658 THE SENSES. 



vision. The sonorous impulses, first communicated by the atmosphere 

 to the membrana tympaui, are thence transmitted through the bony 

 tissue of the malleus, incus, and stapes. From the base of the stapes 

 they pass to the perilymph of the vestibular cavity ; from that, through 

 the floating wall of the membranous sac, to the endolymph or the fluid 

 contained in its interior ; and it is the vibration of this internal fluid 

 which finally acts upon the sensitive nervous terminations in the audi- 

 tory spot. It is thus through a series of intermediate vibrations, that 

 sounds coming from the exterior produce their impression upon the 

 internal ear. 



Office of the Semicircular Canals. These singular appendages of 

 the bony and membranous labyrinth have attracted attention, especially 

 on account of the constancy of their occurrence and the peculiarity of 

 their position. The principal features of their anatomical history are 

 the following : 



1. They are universally present, as portions of the internal ear, in 

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

 absent only in amphioxus, where there is no organ of hearing whatever. 



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 struc- 

 tural development, especially in the organs of sense, is very incomplete. 1 

 In the lamprey there are two, and in the hag one only, the cavity of 

 which is confounded with that of the utricle ; the whole forming a mem- 

 branous canal bent upon itself like a ring. 



3. The three canals stand in three different planes, which are all per- 

 pendicular to each other. Thus one is vertical and longitudinal, in 

 respect to the axis of the petrous bone ; another vertical and transverse; 

 and the third transverse and horizontal. They represent accordingly, 

 by their position, the three dimensions of space ; and from this circum- 

 stance the idea was earty suggested that they might serve in some way 

 to indicate the direction in which sounds arrive from the exterior. 

 But subsequent researches have yielded nothing to corroborate this 

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

 sonorous impulses originally come, they must traverse the membrana 

 tympani and chain of bones, and finally reach the internal ear by the 

 same course. This view of the office of the semicircular canals is 

 therefore no longer entertained. 



Lastly, an essential point in their anatomical structure is that they 

 are destitute of nerve fibres, and consequently are w r anting in sensi- 

 bility. The only nervous distribution connected with them is that to 

 the ampullae situated at one of their extremities, but no nerve fibres 

 extend to the semicircular canals themselves. The function which they 

 perform must therefore in all probability be one of a mechanical or 

 physical kind. 



1 Owen, Anatomy of the Vertebrates. London, 1868, vol. iii. p. 222. Wagrner, 

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

 1845, p. 227. 



