vibrations, that sounds 

 coming from without finally produce their impression on the internal 

 ear. 



Office of the Semicircular Canals. These singular appendages 

 have attracted special attention, owing to the constancy of their occur- 

 rence and the peculiarity of their position. The principal features of 

 their anatomical history are the following : 



1. They are always present, as portions of the internal ear, in mam- 

 malians, birds, and reptiles, and nearly always in fish ; being entirely 

 absent only in amphioxus, where there is no organ of hearing what- 

 ever. 



2. They are always three in number. The only exception to this 

 rule is found among fishes, in the lamprey and the hag ; where the 

 entire structural development is very incomplete.* In the lamprey 

 there are two, and in the hag only one, the cavity of which is con- 

 founded with that of the utricle, forming a ring-like membranous canal. 



3. The canals stand in three different planes, perpendicular to each 

 other. One is vertical and longitudinal, in relation to the petrous bone ; 

 another vertical and transverse ; and the third transverse and horizon- 

 tal. They represent accordingly, by their position, the three dimen- 

 sions of space ; and from this circumstance it has been surmised that 

 they serve to indicate the direction from which sounds are perceived. 

 But subsequent researches have yielded nothing to corroborate this 

 view; and it is evident, furthermore, that, from whatever quarter 

 sounds may originally come, they must reach the internal ear, through 

 the membrana tympani and chain of bones, by the same course. 



Lastly, an essential point in the structure of the semicircular canals 

 is that they are destitute of nerve fibres, and consequently wanting 

 in sensibility. The only nervous distribution connected with them is 

 that to the ampullaB at their extremities, but no fibres extend to the 

 canals themselves. Their function must therefore in all probability 

 be of a mechanical or physical nature. 



In experimenting upon the internal ear in animals, it has been re- 

 marked that division or injury of the semicircular canals is followed 



* Owen, Anatomy of the Vertebrates. London, 1868, vol. iii., p. 222. Wagner, 

 Comparative Anatomy of the Vertebrate Animals, Tulk's translation. New York, 

 1845, p. 227. 



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