A similar exaltation may manifest itself, under the like circumstances, in the general tactile sensibility, both of the surface and in terior of the body, especially as affected by the vibrations transmitted through solid sub stances ; whereby a deficiency in the sense of hearing is in some degree supplied. Thus the visitor to a school for the deaf and dumb remarks with surprise that a slight rap given by the master on the table or floor is sufficient to excite the attention of the pupils ; and finds on examination that this is not heard, but is felt by them. A minute account of his personal experience on this head is given by Dr. Kitto ; and as it involves several interest ing physiological considerations, the principal facts mentioned by him will be here brought under the notice of the reader.—" In the state of entire deafness," he remarks, "a peculiar susceptibility of the whole frame to tangible percussion supplies the only intimations which have the slightest approximation to those which hearing affords. I was about to call this a peculiar susceptibility of the sense of touch ; but this would unduly limit a kind of vibration, which, in certain of its develop ments, seems to pervade the whole frame, to the very bones and marrow. I do not at all imagine that there is in this anything essen tially different from that which is experienced by those who are in possession of their hear ing ; but it would seem that the absence of that sense concentrates the attention more exclusively upon the sensation which is through this medium obtained ; and the in timation of which, being no longer checked and verified by the information of higher organs, assume an importance which does not naturally belong to them." This sense of percussion is but little excited in the human body by the vibrations of air ; obviously be cause there is no expanded surface adapted to receive their influence. Thus Dr. Kitto mentions that the loudest thunder makes no impression upon him, unless it shakes the house in which he is ; in which case it communicates a sensation resembling that produced by the removal of a piece of fur niture in an adjoining room.* In like man ner, he is utterly unconscious alike of the sound of bells, and of the vibration produced by their percussion, unless the latter be pro pagated through solid bodies, as when he places himself in direct contact with a tower in which a powerful peal is being rung. " I re member," he says, "that once when I was showing a young friend from the country over St. Paul's, we happened to be up examining the great clock, at the very time it began to strike. The sensation which this occasioned was that of very heavy blows upon the fabric in which I stood, communicated to my feet by contact with the floor, and by the feet diffused over the whole body. So," he con tinues, " guns — even powerful cannon make no impression upon this sense, unless I happen to be very near when they are fired ; in that case, I can compare the effect to no thing better than the sensation produced by a heavy blow upon the head from a fist co vered with a boxing glove. This effect could only be produced by the tangible percussion of the air, and by the percussion upon the ground transmitted by the feet." So, again, Dr. Kitto states that he is not conscious of even a very loud knock at the door of the room in which he is, unless the door be in such connection with the floor that the per cussion is communicated through the latter, or unless he be himself in contact with some part of the wall to which it is hung. But, on the other hand, he states,— " The drawing of furniture, as tables and sofas, over the floor above or below me, the shutting of doors, and the feet of children at play, distress me far more than the same causes would do if 1 were in actual possession of my hearing. By being to me unattended by any circumstances or preliminaries, they startle dreadfully ; and by the vibration being diffused from the feet over the whole body, they shake the whole nervous system, in a way which even long use has not enabled me to bear. The moving of a table is to me more than to the reader would be the com bined noise and vibration of a mail coach drawn over a wooden floor ; the feet of children, like the tramp of horses upon the same floor ; and the shutting of a door like a thunderclap, shaking the very house. It is by having once heard, that I am enabled to make such comparisons as these, for the illustration of a sensation which one who has never heard, and one who is not deaf, would be alike unable to describe." The fact that the shutting of a door is felt with painful distinctness (as Dr. Kitto elsewhere mentions), even when upon a different floor, whilst the loudest ordinary knocking at the same door is not perceived, very curiously illustrates the necessity for the transmission of the vibrations along a solid medium. "The valve of the door on which the percussion is made by knocking, is a detached frame of wood hung upon hinges, and the vibration is therefore comparatively isolated and not pro pagated throughout the frame of the house, as is the case when, in shutting the door, the valve itself strikes the door-post, which is identified with the framework of the build ing." In illustration of the acuteness of this sense in his own person, Dr. Kitto states that the lightest footfall upon the same floor is quite sufficient to attract his attention, or even to arouse him from sleep. " If any small article," he continues, " such as a thimble, a pencil, a penknife, or even a more minute object, falls from the table to the floor, I am often aware of it, even when other persons sitting at the same table have not been apprised of it by the ear. The greater the number of my points of contact with the floor, the stronger are the impres sions I receive : hence they are more vivid and distinct when I sit than when I stand ; be cause, in the former case, not only my own two legs, but the four legs of my chair, are concerned in conveying the percussion to my sensorium. And when the chair itself on which I am seated has been subject to the percussion, the sensation is such as baffles description. For instance, a few days since, when I was seated with the back of my chair facing a chiffoniere, the door of this re ceptacle was opened by some one, and swung back so as to touch my chair. The touch could not but have been slight, but to me the concussion was dreadful, and almost made me scream with the surprise and pain, the sensa tion being very similar to that which a heavy person feels on touching the ground, when he has jumped from a higher place than he ought.
Even this concussion, to me so violent and distressing, had not been noticed by any one in the room but myself. * * # If these per ceptions are so acute in carpeted rooms, it will be easily understood by how much more intense they become upon a naked wooden floor. The sensation then amounts to torture —as every movement or concussion, in any part of the room, then comes with an in tensity of effect, far more than proportioned to the difference in the impression which would, under the same circumstances, be pro duced upon the auditory sense." It is interesting to remark that, notwith standing this acuteness of the sense in ques tion, it does not seem to convey (in Dr. K.'s case at least) any information of the direction or distance of the percussions, except such as is afforded by their relative intensity. Thus he says ;—" I am unable to determine from the information of the sensation itself, whether it has occurred upon the floor above, or in that below me, or in the passage or room ad joining that in which I may be at the time. I am not aware that the impression is more distinct from the floor above than from that below ; but it certainly is more distinct in another room of the same floor, than from either the one above or below ; whence I am much in the habit of referring to the next room the percussions which make the strongest impression on me. In this I am not seldom mistaken. ' * * The information is equally defective, even in the very room I may happen to occupy. If a book or other object falls in any part of the room, the sen sation is painfully distinct, the percussion being upon the very boards on which I stand ; but even in this case, 1 am at a loss for the quarter in which the circumstance has oc curred, and generally look for it in the wrong direction, and have to scan the whole room with my eye before I can make it out." It is probable that the want of power to estimate direction arises from the circumstance that the communication of the percussions takes place, in this and similar cases, through the same channel (the floor) to the same parts of the solid mass of the body, through which the vibrations immediately spread in every direction. It can easily be conceived that if the percussions were transmitted through a liquid medium, its vibrations, being propagated in a more determinate direction, might affect one or another part of the surface in such a manner as to suggest the direction of their source ; and that in this mode aquatic animals endowed with a nervous apparatus at their surface, specially adapted to be impressed by such vibrations, might communicate with each other through great distances. This appears to be the case with regard to the Spermaceti Whale, and probably others of the Cetacea. It has been observed by the whale-fishers, that when a straggler from a " school" is attacked, even at a distance of several miles from it, a number of its fellows bear down to its assistance, in an almost incredibly short space of time. It can scarcely be doubted that this communication must be made through the medium of the vibrations of the water, excited by the struggles of the animal, or per haps by some peculiar instinctive movements especially adapted for this purpose, and pro pagated through the liquid medium to the large cutaneous surface of the distant whales. And this idea is confirmed by the fact, that the nerves which proceed to the surface of the body pass through the layers of blubber (which form the inner part of the true skin) with scarcely any division, and then spread out into a network of extreme minuteness as soon as they approach the exterior of the in tegument. The expanse of such a network over a thick layer of elastic tissue, whose meshes are distended with oleaginous fluid, obviously affords a condition peculiarly fa vourable to the reception of impressions ori ginating in percussion.
After the details which have been given in proof of the degree of exaltation of which the general tactile sensibility is capable in the human subject, we shall have less difficulty in understanding that even the vibrations of air, 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 Bat 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 Rhinolophidce 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." t 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 iby 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 disk.