27G MODE IN WHICH LIGHT ACTS ON THE RETINA. 



ago, to the structure and physiological signification of the 

 columns of \ho retina, by the observations of H. Miiller and 

 Kolliker, I became satisfied that those structures are not, as the 

 latter asserted, nervous structures, properly so called, but 

 special structures, of the same nature as the Pacinian bodies 

 and the tactile corpuscles. I stated and explained my ojiinion 

 of the nature of these bodies in a lecture on the retina delivered 

 and reported in 1854.* But I had generalised these relations 

 of nervous filaments to special terminal exciting structures, 

 still further, in the zoological lectures which I delivered in 

 1853, for my late distinguished colleague and preceptor, Pro- 

 fessor 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 generalisation which appears to me of primary importance 

 in the general physiology of the nervous system. I assume, 

 as established the doctrine of Du Bois Eeymond, that a nerve- 

 filament is capable of propagating the nervous current equally 

 well in both directions ; and that the physical and physiolo- 

 gical 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 converted at the central extremity of the fila- 

 ment — that is to say, is ph}'Tsiologically reflected into the motor 

 filaments, or psychically interpreted as sensation — depends 

 upon the physiological or psychical endowments of the differ- 

 ent portions of the nervous centre with whicli the filaments 

 are connected. These two positions being assumed, then, I 

 hold that, although the ultimate nervous filament may have 

 * Edinburgh Medical Journal, p. 377, 1855 ; and No. XII. in this volume. 



