74 Dr Graves on the Sense of Touch. 



tion, a fact too familiar to require any elaborate illustration. As 

 to the idea of direction which we derive from the sensation impart- 

 ed to the skin by any minute substance, he justly observes, that 

 it is always judged to be perpendicular to the surface of the skin 

 at the point of contact. Of this there can be no doubt, and here 

 we have a very striking analogy between the sense of vision and 

 of touch, for it is a primary law, that rays of light impingent on 

 the retina always produce a sensation, i. e. are seen in a direction 

 perpendicular to that point ; it would be well worth while exa- 

 mining whether the same law of perpendicularity is extended also 

 to the ear. In the case of the eye this law is strikingly useful, as 

 it enables many rays, originally diverging from the same luminous 

 point, all to create a sensation in the same direction, although 

 in converging they strike the retina from very different direc- 

 tions; in the eye all these perpendicular lines intersect at a com- 

 mon point, thence called the centre of visible direction, and this 

 result derived from the spherical shape of the retina is attended 

 with the most important consequences. No one has as yet at- 

 tempted to investigate the question, whether any similar provi- 

 sion or contrivance exists with regard to the lines of direction, 

 to which each part of the auditory nerve receiving vibrations re- 

 fers sound ; any given point of the hearing surface of the acoustic 

 nerve receives impulses from the vibration essential to this sense, 

 conveyed either through the fluid of the vestibule and semicircu- 

 lar canals, or through the solid bone surrounding the cochlea ; the 

 question arises, whether vibrations excited originally by the sound- 

 ing body arrive by different routes simultaneously at the same 

 point of the nerve, so as materially to reinforce and strengthen 

 each other. Is there in this case any provision made to pre- 

 vent vibrations, which arrive in different directions, from inter- 

 fering with each other, with reference to the sensation they pro- 

 duce? Or are both, as impinging on a common point, referred 

 to one common direction ? If this were the case, the analogy be- 

 tween the perceptive properties of the retina and auditory nerve 

 would be perfect, and nothing w^ould remain to the philosophical 

 examiner of the mechanism of the sense of hearing but to dis- 

 cover what relation these lines of common direction bear to the 

 surface of the auditory nerve, and to each other ; are they, as 

 in the case of the retina, perpendicular to the nervous surface, 



