No. XVII.] APPENDIX. 263 



Having developed photo-voltaic circuits, the eye itself next demands 

 our attention ; and we find nerve and blood to be abundantly supplied to 

 that organ. The electro-voltaic test is best applied by the insertion of one 

 needle into the choroid, the second into the muscles of the eyeball, and 

 I found a slight deflection of the galvanometer when a strong light was 

 thrown into the eye, proving that vision was a voltaic phenomenon. 



The essential part of the organ of hearing is encased in textures of 

 such extreme hardness, that it will probably be for ever prevented from 

 being the subject of direct experiment. In the cochlea, I believe we 

 may reasonably assume that the pitch of the note is determined ; and 

 in the semicircular canals which are placed in the three orthogonal planes 

 of a cube, physiologists are pretty generally agreed that animals learn the 

 direction of sound. Blood and nerve essentials to voltaic action are 

 here distributed, and no physical difficulty is presented to the probability 

 of a voltaic circuit being determined by sounds. 



The nasal organ is, like the ear and eye, liberally supplied with blood 

 and nerve-fibres. The voltaic circuit is easily demonstrated by the electro- 

 voltaic test; but the animal has an extraordinary repugnance to the 

 operation, and you must be extremely careful not to be deceived by other 

 secretions which are competent to set up the voltaic action. I can very 

 readily show you that it is not at all difficult to form voltaic circuits, 

 in which odours should excite the electric action. The tube which I hold 

 in my hand contains two iron plates, which are separated by a mem- 

 brane ; and on each side pieces of sponge, dipped in very dilute muriatic 

 acid, are arranged. Now, if ammoniacal vapour, which produces the 

 most powerful action on the natural nose, be brought under one side 

 of the diaphragm, you perceive that a very strong action of the needle is 

 immediately produced. The experiment which I have selected is one 

 which shows the result easily, rapidly, and in a very marked manner; 

 but I should not think it a bold assertion to declare, that with a little 

 trouble and patience I could exhibit voltaic effects, although perhaps to 

 a less marked extent, with every other odoriferous body. 



When an animal tastes, the matter which contains the savour comes 

 in immediate contact with the tongue, and is there probably absorbed. I 

 need hardly state that the essentials for sensation, blood and nerve, 

 are abundantly supplied to that organ. With respect to physical con- 

 trivances analogical with the tongue, it is very easy to show voltaic 

 force excited by savours ; and I have here a Y-shaped tube, containing 

 a solution of pernitrate of iron, and two platinum poles, which exhibit 

 by themselves no signs of electric action. As soon, however, as I drop 

 a little infusion of meat into one side of the tube, you will instantly 

 perceive that the galvanometer shows signs of action. There is no 

 mystery about the meat, as sugar, or in fact any other savour, would 

 have had a similar property in a greater or less degree. The direct 

 examination of the tongue in the living animal affords unsatisfactory 

 results, inasmuch as secretions in the mouth are very apt to give wrong 

 results a circumstance which should be very carefully guarded against. 



The last organ of sensation to which I have to beg your attention is 

 the skin. Now, by the ordinary sensor nerves, we derive two sets of 

 impressions of somewhat different characters for instance, we are enabled 

 to judge of impressions upon the body by either heat or force, or what 



