That anesthesia may proceed from various causes whose operation is limited to the sen sorial centres, is a matter of every-day ex perience. It is, however, where they have suffered from some obvious lesion of a com paratively restricted character, that the proof of this is most complete ; for although there is strong ground for believing that the ordi nary operation of anaesthetic agents and nar cotic poisons is confined to the cerebrum and sensorial% yet we could not positively affirm such to be the case, since, when taken into the blood, they may act not only on the sen sorial centres, but on the entire nervous system. All the phenomena of narcoctic poisoning, however, indicate that opium, al cohol, &c., single out the cerebrum and sen sory ganglia for their special action, just as strychnia singles out the spinal cord ; the suspension of the functional activity of the former being usually complete, before there is the slightest affection of the latter. That a failure of the circulation in the encephalon produces complete and universal anesthesia while it lasts, was fully proved by Sir A. Cooper's well-known experiment; and it seems probable that many of the structural lesions which manifest themselves in paralysis of motion and sensation produce this suspen sion of functional power in parts not them selves affected by disease, chiefly in virtue of the derangement of the intra-cranial circula tion which they invole.
There is one of the phenomena of the anes thesia produced by the accidental or inten tional introduction of poisonous substances into the blood, which seems deserving of more special notice ; viz., the suspension of the power of receiving painful impressions, with out the obliteration of the ordinary tactile sensibility. This is a frequent result of the exhibition of ether and chloroform ; and does net seem to depend upon a mere blunting of the ordinary sensibility. It has been especi ally noticed, also, in cases of lead poisoning, in which state it seems to be more frequent than complete anesthesia. According to M. Beau, the insensibility to pain, which he terms analgesia, may be observed in a large propor tion of cases of "saturnine intoxication." " We must not confine ourselves," he remarks, "to asking the patient whether he feels, hut limit our question to the sensation of pain. Parts which are thus insensible to pain are so also to tickling. This form of anesthesia may affect the entire surface, being, however, most remarkable in the extremities, and especially the upper ones. It may extend even to the mucous membranes, and especially those which are normally endowed with great sen sibility, — as the uvula, isthmus faucium, nares, or conjunctiva, — any of which parts may be tickled without the usual conse quences, the patient still being quite con scious of the mere contact." Reference has been already made to the influence of the attention on the acuteness of sensations ; and to the slight degree in which they are felt, when the mind is completely engrossed in some other feeling or idea. This is sometimes seen in spontaneous reverie ; and there are individuals who can exert such a power of mental abstraction, as voluntarily to concentrate their attention on some ex ternal object, or internal idea, so as to escape all suffering from a severe operation. This, however, is much better seen in some of those cases of somnambulism (see SLEEP) in which the mind is completely under the guidance of the suggestions received from without, its whole spontaneous directing power being suspended. For it is frequently possible, in such cases, to withdraw the patient's atten tion from any part of the body, to such an extent that the anesthesia is complete as regards that part, whilst every other portion enjoys the ordinary sensibility. Thus a tem porary loss of sensation on the whole of one side may be induced, or a single limb may be rendered anaesthetic ; and the sensibility of the parts may be instantaneously restored, merely by directing the patient's attention towards them.
With regard to hypercesthesia, or increased sensibility, we have much less definite in formation. There can be no doubt, however, that it, too, may proceed from changes either in the periphery, or in the central organs, and perhaps also from an alteration in the trunks of the nerves in their course. The acute
sensibility of an inflamed or irritated part is an example of the first of these conditions ; and the extraordinary exaltation of sensibility in the incipient stage of phrenitis may serve as an illustration of the second. In some cases the entire nervous system would seem to partake of this undue excitability ; this we especially see in hysterical subjects, in whom the slightest contact frequently occasions in tense suffering, so that even the mere pointing of a finger at any part of the body will cause a scream of alarm. The sufferings of such persons are not rightly designated as ima ginary; they are as real to them as are those proceeding from far more serious causes to persons of less excitable temperament. The fault partly lies in the habitual attention which they pay to the most trivial feelings ; but in part also, it may be surmised, to an abnormal state of nutrition of the entire nervous system,—both centres and trunks,— from depravation of the blood. This view harmonizes well with the fact just now stated, that in the Epidemic. de Paris, a temporary hypermsthesia (afterwards giving place to an msthesia) was commonly among the earliest symptoms. And it is not a little curious that in the remarkable series of cases of lead poisoning which recently occurred in the ex royal family of France, during their residence at Claremont, the same symptom presented itself, and was in some instances the only symptom which indicated the morbid con tamination of the blood.* In most cases of this form of hypermsthesia, the exaltation of sensibility seems confined to the surface, being much more excited by a slight touch than by hard pressure ; and this difference will frequently serve to distinguish the "hys terical" tenderness from that of inflammation, in which the pain is augmented the more severe the pressure. The writer has had opportunities of noticing an extreme sensi tiveness to changes of temperature in certain cases of somnambulism, both natural and artificial ; and he believes that this fact affords a ready solution of various marvels which have been narrated touching the power of " mesmerized" subjects to distinguish a piece of money which had been held in the mes merizer's hands, or a glass of water in which his finger had been immersed.
It has recently been proposed to apply Prof. Weber's method of estimating the re lative acuteness of the tactile sense in dif ferent parts of the body, to the determination of the degree of anmsthesia or of hyper msthesia, in patients affected with these dis orders. Thus it was found by M. Brown Sequard that in one case of nearly complete anaesthesia of the lower extremities, the pa tient only felt a single impression on the skin of his legs, when the points of the com passes were from 10 to 20 centim. apart ; the normal "limit of confusion" for this portion of the surface being from 3 to 5 centim. In another case of slighter anmsthesia, the " limit of confusion" in the same part was from 9 to 15 centim. And in a third case of very slight anmsthesia, it was from 6 to 7 centim. In a case of hypermsthesia, on the other hand, which accompanied paralysis of the motor power, the patient could perceive the distinctness of the two points on the foot, when they were separated to the distance of only 5 millim., although the normal " limit of confusion" in that part was from 25 to 30 millim. The sensibility to pain, in this case, was as much exaggerated as was the tactile sensibility.
Of the depravation of tactile sensibility, manifested in a variety of morbid phenomena, —such as the sense of heat or even of burning (without any real elevation of temperature), of fornication, of tickling, of itching, Sze., it must suffice here to remark, that this, like the preceding affections, may be due to causes acting on the peripheral nerves, or on the nervous centres, or on the connecting trunks. Of the latter we have a good example in the formication which generally succeeds complete anmsthesia, when a nerve has been pressed upon for a time, and the pressure is then re moved.