Touch has a far more complicated mechanism than tact. It one of the senses properly so called, or the special senses, and like the others of its class is distinguished by its requiring the assistance of muscles. Its sensations are com pounded of tact and muscular resistance, and the organ is that wonderful instrument the hand. The imperfect state of this organ in infants must have been noticed by every one ; it is generally closed and capable of grasping but very feebly ; at all events a long time occurs before the little being learns to arrange the sensitive tips of the fingers, and to adjust the thumb in such a manner as to ascertain with nicety the form, consistence, and other properties of bodies. Whether the skin is less sensitive in these subjects we cannot say, but it is quite certain that the muscles, which effect the digital motions alluded to, are not developed any more than those in other parts of the body. Fine ness of touch, tactus eruditus, is one of the most difficult attainments of manhood.
Concerning the alterations in the olfactory apparatus we have already spoken, when the development of the facial bones was under consideration. The sense of smell is mani fested pretty early, but there can be no great precision and nicety in its exercise, both from defect of surface, and from the want of mus cular power and command, in adjusting the quantity and impetus of the air that conveys the odorous particles. Thus, some agents are only appreciated by a sudden inhalation through the nostrils, as if to bring the particles with a certain degree of force upon the Schneiderian membrane. This art the child does not under stand.
Taste being a sense so essential to the main tenance of the system, whether by inducing the animal to take the trouble of eating, or by warning him of improper aliment, is mani fested very early. The usual description of the mechanism of taste would give just cause for questioning what was said respecting the ne cessity of a co-operation of muscular action with the five senses. Taste is described as the result simply of the application of sapid bodies to the tongue, palate, velum palati, &c. But these bodies excite no sensation without the aid of muscles. A certain degree of com pression is necessary, which is accomplished by pressing the tongue against the roof of the mouth. Any one may assure himself of this fact by placing a strongly flavoured substance on the tongue when projected from the mouth ; no taste will ensue till the member is with drawn and then pressed against the palate.
This observation applies not only to the tongue but also to the palate itself, and that sensitive surface the velum. In each instance, however, the effect may be imitated, by compressing with the finger the part where the substance is applied.
Taste must undergo a progressive develop ment correspondently with the muscular or gans. It is, to say the least, very doubtful if a child could perform those delicate manoeuvres of the tongue and palate, which are practised by gourmands or professed vine-tasters. There is something more than this muscular action, however, to be taken into consideration. The more refined flavours are probably felt and estimated by the lining membrane of the nasal passages. It is common to remark that the scent of a substance is similar to its taste, but in all probability the two sensations are iden tical ; for the taste in question is not perceived if the nostrils be closed ; witness the abolition of taste during a catarrh. If therefore so close a connection exists between the two senses, it is clear that the development of the organi zation belonging to the one must influence the other function ; and it has been already pointed out that the olfactory surface increases with growth.
The new-born infant is probably all but deaf; even the loudest sounds produce no sensible impression. The nurse's lullaby, therefore, is for some time superfluous ; by degrees, how ever, the shrill tones, of which such strains for the most part consist, begin to be appreciated ; the precise period however we do not know. In correspondence with this obtusity we find the organ incomplete, but the incompleteness has reference rather to the external than to the internal ear. Thus the pinna is very inelastic, and therefore unfitted for collecting vibrations ; the same may be said of the meatus auditorius. In like manner, the membrana tympani is very oblique, and scarcely more than a continuation of the superior surface of the meatus, and thus little calculated to receive the vibrations. These parts are also covered with a soft matter very unfavourable to vibrations ; the tympanum is very small, and the mastoid cells do not exist. In the progress of age all these parts gradually increase in hardness, and consequently are bet ter adapted to their function. There are mus cles attached to this sense also, but we are deficient in observations on their degree of de velopment, though we may infer their condition from analogies in the rest of the muscular system.