THE SEMICIRCULAR CANALS. 



409 



sidered by Dr. Jackson as corresponding, in function, with the pig- 

 mentum nigrum of the choroid coat in the organ of vision. The pre- 

 cise mode in which this suppression is accomplished, will be better 

 appreciated by following up Dr. Jackson's comparison: 



" The two senses and their apparatus are homologous. The essen- 

 tial phenomena and laws of each are identical. The knowledge of 

 those of the one sense demonstrates those of the other. The condi- 

 tions of perfect vision and perfect hearing are the same. They are, 

 1st, The existence of separate, independent, sensitive spaces or sec- 

 tions of the retina for distinct images and perceptions of visual impres- 

 sions. Volkman estimates these to be 0.0005 mm. ; and others at 

 suouooo f an i llcn ' 2d, A single distinct impression made by the 

 molecular vibration of the ether the excitor of the sense of sight. 



" The above conditions are obtained (a) by the special anatomical 

 arrangement of the retina ; (b) by the refracting apparatus of the 

 globe of the eye, that concentrates the undulatory rays of the ether, 

 proceeding from every point of a visual object on the distinct, sensitive 

 points or spaces of the retina; (c) by the suppression of the undulatory 

 vibrations immediately they have excited an impression on the retina, 

 by the black pigment of the choroid coat. Their reflection from the 

 exterior surface of the sclerotic coat, and reiterated excitement of the 

 retinal surfaces, is thus prevented. In Albinos the pigment of the 

 choroid is either deficient or absent, and the consequence is indistinct 

 vision in daylight, from the general excitement of the retina by the 

 reflected undulations of the ether occupying the globe of the eye. 



"The same conditions are obtained in hearing: 1st, By the auditive 

 nerve being decomposed into its separate filaments and ganglionic ves- 

 icles, amounting to some thousands, and spread out in a manner to 

 receive single, individual impressions in the membranous vestibule, 

 ampullae, and on the lamina spiralis of the cochlea. 2d, By the mole- 

 cular undulations or vibrations excited in the fluids, peri- and endo- 

 lymph, by the sonorous undulations communicated by the stapes, occu- 

 pying the fenestra ovalis. From this point they radiate in expanding 

 waves of undulations, strike on, and pass through the membranous 

 vestibule arid ampullae, on which the filaments of the vestibular branch 

 of the auditive nerve are arranged, producing a single, distinct impres- 

 sion, reinforced by the resonance of the superimposed otoconiae, and 

 exciting a single and distinct impulse and perception of sound. These 

 bodies act like the sounding-board of the piano. The sonorous vibra- 

 tion having thus completed its office, the specific excitation of the sense 

 of hearing must, like that of the visual vibration, cease, or be sup- 

 pressed. This occurs, in part, in the ampullae, but mostly in the 

 semicirculaY canals. 



"The vibrations of the endolympb, reaching the ampullae, are par- 

 tially broken and weakened at their openings: those entering the 

 ampullae again expand, losing thereby their impetus, and either die 

 away against the membranous walls, or come in contact with the 

 vibrations of the perilymph on their exterior. The two can scarcely 

 be in perfect consonance of expansion or condensation, and interference 

 ensues, by which they are suppressed. In this mode all ,the feebler 



