792 OP THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



blindness" formerly described ( 631). It is not a little curious, that the 

 two defects are occasionally coexistent in the same individuals. 1 



[The late Dr. Samuel Jackson, Professor of the Institutes of Medicine in 

 the University of Pennsylvania, thought that the semicircular canals acted 

 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 inter- 

 nal ear of the Invertebrata. The production of mere sound or noise of dif- 

 ferent intensities would result from reflected undulatory vibrations main- 

 tained in the labyrinthic fluid, while the perception of immense numbers 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, were considered by Dr. Jackson as corresponding, in 

 function, with the pigmentum uigrum of the choroid coat in the organ of 

 vision. The precise 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 essential phe- 

 nomena and laws of each are identical. The knowledge of those of the one 

 sense demonstrates those of the other. The conditions of perfect vision and 

 perfect hearing are the same. They are, 1st. The existence of separate, in- 

 dependent, sensitive spaces or sections of the retina for distinct images and 

 perceptions of visual impressions. Volkmann estimates these to be 0.0005 

 mm.; and others at ^oooooo of an inch. 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 arrange- 

 ment of the retina ; (b) by the refracting apparatus of the globe of the eye, 

 that concentrates the uudulatory 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 cho- 

 roid coat. Their reflection from the exterior surface of the sclerotic coat, 

 and reiterated excitement of the retinal surfaces, is thus prevented. In Al- 

 binos the pigment of the choroid is either deficient or absent, and the conse- 

 quence 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 vesicles, amount- 

 ing to some thousands, and spread out in a manner to receive single, indi- 

 vidual impressions in the membranous vestibule, ampullse, and on the lamina 

 spiralis of the cochlea. 2d. By the molecular undulations or vibrations ex- 

 cited in the fluids, perilymph and endolyrnph, by the sonorous undulations 

 communicated by the stapes, occupying the fenestra ovalis. From this point 

 they radiate in expanding waves of undulations, strike on, and pass through 

 the membranous vestibule and ampulla?, on which the filaments of the ves- 

 tibular branch of the auditive nerve are arranged, producing a single, distinct 



1 See a collection of such cases by Dr. Pliny Earle, in Amer. Journ. of Med. Sci., 

 vol. xxxv. 



