OF SENSATION IN GENERAL. 393 



individual, between the sensations thus originating, and those which are pro- 

 duced 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 retinte. 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 

 sensation 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 

 ordinary situations of the peripheral terminations of those nerves ; even 

 though these should not exist, or should be destitute of the power of receiv- 

 ing impressions. 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 man- 

 ner; for until the flap of skin, from which the new nose is formed, obtains 

 vascular and nervous connections 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 pro- 

 duced, 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 con- 

 siderable distance. 



519. 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 ano- 

 ther channel, the irritation is no longer felt. Every one is aware how vividly 

 sounds are perceived, when they break in upon the stillness of the night; being 

 increased in strength, not only by the contrast, but by absorbing the whole 

 attention. An interesting experiment is mentioned by Miiller, which shows 

 how completely the mind may be unconscious of impressions communicated 

 to it by one organ of sense, when occupied, even without a distinct effort of 

 the will, by those received through another. If we look at a sheet of white 

 paper through two differently-coloured glasses at the same time one being 

 placed before each eye, the resulting sensation is seldom that of a mixture 

 of the colours : if the experiment be tried with blue and yellow glasses, for 

 example, we do not see the paper of an uniform green; but the blue is pre- 

 dominant at one moment, and the yellow at another ; or blue nebulous spots 

 may present themselves on a yellow field, or yellow spots on a blue field. 

 We perceive from this experiment, that the attention may not only be directed 

 to the impressions made on either retina, to the complete exclusion of those of 



