OF SENSATION IN GENERAL. 231 



ties 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 localities in 

 the Sensorium, in which the changes to which they give rise are performed. 

 This may be judged of from the fact, that the phenomena of subjective sensa- 

 tion 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 phenomena of spec- 

 tral illusions are very nearly connected with those of dreaming ; both may be 

 in some degree influenced by external causes, acting upon the organs of sen- 

 sation, which are 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 individual, 

 between the sensations thus originating, and those which are produced in the 

 usual manner; for we find that, unless otherwise convinced by their own 

 reason, persons who witness spectral illusions believe as firmly in the reality 

 of the objects that come before their minds, as if the images of those objects 

 were actually formed on their retina. This is another proof, if any were 

 wanting, that the organ of sense, and the nerve belonging to it, are but the 

 instruments by which certain changes are produced in the sensorium; of 

 which changes, and not of the immediate impression of the object, the sensa- 

 tion really consists. It seems to be by an innate law of our constitution, that 

 these subjective sensations, whether originating in the central organs, or in 

 the course of the nervous trunks, should be referred by the mind to the ordi- 

 nary situations of the peripheral terminations of those nerves ; even though 

 these should not exist, or should be destitute of the power of receiving impres- 

 sions. Thus, after amputations, the patients are for some time affected with 

 sensations (originating probably in the cut extremities of the nerves), which 

 they refer to the removed extremities ; the same has been noticed in regard to 

 the eye, as well when it has been completely extirpated, as when its powers 

 have been destroyed by disease. The effects of the Taliacotian operation also 

 exhibit the operation of this law in a curious manner ; for until the flap of 

 skin, from which the new nose is formed, obtains vascular and nervous con- 

 nections in its new situation, the sensation produced by touching it is referred 

 to the forehead. Another interesting illustration of it may be obtained by the 

 following very simple experiment : if the middle finger of either hand be 

 crossed behind the fore-finger, so that its extremity is on the radial side of the 

 latter, and the ends of the two fingers thus disposed be rolled over a marble, 

 pea, or other round body, a sensation will be produced, which, if unconnected 

 by reason, would cause the mind to believe in the existence of two distinct 

 bodies ; this is due to the impression being made at the same time upon the 

 radial side of the fore-finger, and the ulnar side of the middle finger, two 

 joints which, in the natural position, are at a considerable distance. 



313. The acuteness of particular sensations is influenced in a remarkable 

 degree by the attention they receive from the mind. If the mind be entirely 

 inactive, as in profound sleep, no sensation whatever is produced by ordinary 

 impressions ; on the other hand, when the mind is from any cause strongly 

 directed upon them, impressions very feeble in themselves produce sensations 

 of even painful acuteness. Every one knows how much a slight itching of 

 some part of the surface may be magnified, by the direction of the thoughts 

 to it ; whilst as soon as they are forced by some stronger impression into an- 

 other channel, the irritation is no longer felt. To the traveler in warm coun- 

 tries, the shrill but feeble buzz of a single mosquito, accidentally enclosed 



