Some further experiments have recently been made by Professor Weber, to determine whether the sense of temperature is received through any other channel than the sensory apparatus contained in the integuments. The first means of which he availed himself for deciding this question, was that afforded by the results of accident or surgical operations, in which a portion of skin has been left defi cient. Thus, in three cases in which a large portion of the skin had been destroyed by a burn, and in which healing had not advanced so far as to renew the organ of touch, it was found that no correct discrimination could be made between two spatulas, one of them at a temperature of from 48° to 54°, the other of from 113° to 122°, which were brought into contact with the denuded surface ; so that one of these patients thrice affirmed that he was being touched with the cold body when it was the warm, and the reverse. But when the spatula was in one instance made some what warmer, and was brought into contact with the unskinned surface, the patient felt, not heal but pain. Another means of gaining information on this point is afforded by the ingestion or injection of a large quantity of warm or cold fluid into the stomach or intes tinal canal. Thus Professor Weber states, that after drinking a tumbler of water at 32°, he felt the cold water in the mouth, in the palate, and in the pharynx, as far as the limits of the sense of touch ; but that the gradual passage of the cold water into the stomach could not be perceived. There was, it is true, a slight sensation of cold in the gastric region; but as it only occupied the situation of the anterior wall of the stomach, it was at tributable to the abstraction of heat from the abdominal integuments in contact with this. In an opposite experiment, the author drank quickly three glasses of milk, the temperature of the first of which was 158°, that of the second 145°, whilst that of the third was in termediate between the two. The sensation of heat could not be traced lower down than that of the cold in the previous experiment. At the moment when the fluid entered the stomach, there was a feeling which remained for some time, but which could not be dis tinguished as heat, being mistakeable for cold. In order to ascertain the sensations produced in the large intestine by cold water, an in jection of 14 ozs. of water of the temperature of 65° was thrown up the rectum, but scarcely any sensation of cold could be perceived from it. In another instance, 21 ozs. of water at the same temperature was thrown up, without any resulting sensation of cold. In both these cases, on the return of the enema a few mi nutes afterwards, a distinct feeling of cold was experienced at the anus. When water of so low a temperature as was injected, the first feeling excited was a sensation of cold in the immediate neighbourhood of the anus, and then a feeble movement in the bowels ; but a little time afterwards, there was a faint sensa tion of cold, especially in the anterior wall of the abdomen. This sensation, however, re mained after the return of the water ; and may hence be attributed to the abstraction of warmth from the abdominal integuments, which was proved to take place, the temper ature of the surface being lowered 3 degrees. So, again, if the cavity of the nose be filled with cold water, the coldness is only perceived in the parts of the cavity which are most en dowed with the proper tactile sense, namely, the neighbourhood of the nostrils and of the pharynx ; and it is not at all discernible in the higher part of the cavity, which is espe cially subservient to the olfactive sense. [See SMELL.] But when the water injected is very cold (e. g. 41°), a peculiar pain is felt in the upper part of the nasal fossm, extending to the regions of the forehead and lachrymal canals ; this pain, however, is altogether dif ferent from the sense of coldness.
From the foregoing experiments it appears fair to conclude, that the sensory nerves have no power of receiving impressions arising from difference of temperature, unless those im pressions are communicated through a special organ ; but they afford no adequate ground for the supposition, that a set of nerve-fibres is provided for their transmission, distinct from those which minister to common sensation.
This conclusion is confirmed by the fact, that we cannot excite impressions of heat or cold by direct application to the trunks of nerves which wl know must conduct such impres sions. Thus the parts of the skin immediately beneath which lie large nerve-trunks, are not more sensitive to moderate heat or cold than any other part ; whilst a greater degree of either is felt as pain, not as a change of tem perature. Thus, as we have already seen, a mixture of ice and water, applied over the ulnar nerve, affects it in fifteen seconds, and produces severe pain, having no resemblance to cold, such as cannot be excited by the same cold applied to any other region. So the nerve of the tooth-pulp is equally and similarly affected by water of 43° and of either application causing a pain exactly similar to that excited by the other, or that produced by pressure.% We have now to consider those more com plex modes of exercise of the sense of touch which require the conjoint exercise of the " muscular sense ;" and as this is a modifi cation of the general sensibility, which may perhaps be regarded as being as special or peculiar in its relations to the muscular sys tem as the sense of touch (properly so called) is to that of the skin,",it will be desirable to examine, in the first instance, into its modus operandi.
Muscular Sense. — It may be stated as a general fact, that all voluntary muscular con traction must be guided and controlled by sen sation; and in the majority of cases, the con trolling sensation is derived from the muscles themselves, of whose condition we are ren dered cognisant by the sensory nerves with which they are furnished. The proof of this necessity is furnished by the entire want of power to make or sustain voluntary efforts, when the guiding sensation is deficient. Thus, in complete anmsthesia of the lower extre mities, without loss of muscular power, the patient is as completely unable to walk, as if the motor nerves had also been paralysed, unless the deficient sensorial guidance be re placed by some other ; and in similar affections of the upper extremities, there is a like in ability to raise the limb or to sustain a weight. But in such cases, the deficiency of the " mus cular sense" may be made good by the visual; thus, the patient who cannot walk, because he cannot feel either the contact of his foot with the ground, or the muscular effort he is mak ing, can do so if he looks at his limbs ; and the woman who cannot feel the pressure of her child upon her arms, can yet sustain it as long as she keeps her eyes fixed upon it, but no longer,—the muscles ceasing to contract, and the limb dropping powerless, the moment that the eyes are withdrawn from it. There are two groups of muscular actions, however, which, although as voluntary in their character as the foregoing, are yet habitually guided by other sensations than those derived from the muscles themselves. These are, the move ments of the eyeball, and those of the vocal apparatus. The former are directed (as Dr. Alison has well shown %) by the visual sense, by which the action of the muscles is guided and controlled in the same manner as that of other muscles is directed by their own "mus cular sense ;" and hence it happens that, when we close our eyes, we cannot move them in any required direction, without an effort that strongly calls forth the muscular sense, by which the action is then guided. In persons who have become blind after having once enjoyed sight, an association is formed by habit between the muscular sense and the con tractile action, that enables the former to serve as the guide after the loss of the visual sense ; but in those who are born perfectly blind, or who have become so in early infancy, this association is never formed, and the eyes of such persons exhibit a continued indefinite movement, and cannot by any amount of effort be steadily fixed in one spot, or be turned in any definite direction. A very small amount of the visual sense, however, such as serves merely to indicate the direction of light, is sufficient for the government of the move ments of the eye-ball.