TOUCH. 



11C7 



Besides the papillary apparatus, however, 

 we find certain animals endowed with special 

 organs of touch, which are constructed upon 

 a very different plan ; consisting of a rod or 

 filament, which is in itself insensible, but 

 which is connected at its base with nervous 

 fibres, in such a manner that any motion 

 or vibration communicated to it must be 

 transmitted to them. Such are the long stiff 

 hairs which are known as the " whiskers " of 

 the feline tribe, and which are so particularly 

 large in the seal ; these are also highly deve- 

 loped in many of the rodentia, such as the 

 hare and rabbit ; and it has been proved by 

 experiment, that if they be cut off, the animal 

 loses in great degree its power of guiding its 

 movements in the dark. Thus Mr. Broughton 

 found that whilst a kitten whose whiskers 

 were entire was capable of threading its way 

 blindfold out of a labyrinth in which it was 

 designedly placed, it was totally unable to do 

 so when its whiskers were cut off; for it then 

 struck its head repeatedly against the sides, 

 ran against all the corners, and tumbled over 

 steps placed in its way, instead of avoiding 

 them as it did prior to the loss of its whis- 

 kers.* In animals whose hairs are their im- 

 portant instruments of touch, a true papilla, 

 copiously furnished with nerves and blood- 

 vessels, is found to project into the bulb of 

 each hair. The jointed appendages to the 

 head, known as antenna and palpi, which are 

 possessed by most articulated animals, are un- 

 doubtedly instruments of touch, whatever 

 sensory impressions they may receive in addi- 

 tion. The antennae, when prolonged, serve 

 to guide the movements of the animal ; the 

 impressions which they receive at their ex- 

 tremities being communicated to the nerves 

 at their base, just as a blind man judges by 

 the stick held in his hand of the proximity of 

 obstacles to his progress.f On the other hand, 

 the palpi appear to minister to the cognisance 

 of objects brought into the neighbourhood of 

 the mouth, and to have for their chief office 

 to guide in the selection of food. But while 

 there are many facts which seem to indicate 

 that the antennae minister to the sense of 

 hearing, there are others which appear to 

 point to the palpi as the special instruments 

 of that of smell. 



Conditions of the Sense of Touch. The 

 sense of touch, strictly so called, is exercised 

 under conditions essentially the same as those 

 through which the general sensibility of the 



* London Medical and Physical Journal, 1823. 



t The author is acquainted with a blind gentle- 

 man who exhibits a remarkable dexterity in the use 

 of his stick in guiding his movements ; and he has 

 been informed by him, that much of his power of 

 discrimination depends upon the flexibility, elasti- 

 city, &c. of this instrument, so that, when he has 

 chanced to lose or break the one to which he has 

 been accustomed, it is often long before he can ob- 

 tain another that shall suit him so well. This cir- 

 cumstance seems to throw some light upon the re- 

 makable varieties of conformation in the antenna? 

 of insects ; as it may well be imagined that each is 

 adapted to receive and to communicate impressions 

 of a particular class, adapted to the wants of the 

 species. 



body is affected. It is requisite, in the first 

 place, that the bodies, of whose' presence it 

 takes cognisance, should be brought into 

 actual contact with the tactile surface ; the 

 only exception being in regard to the tempe- 

 rature of objects, the influence of which may 

 be communicated by radiations from a distance. 

 This difference, however, does not indicate 

 any fundamental diversity such as some have 

 imagined to exist, between the sense of tem- 

 perature and that of resistance ; for, in each 

 case, that which is perceived by the mind is 

 the impression made upon the sensory organ ; 

 and the change in this is excited in the one 

 case by pressure, and in the other by heat or 

 cold. The same organ appears to be adapted 

 to take cognisance of both classes of impres- 

 sions ; a feeling of one kind being excited, 

 when its condition is altered by pressure ; and 

 a feeling of a different kind, when its tempe- 

 rature has undergone a change under the in- 

 fluence of calorific radiations. And the dif- 

 ference between these classes of sensations is 

 not greater than that which exists among 

 others, whether of a general or a special 

 kind, which we know to be transmitted by 

 the same nerve-fibres. Yet it would seem 

 that, whilst there is no sufficient reason for 

 supposing impressions of contact and of tem- 

 perature to be transmitted by different nerve- 

 fibres, we must admit that some of these 

 fibres, either in virtue of their own constitu- 

 tion, of the locality of their central termina- 

 tion, or of the apparatus with which they are 

 furnished at their peripheral origin, are en- 

 dowed with a greater readiness to receive and 

 transmit one or the other class of impressions. 

 For we find that the parts whose tactile sen- 

 sibility is the most discriminating, are not 

 always those by which the keenest apprecia- 

 tion of changes of temperature is obtained. 

 And, in like manner, the occasional occur- 

 rence of cases of paralysis, in which there is a 

 total loss of one kind of sensation, whilst the 

 other is preserved, or in which one is di- 

 minished beyond all proportion to the other, 

 seems to show that such a change may take 

 place in the nerve-fibres, as may indispose 

 them to the reception and transmission of one 

 class of impressions, whilst they are still 

 capable of actively responding to the other. 



After what has been said of the necessity of 

 the supply of blood, for the active exercise of 

 common or general sensibility, and of the vas- 

 cularity of the special tactile organs, it is not 

 requisite to lay any further stress on this 

 point, in relation to the sense of touch, strictly 

 so called. Another important condition, which 

 is probably common to the whole sensory 

 apparatus of the warm-blooded animal, and 

 which has been already noticed under the 

 head of Taste, is a temperature not too far 

 removed from that which is natural to the 

 body. It has been shown by Professor E. H. 

 Weber, that if the fingers or the lips be im- 

 mersed for half a minute or a minute in water 

 heated to 125, or cooled to 32, the power 

 of tactile discrimination is so much impaired, 

 that the power of distinguishing between a 



