Sense of Weight. — This is usually derived from a double source; namely, the impression made upon the cutaneous surface by the sim ple pressure of the body; and the conscious ness of the muscular effort employed to re sist that pressure. The latter enables us to compare the weights of different bodies much more accurately than the former, which is li able to excite fallacious ideas. The extent of surface, for example, which is in contact with the skin, greatly modifies the estimate of the pressure of a heavy body ; the body feeling lightest when its pressure is distributed over a larger surface, and vice versa. Thus, a trun cated cone seems heavier when it rests (with out any effort being made to raise or support it) upon its small extremity, than when it rests upon its large extremity on the same part of the surface. At first sight this fact appears altogether antagonistic to the one just stated with regard to the sense of temperature, the impressions on which are more powerful when they are made over a large than over a small surface. But it is to be borne in mind, that in the latter case an absolutely larger amount of calorific influence is exerted, when the large surface is exposed to the action of heat ; whilst in the former, the amount of pressure is really the same, whether it be distributed over a larger or a smaller area. The experi ments of Weber on the relative information derived from the mere sense of pressure, and from the sense of muscular effort, in the ap preciation of weights, are very instructive. He found that if the two hands of the same individual be placed upon cushions, and un equal weights be placed upon the right and left hands respectively, the eyes being kept shut, it will not be possible to say on which hand the heavier weight lies, unless the dif ference be very considerable ; but a compara tively small amount of difference is at once dis criminated, when a muscular effort is made to lift the hands from the cushions. This power of comparison is capable of being rendered more exact by practice; so that men accus tomed to estimate weights by poising them in their hands, will readily distinguish between two which differ only by one-thirtieth part. It is found that the power of comparison is much greater (as in the estimation of tempe rature) when the impressions are successively, than when they are simultaneously made, pro vided that the interval be not too long. Thus in the comparison of two weights, the greatest nicety is attained by poising the one, and immediately afterwards the other, in the same hand ; but the intervention of a few seconds between the poising of the first and that of the second does not prevent their accurate com parison. The interval may amount to twenty seconds, and yet a just estimate may still be made ; but when it amounts to forty seconds all accuracy is lost. Professor Weber has further ascertained, that, in the estimation of weights by their simple pressure on the sur face, the left side and extremities have usually a more acute perception than the right ; for out of fourteen individuals, lie found this to be the case in eleven; in two, the contrary was observed ; whilst in one, no difference was perceptible.
Sense of Direction.— The combination of the muscular with the simply tactile sense enables us also to judge in some degree of the direction of the pressure. Of this we gain no information whatever from the tactile sense alone, which always suggests the idea that the pressure is made vertically to the surface, when it is not corrected by the sense of mus cular effort called forth to antagonise it. The following example, given by Weber, shows how completely involuntary may be this effort, yet how large a share it has in communicating to us the information we derive from an im pression, of whose direction we are rendered cognisant. When a hair of the head is pulled,
he remarks, we can judge perfectly well of the direction of the traction ; this power of discrimination is not, however, derived (as might at first be supposed) through the sen sation originating in the bulb of the hair, but from the sense of the muscular effort which is called forth to antagonise the traction, and to keep the head steady during its continuance. If we prevent these muscles from being called into play, by steadily holding between the hands the head of the person operated on, and if we also prevent the traction from call ing forth the muscular action of the scalp, by surrounding the point from which the hair is pulled with a firm pressure by the fingers, we find that the discriminating power is completely lost ; the subject of the experi ment being totally unable to distinguish the direction from which the hair is pulled.
It is by this combination, too, that we judge of the rate and direction of the passive motion of our bodies, when we have no other means of guidance. If, for example, a person be seated in a carriage, with his eyes closed, and the carriage be suddenly put in motion, the inertia of his body causes it to be thrown in the contrary direction ; and; in order to re cover and sustain its equilibrium, a muscular effort is required, which is greater in propor tion to the rate of motion. If the motion continue uniform, however, this effort becomes so habitual that he ceases to be conscious of it ; and he only becomes cognisant of the mo tion by its cessation, the equilibrium of the body being then again disturbed by its inertia, which tends to impel it in the direction in which it was previously moving, so as to re quire the effort of a contrary set of muscles for the maintenance of the erect position.
Mental Phenomena connected with the Sense.
— The interpretation which the mind puts upon the impressions made by external ob jects upon the tactile organs, is partly the re sult of intuition, partly of experience. Thus we intuitively refer an impression made upon any part of a sensory nerve in its course, to the peripheral extremities of that nerve, or (as in cases of amputation) to the part from which they should normally arise. So, again, if a part of the body be removed from its usual position and connections (as in the Taliacotian and various other operations of plastic surgery), impressions made upon it continue to be referred to its original seat, so long as it retains any nervous connection with it, and until new connections have been formed with the nerves of the part to which it has been transferred. So, again, when our members are in an unaccustomed position, we still, unless our attention be directed to the fact, interpret impressions made upon them as if they were in their ordinary relation to each other, and may thus be altogether misled : — as in the experiment mentioned by Aristotle, of rolling a pea or other globular body between two fingers of one hand, which are crossed instead of lying parallel, so that the surfaces that are usually most distant are brought into proximity with each other ; the sensation then received is that of a dis tinct convex body opposed to each of these surfaces, so that the single body seems to be double; whereas, if the pea were rolled be tween the two surfaces which are usually and normally approximated, it is felt but as a single globe. This intuitive reference is obviously analogous to that by which we judge of the relative situations of visual objects, from the direction in which their rays impinge upon the retina, or from the muscular sensations re ceived from the muscles of the orbit.