MORBID CONDITIONS OF TI1E • SENSE OF Toucn.
Like most other vital functions, the sense of touch may become disordered in the way of deficiency, excess, or depravation.
The state of complete deficiency is known as Anaesthesia; 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 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" anesthetic 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 f'rom 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 anesthesia 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 anesthesia, 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 anesthesia 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
usually referrible, in the first place, to the influence of cold and damp; and it is especi ally liable to occur in persons of a rheumatic or gouty diathesis. That even the sedative influence of cold may be propagated along the nerve-trunks, and that its anesthetic effect is not clue to its peripheral influence alone, appears from the circumstance remarked by Dr. Graves (loc. cit.), that the paralysis induced by handling snow, or by immersing the hands in freezing mixtures for some little time, is not confined to the hands and fingers, but extends to the muscles and surface of the fore-arms. And it was also remarked by the same eminent physician, that in a case in which the inside of the ring finger had been wounded by a blunt needle, and a partial anaesthesia induced, the same effect was per ceived in the little finger (alike supplied by the ulnar nerve), obviously through the extension of the paralysing influence towards the centre, so as to affect the trunk higher up than the point at which its branch to the little finger was given off. Dr. Graves further cites, as an example of anesthesia having its seat in disordered nutrition of the peripheral nerves, and gradually advancing along their trunks towards the centres, the curious Epidemie de Paris, which occurred in the spring and sum mer of 1828. " It began (frequently in per sons of good constitution) with sensations of pricking and severe pain in the integuments of the hands and feet, accompanied by so acute a degree of sensibility, that the patients could not bear these parts to be touched by the bed-clothes. After some time, a few days, or even a few hours, a diminution, or even abolition of sensation took place in the affected members ; they became incapable of distin guishing the shape, texture, or temperature of bodies, the power of motion declined, and finally they were observed to become al together paralytic. The injury was not con fined to the hands and feet alone, but, ad vancing with progressive pace, extended over the whole of both extremities. Persons lay in bed powerless and helpless, and continued in this state for weeks and months. Every remedy which the ingenuity of the French practitioners could suggest was tried, and proved ineffectual. In some, the stomach and bowels were deranged, and this affection terminated in a bad state of health, and even in death ; in others, the vital organs, cerebral, respiratory, and digestive, were in the same state as before their illness, and their appetites were good, but still they remained paralytics. At last, at some period of the disease, motion and sensation gradually returned, and a re covery generally took place, although, in some instances, the paralysis was very capricious, vanishing and again re-appearing. In the fatal cases, no evidence could be obtained, from the most diligent search, of any lesion, functional or organic, in the brain, cerebellum, or spinal marrow."# These phenomena are scarcely explicable on any other hypothesis than that of some general cause (probably a morbid matter circulating in the blood) affect ing the nutrition and functional activity of the nerve-trunks, rather than of their centres.