230 OF SENSATION, AND THE ORGANS OF THE SENSES. 



of resistance, is well known to occasion, when exerted on the eye, the sensa- 

 tion of light and colours; and, when made 4 with some violence on the ear, to 

 produce tinnitus aurium. It is not so easy to excite sensations of taste and 

 smell by mechanical irritation; and yet, as Dr. Baly* has shown, it may 

 readily be accomplished in regard to the former. The sense of nausea may 

 be easily produced, as is familiarly known, by mechanical irritation of the 

 fauces. The stimulus of electricity still more completely possesses the power 

 of affecting all the sensory nerves, with the changes which are peculiar to 

 them ; for, by proper management, an individual may be made conscious at 

 the same time of flashes of light, of distinct sounds, of a phosphoric odour, of 

 a peculiar taste, and of pricking sensations, all excited by the same cause, the 

 effects of which are modified, according to the respective peculiarities of the 

 instruments through which it operates. But although there are some stimuli 

 which can produce sensory impressions on all the nerves of sensation, it will 

 be found that those, to which any one organ is peculiarly fitted to respond, 

 produce little or no effect upon the rest. Thus the ear cannot distinguish the 

 slightest difference between a luminous and a dark object. A tuning-fork, 

 which when laid upon the ear whilst vibrating, produces a distinct musical 

 tone, excites no other sensation when placed upon the eye than a slight jarring 

 feeling. The most delicate touch cannot distinguish a substance which is 

 sweet to the taste, from one which is bitter; nor can the taste (if the commu- 

 nication between the mouth and the nose be cut off) perceive any thing peculiar 

 in the most strongly-odoriferous bodies. 



311. It may hence be inferred that no nerve of special sensation can, by any 

 possibility, take on the function of another. How far the nerves of common 

 sensation can, under any circumstances, perform the offices usually delegated 

 to those of special sense, we are not yet in a condition to determine. Com- 

 parative Anatomy seems to show that, in the lowest animals in which the 

 rudiments of eyes can be detected, there is no distinction between the nerves 

 proceeding to these organs, and the rest ; and there would appear some ground 

 for the belief that, as in other cases, the special organs of sensibility are 

 gradually elaborated, in ascending the Animal scale, from the more general 

 apparatus, and are not merely superadded to it. Hence we may conceive the 

 possibility (though there is no proof of the fact) that states of the system might 

 occur, in which a change in the common sensory nerves might produce the 

 sensation of light, sound, &c. But it is quite impossible (so far at least as our 

 present knowledge of physical phenomena permits us to decide upon the im- 

 possibility of any thing) that distinct visual impressions should be communi- 

 cated to a nerve, except through the mediation of such an optical instrument 

 as the eye ; or distinct sonorous impressions, except through such an acoustic 

 instrument as the ear. Hence we must receive with the greatest caution 

 the wonderful accounts of transference of sensation, many of which have 

 undoubtedly been the offspring of deception. Still it may be objected that, as 

 we are so totally destitute of real knowledge as to the mode in which vision 

 is ordinarily produced by inverted images upon the retina, we have no right to 

 assert that it may not take place in some other way, and perhaps this objec- 

 tion should lead us to consider the phenomenon rather as extremely improbable, 

 than as impossible. But the improbability maybe compared to that of a stone 

 ascending like a balloon, or a piece of lead floating on the water ; for we have 

 no more knowledge of the ultimate cause of that which \ve term the force of 

 Gravitation, than we have of the nature of Sensation. 



312. The peculiar aptitudes of the different Sensory nerves, to receive and 

 convey impressions of various kinds, must be regarded as the result of proper- 



* Translation of Mailer's Physiology, p. 1062, note. 



