Tactile sensations, like all others, are more or less complete according as the attention is, or is not, directed to them. The contact of a foreign body with any sensitive portion of the organism is revealed to us by feeling, and it is by touch that we discover the form, resistance, and temperature of this body. Feeling may be involuntary, touch is an act of the will; there is therefore the same difference between feeling and touch that there is between seeing and looking, between hearing and listening, scenting an odour and smelling it, perceiving a flavour and tasting it.
We must also distinguish between impressions due to general sensibility and tactile sensations proper; thus, if we knock the elbow we feel an acute pain along the course of the ulnar nerve, but the impression produced on the skin is perfectly distinct from this deeper suffering when not entirely masked by it It is the same in sensations resulting from a shock to the right hypochonder which produces a pain in the liver. All the tissues which receive nerves of sensation may be the seat of impressions which may be referred to the general sensibility, and which are for the most part painful; impressions on the sense of touch are only produced in certain tissues specially endowed with this sense. General and tactile sensibility are independent of each other, and they are not developed proportionally; as, for instance, the palmar surface of the fingers is endowed with an exquisite sense of feeling, but it is almost insensible to a blow which would be very painful to the cheek, in which this sense is much less developed.
Touch has its seat in the skin throughout its whole extent, and in some of the mucous membranes. It is by the nervous papillæ containing the tactile corpuscles that the impression is perceived, and the tactile sensibility of any region is in proportion to the number of nervous papillæ existing in it.
We receive three distinct impressions at a time by the sense of touch—that of contact from a foreign body, that of the pressure which it exercises on the skin, and that of its relative temperature.
The sensation of contact is not equally distinct and precise in all parts of the body, and the reason of this we will endeavour to explain. If we apply simultaneously the two points of a compass to the skin, they must be more or less separated according to the region experimented on, in order that their contact may cause one or two distinct sensations, and we may in this way measure the delicacy of the sense at any given point on the skin. It is evident that the less sensitive the skin is, the more widely we must separate the points of the compass to produce a double sensation. E. Weber after many experiments classes the regions in the order of their sensibility as follows:—The tip of the tongue gives a double sensation when the feet of the compass are separated about half a line; the palmar surface of the ends of the fingers, one line; the red surface of the lips and the surface of the second joint of the fingers, two lines; the end of the nose and the palm of the hand near the fingers, three lines; the back and edges of the tongue at about an inch from the tip, and the skin of the lips, four lines; the palm of the hand, the cheek, and the eyelids, five lines; the palate, six lines; the prominence of the cheek, and the sole of the foot near the great toe, seven lines; the back of the hand near the fingers, eight lines; the gums, nine lines; the lower portion of the forehead, ten lines; the lower part of the occiput, twelve lines; the back of the hand, fourteen lines; the throat under the jaw, fifteen lines; the shoulder, fore-arm, and knee, eighteen lines; the chest over the sternum, twenty lines; the loins, the upper part of the back, and the neck on the line of the spine, twenty-four lines; the middle of the back, of the neck, of the arm, and of the thigh, thirty lines.
Gratiolet found by oft-repeated experiments that the distances recognized by the pulp of the fingers might be very much less. By touching two points on the same papillary ridge on the pulp of the last joint of the middle finger, separated only by the orifice of a sudoriferous duct, the two sensations were plainly distinguished at the distance of less than one quarter of a line.
The experiments of Valentin, on the other hand, prove that tactile sensibility varies from single to double in corresponding regions in different individuals; we can only accept the measurements of Weber, therefore, as indicating the relative sensibility. And lastly, we are indebted to M. Belfield-Lefevre for the experiments which led him to the following conclusions. The distance between two points of contact is better appreciated, if these points are placed on a line transverse to the axis of the body, than when on a line parallel to it or longitudinal. According to Weber, on the ends of the fingers, and on the tip of the tongue, the distance is better appreciated on a longitudinal than on a transverse line. The distance between two points of contact, distinct and simultaneous, is greater in proportion to the delicacy of feeling in the region experimented on; it seems to be greater also when the contact takes place successively in the two points, than when it takes place simultaneously, and greater also if the two contacts are separated by a longer interval of time. If the two points of contact are separated by the median line, the distance between them seems greater than if they are placed on the same side of the body.—If two points are touched, which are subject to variation from functional displacement, the eyelids or the lips for example, the distance is greater than if the two contacts take place on one eyelid, or one lip.—This sense is also increasingly developed on the surface of the limbs in proportion to the distance from the body.
The sensation of contact varies according as it results from a simple application of a foreign body to the skin, or from a shock, or a succession of shocks repeated at short intervals, like that which produces vibration in a body. In the latter case the region of contact receives a shock in proportion to the intensity of the vibrations; as when we touch the skin with a tuning instrument while vibrating, or if we grasp a vibrating metallic body or wooden rod, or close the lips against the reed of a basoon while it is being blown into, it produces on the surface in contact an impression, varying from a painful shock to a simple tickling or pleasurable sensation. It is a sensation of the same nature, though diffused through the whole body, that we feel from the vibrations impressed on the atmosphere by the explosion of artillery, the roll of thunder, or the ringing of a great bell. The sense of touch then gives us an idea of the sonorous waves which excite the auditory nerve, and furnishes us with the proof, that the same cause acts differently on the special nerves of the different senses. In fact, the nervous papillæ of touch transmit a sensation of motion and of shock; the tympanum perceives neither tickling nor shock, the impression which it transmits to the auditory nerve is not that of a vibratory movement, but of the sound which results from it.
The sensation of pressure is distinctly perceived by touch; but we must distinguish between the pressure of a body against the skin, and the resistance which this body offers to a muscular effort tending to displace it. If when the hand is supported, and a weight placed in it, no effort is made to raise it, and the muscles remain inactive, we feel a sensation of pressure, the force of which may be judged of with more or less exactness; but the moment we endeavour to appreciate the weight the sensation becomes complex, and we have the idea of pressure, and that of the muscular effort which we oppose to it; and great attention is necessary to prevent an instinctive contraction of the muscles of the hand to resist the weight with which it is charged. We must also take into account habit and the relative strength of the hands; as the right, which is generally more used than the left; may be less sensitive to pressure, and appreciate its degree less exactly.
Those portions of the skin where the touch is most acute, and where two points of contact are perceptible at slight distances, are, according to Belfield-Lefevre, those which estimate most correctly the degree of pressure; as the lips, the palmar surface of the fingers, the under surface of the toes, and the skin of the forehead, are better endowed in this respect than the rest of the body. But neither touch nor pressure alone can give an exact notion of the weight, we must support the body on which the experiment is made by muscular effort; the inequality of the two hands in this respect is so well known, that we use one and the other alternately in order to ascertain correctly the weight of the object It is estimated that pressure alone will not enable us to appreciate more than an eighth of difference between two weights, but by lifting we can appreciate a sixteenth.
The form of bodies also influences the sensation of pressure; when an object with only a small surface is applied to the skin the pressure seems greater than when it is spread over a greater extent; and it may even become painful when supported on a restricted point; thus, the weight which we carry with ease on the whole breadth of the shoulder, is intolerable when resting only on one of its angles. A truncated cone laid on the forehead seems heavy or light according as it rests on its smaller or larger base. Soldiers and travellers know very well that they cannot with impunity exchange the broad bands of their knapsacks for narrow straps or cords. It is unnecessary to remark, that if the weight be distributed over a large surface, each point of that surface supports but a fraction of the entire weight; the whole mass, on the contrary, presses on a limited space.