392 ON SENSATION, AND THE ORGANS OF THE SENSES. 



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 can- 

 not distinguish the slightest difference between a luminous and a dark object. 

 A tuning-fork, which, when laid upon the ear whilst vibrating, produces a dis- 

 tinct musical tone, excites no other sensation when placed upon the eye than 

 a slight jarring feeling. The most delicate touch cannot distinguish a sub- 

 stance which is sweet to the taste from one which is bitter; nor can the taste 

 (if the communication between the mouth and the nose be cut off) perceive 

 anything peculiar in the most strongly odoriferous bodies. 



517. 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 com- 

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

 delegated to those of special sense, we are not yet in a condition to deter- 

 mine. Comparative 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 sensi- 

 bility are gradually elaborated, in ascending the Animal scale, from the more 

 general apparatus, and are not merely superadded to it. Hence we may con- 

 ceive 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 impossibility of anything) that distinct visual impressions should be 

 communicated 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, since 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 objection should lead us to consider the phenomenon rather as 

 extremely improbable, than as impossible. But the improbability may be 

 compared to that of a stone ascending like a balloon, or a piece of lead float- 

 ing on the water ; for we have no more knowledge of the ultimate cause of 

 that which we term the force of Gravitation, than we have of the nature of 

 Sensation. 



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

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

 properties inherent in themselves; just as we consider the difference between 

 the afferent nerves in general, and the motor nerves, to be one belonging to 

 their own constitution. But it is probable that there are also different locali- 

 ties in the Sensorium, in which the changes to which they give rise are per- 

 formed. This may be judged of from the fact, that the phenomena of sub- 

 jective sensation frequently originate in peculiar conditions of the encephalon 



itself, and not in the nervous trunks or organs of sense ; thus, in dreaming, 

 we have frequently very vivid pictures of external objects presented to our 

 minds; and we sometimes distinctly hear voices and musical tones, or have 

 perceptions (though this is less common) of tastes and odours. The phe- 

 nomena of spectral illusions are very nearly connected with those of dream- 

 ing; both may be in some degree influenced by external causes, acting upon 

 the organs of sensation, which arc misinterpreted (as it were) by the mind, 

 owing to its state of imperfect operation ; but both also may entirely originate 

 in the central organs. There seems to be no difference, in the feelings of the 



