408 SPECIAL PHYSIOLOGY. 



in the same manner as the dampers of a piano stop the vibrations of 

 the strings, so as to prevent the confusion of successive notes, thus 

 performing, as it were, an office corresponding with that of the choroid 

 coat in the eye, which absorbs the rays of light after they have acted 

 on the retina, and thus prevents the confusion of successive images. 

 The idea that the cochlea is an organ for distinguishing pitch, was 

 suggested by Duges, and considered by him to be supported by the 

 general concurrence of the development of the cochlea with the rela- 

 tive extent of the vocal sounds, in the same class of animals, as may 

 be understood by comparing the cochlea and the voice in Mammalia, 

 Birds, and Reptiles. Helmholz further attributes to the graduated 

 structures of the lamina spiralis of the cochlea, the office of receiving 

 the impressions which produce the so-called sound colors already re- 

 ferred to, and so of aiding in the recognition, not only of the pitch, 

 but of the timbre or quality of sounds. Each nervous filament is sup- 

 posed to receive single vibrations; and the combinations of these in 

 harmonic groups, with the fundamental notes, in the production of 

 ordinary sounds, are recognized with greater or less facility, by dif- 

 ferent persons. There are individuals as unable to appreciate musical 

 sounds, as others are to distinguish colors; whilst persons possessed of 

 an acute musical ear, may be compared with those who are remarkable 

 as colorists. Supposing that the cochlea may be the part through 

 which we receive impressions concerning pitch and timbre, and so of 

 melody and harmony, the membranous labyrinth may be the part which 

 informs us of the intensity, quality, or loudness of sounds. 



The semicircular canals, or rather, their contained membranous 

 canals, assist in the reception of sounds from the cranial walls. They 

 are also supposed to be specially concerned in distinguishing the direc- 

 tion of sounds. The relative position of these canals favors this idea; 

 for in Man, and in almost all animals in which they exist, they occupy 

 three planes nearly at right angles with each other, and therefore cor- 

 responding with the three dimensions of a cube. 



[The semicircular canals, evidently intended to perfect the sense of 

 hearing in man and the higher animals, are believed by Dr. Samuel 

 Jackson. Emeritus Professor of the Institutes of Medicine in the Uni- 

 versity of Pennsylvania, to act by suppressing the sonorous undulations 

 or vibrations of the lymph of the vestibule, which are the immediate 

 excitants of the sense of hearing. They arrest the waves of reflexion, 

 which would necessarily occur in a simple cavity, wholly limited by 

 plane surfaces, as the vestibule would be without these appendages, 

 and as is the case with the rudimentary vestibule or internal ear of 

 the Invertebrata. The production of mere sound or noise of different 

 intensities would result from reflected undulatory vibrations main- 

 tained in the labyrinthic fluid, while the perception of immense num- 

 bers of fine and delicate tones, and varying qualities of sound, now so 

 characteristic of the hearing of man and the higher animals, would be 

 rendered impossible in the confusion of vibrations to and fro in the 

 fluid of the labyrinth, but for the semicircular canals, by which they 

 are suppressed. These canals, in the apparatus of hearing, are con- 



