USE OF THE COCHLEA. 407 



dius relaxes those three membranes, their reciprocal, but opposed, 

 actions may be compared to those of the circular and radiating fibres 

 of the iris, the former of which narrow, whilst the latter widen, the 

 pupil, or aperture through which the light enters the eye. It is in- 

 teresting to note, that these two little muscles of the ear are, like the 

 circular and radiating fibres of the iris, supplied, the one by a cranial 

 motor nerve, viz., the facial, and the other by a sympathetic branch, 

 viz., from the otic ganglion, the two sets of the fibres of the iris being 

 supplied, the former by the third pair, the latter by branches from the 

 ophthalmic ganglion of the sympathetic. 



Concerning the functions performed by the various parts of the 

 labyrinth, nothing is positively known. The fluid contained in its 

 chambers, serves to support the various membranes and nervous struc- 

 tures within it, in a certain degree of tension; and it is, as already 

 stated, the last medium by which the sonorous vibrations are finally 

 conveyed to the auditory nerve, the material particles of which must 

 also be thrown into corresponding mechanical vibrations. 



The vestibule is the part essential to the simplest exercise of the 

 sense of hearing; for, even in the most rudimentary conditions, both 

 of the human ear, and of that of the lowest Vertebrata, the vestibule, 

 or the central chamber of the labyrinth, is the part first developed in 

 connection with the recipient extremities of the auditory nerve. It 

 is, in the lowest Fishes, the only part of the labyrinth which is present. 

 Moreover, so long as it remains unimpaired, the sense of hearing in 

 Man and the higher Vertebrata, is not lost, although all the external 

 and tympanic portions of the ear be destroyed. From its position 

 opposite to the base of the stapes, it must first receive the sonorous 

 vibrations travelling by that path, and may serve to transmit them to 

 the rest of the labyrinth. 



The cochlea and its nerves must undoubtedly receive direct vibra- 

 tions, i. e., vibrations communicated through the bones of the cranium ; 

 for there exists an intimate connection between the lamina spiralis, on 

 which the nerve tubules are distributed, and the osseous walls of the 

 labyrinth, the two being continuous with each other. But the nerves 

 distributed to the membranous part of the lamina, spiralis of the cochlea, 

 must also receive sonorous vibrations transmitted from the tympanic 

 apparatus, either by the fenestra ovalis, or the fenestra rotunda, 

 through the fluid of the cochlea and its special recipient and trans- 

 lating apparatus, which converts the mechanical into the nervous sonor- 

 ous vibrations. It has been conjectured, that the lamina spiralis, with 

 its highly complex, denticulate, rod-like, and nervous structures, is, 

 on account of the graduated lengths of some, at least, of its component 

 parts, connected with the reception of sounds of different pitch, the 

 sonorous undulations of which are themselves of various lengths. The 

 rods of Corti especially, have been supposed to act like vibrating elastic 

 bars of different lengths, just as different sized tuning-forks vibrate in 

 unison only with their own note, or with its harmonics. Another con- 

 jecture may be offered, namely, that in the complex apparatus of the 

 cochlea, there may also exist contrivances for arresting the vibrations, 

 after they have accomplished the due stimulation of the auditory nerve, 



