491 



nomena. Subjective sounds may be produced by various modes of 

 irritation ; but actual sonant vibrations can only excite the acoustic 

 filaments through the medium of the rods of Corti, or the correspond- 

 ing terminal structures in the vestibule. Corresponding terminal 

 structures are in like manner appended to the tactile, olfactory, and 

 gustatory nerves, apparently for a similar purpose, to provide the 

 necessary conditions of the initial excitement of the nervous current 

 by those secondary properties of external bodies to which the organs 

 of touch, taste, and smell, are related. 



When the attention of anatomists was directed, a few years ago, 

 to the structure and physiological signification of the columns of the 

 retina by the observations of H. Miiller and KoUiker, I became 

 satisfied that those structures are not, as the latter asserted, ner- 

 vous structures, properly so called, but special structures, of the 

 same nature as the Pacinian bodies and the tactile corpuscles. I 

 stated and explained my opinion of the nature of these bodies 

 in a lecture on the retina delivered and reported in 1854. But I 

 had generalized these relations of nervous filaments to special ter- 

 minal exciting structures, still further, in the zoological lectures 

 which I delivered in 1853, for my late distinguished colleague and 

 preceptor Professor Jameson. I also expounded it at considerable 

 length in my course of lectures last winter (1855-6). I shall now 

 state the doctrine in general terms, not only because it is necessary 

 for the elucidation of the distinctive characters of the simple and 

 compound forms of eye ; but also because I am anxious to put on 

 record, by submitting it to this Society, a generalization which ap- 

 pears to me of primary importance in the general physiology of the 

 nervous system. I assume, as established, the doctrine of Du Bois 

 Raymond, that a nerve filament is capable of propagating the ner- 

 vous current equally well in both directions ; and that the physical 

 and physiological characters of this current differ in no respect, are 

 in fact identical in the so-called motor and in the so-called sensory 

 filaments, whether special or common, I also assume as established, 

 that the specific manner in which a centripetal nerve current is con- 

 verted at the central extremity of the filament, that is to say, is 

 physiologically refiected into motor filaments, or, psychically inter- 

 preted as sensation, depends upon the physiological or psychical en- 

 dowments of the different portions of the nervous centre with which 

 the filaments are connected. These two positions being assumed, 



