1182 



TOUCH. 



excited by percussion, may become the chief 

 means of guidance to animals possessed of a 

 special apparatus for taking cognisance of 

 them. Such appears to be the case in the 

 Sat tribe, and especially in those species 

 whose habits are most exclusively nocturnal, 

 and whose dwellings admit the smallest quan- 

 tity of light. " The whole surface of their 

 wings, on both sides, may be considered as an 

 enormously-expanded organ of touch, of the 

 most exquisite sensibility to the peculiar sen- 

 sation for which it is intended ; and it is, 

 therefore, by the varied modifications of the 

 impulsion of the atmosphere upon this sur- 

 face, that the knowledge of the propinquity of 

 foreign bodies is communicated." * It would not 

 seem improbable, however, that the remarkable 

 cutaneous expansions with which the nose and 

 ear are furnished in many bats, are subservi- 

 ent to this function. The enormous exten- 

 sion of the external ear may doubtless aug- 

 ment the intensity of the sense of hearing ; 

 but it is scarcely accordant with our know- 

 ledge of the conditions under which the sense 

 of smell is exercised, to suppose that the 

 extraordinary " nose-leaf" of the Ehmolophidce 

 should be in any great degree subservient to 

 olfactive purposes. The bats of this group 

 (to which belong the greater and lesser horse- 

 shoe bats of our own country) " are more 

 completely lucifugous and retired in their ha- 

 bits than any others ; they are found in the 

 darkest penetralia of caverns, and other 

 places where there is not even the imperfect 

 light which the other genera of bats enjoy." \ 

 Some approach to this power of guidance, de- 

 rived either from the impressions made by the 

 air upon the cutaneous surface, or from the 

 radiation of heat, is occasionally seen in blind 

 persons; who can thus distinguish ! by the 

 hands, and even by the face, the proximity of 

 solid bodies (as in approaching a wall, a 

 door, or a piece of furniture) without actually 

 touching them. 



The sense of temperature, also, appears to 

 be capable of considerable improvement, when 

 its indications are habitually and discrimi- 

 natingly attended to, or when the mind is 

 intensely and exclusively fixed upon them. 

 Thus it is related of Dr. Saunderson, that 

 when some of his pupils were taking the sun's 

 altitude, he was able to tell, by the slight 

 alteration in the temperature of the air, when 

 very light clouds were passing over the sun's 



MORBID CONDITIONS OF THE SENSE OF 

 TOUCH. 



Like most other vital functions, the sense 

 of touch may become disordered in the way 

 of deficiency r , excess, or depravation. 



The state of complete deficiency is known 

 as Anesthesia; a term which, strictly speaking, 

 designates the absence of all sensation, but 

 which is more commonly employed as re- 

 ferring to the sense of touch alone. This 



* See CHEIROPTERA, vol. i. p. 599. 

 j- Loc. cit. 



state, which may be either general or local, 

 may arise from an interruption in the func- 

 tional activity of any part of the nervous ap- 

 paratus concerned in the reception of sensory 

 impressions ; and thus may be due to causes 

 acting either (1) at the peripheral origins of 

 the sensory nerves, or (2) on the nerves in 

 their course, or (3) on the sensorial centres ; 

 as well as to such as act on the whole nervous 

 system at once. The causes which act at the 

 peripheral origins of the nerves may be such 

 as affect either the nerves themselves, the 

 capillary circulation, or both. Of the first 

 we have a typical example in the " anassthetic 

 agents," ether, chloroform, &c. ; the applica- 

 tion of whose vapour for any length of time 

 to the cutaneous surface, entirely suspends its 

 power of receiving sensory impressions ; and 

 that this results from the direct action of 

 the substances on the peripheral nervous ex- 

 pansion, appears from the fact of the suspen- 

 sion being precisely limited to the part to 

 which the vapour is applied. But anaesthesia 

 may be induced, also, by the stagnation of the 

 capillary circulation in a part, without any 

 more direct affection of its nervous endow- 

 ments ; as we see when the main artery of 

 a limb has been tied, previously to the re- 

 establishment of the supply of blood by the 

 collateral circulation, or when the flow of 

 blood through it has been impeded by tem- 

 porary pressure. It is probable that cold 

 operates in producing local anaesthesia in both 

 these modes ; namely, by its direct sedative 

 action upon the peripheral nerves ; and by the 

 stagnation which it produces in the capillary 

 circulation. That the local anaesthesia, which 

 is a not unfrequent result of the presence of 

 poisonous substances in the blood, is due to 

 the special action of these substances upon 

 the peripheral nerves of the particular locality, 

 would not seem an improbable supposition ; 

 when it is remembered how frequently poisons 

 of various kinds single out some particular 

 part of a structure apparently homogeneous, 

 for the production of their peculiar effects, 

 lead, for example, in whatever way introduced 

 into the system, acting first on the muscular 

 fibres of the alimentary canal, and afterwards 

 most commonly on the extensor muscles of 

 the forearm, in which its presence has been 

 detected by chemical analysis. 



Of the anaesthesia induced by causes acting 

 upon the sensory nerves in their course from 

 the periphery to the centre, our most frequent 

 examples are those in which it is produced by 

 pressure on these trunks, whereby the con- 

 veyance of the sensory impressions to the 

 encephalon is effectually checked. Anaesthesia 

 may also arise, however, from diseased con- 

 ditions of these trunks, brought about by 

 perverted nutrition; and there is a form of 

 paraplegia, in which the lesion of sensibility 

 (which is more completely lost than the motor 

 power) appears to commence in the peripheral 

 expansions of the nerves, and to extend along 

 the trunks to the central organs.* This is 

 * Graves's Clinical Medicine, vol. i. p. 503. 



