approach the middle line; and the tip of the nose and red parts of the lips are acutely so, and only inferior to the tip of the tongue. This last, in a space of a few square lines (a line is of an inch), exceeds the most sensitive parts of the fingers; and points of contact with it may be generally perceived distinctly from one another, when only onethird of a line intervenes between them. [The superior sensibility of the tip of the tongue to the finger, is illustrated by the familiar observation, that a hole in a tooth seems very much exaggerated when felt by the tip of the tongue.] As we recede from the tip along the back or sides. of the tongue, we find the sense of touch much duller.

'The sensibility of the surface of the trunk is inferior to that of the extremities or head. The flanks and nipples, which are so sensitive to tickling, are comparatively blunt in regard to the appreciation of the distance between points of contact. Points placed on opposite sides of the middle line, either before or behind, are better distinguished than when both are on the same side.

The above are the results obtained by making the several parts mere passive and motionless recipients of impressions. They evince the precision of the sense in so far only as it depends on the organization of the tactile surface. The augmented power derived from change of position of the object with regard to the surface, is well illustrated by keeping the hand passive, while the object is made to move rapidly over it. In this case the contact of the two points is separately perceived, when so close that they would, if stationary, seem as one. If, still further, the fingers be made to freely traverse the surface of an object, under the guidance of the mind, the appreciation of contact will be far more exquisite, in proportion to the variety of the movements, and the attention given to them. We are then said to feel, or to examine by the sense of touch.'-TODD and BOWMAN, I., 429-30.

These observations of Weber have been deservedly celebrated by physiologists, as the foundation of an accurate

WEBER'S EXPERIMENTS.

175

mode of estimating the tactile sensibility of the skin. They have been extended by other observers, as may be seen in Dr. Carpenter's article on Touch in the Cyclopædia of Anatomy.* It is necessary, however, for us to discuss more closely the matters involved in them, and especially to discriminate the tactile from the muscular element of the sensations.

Whenever two points produce a double sensation, we may imagine that one point lies on the area supplied by one distinct nerve, while the other point lies on the area of a second nerve. There is a certain stage of subdivision or branching of the nerves of touch, beyond which the impressions are fused into one on reaching the cerebrum. How many ultimate nerve fibres are contained in each unit nerve, we cannot pretend to guess; but on the skin of the back, the middle of

• The following are a selection from Weber's measurements. The intervals are expressed in lines, a line being the twelfth part of an inch. The range according to Weber is from the twenty-fourth of an inch, in the tip of the tongue, to two and a-half inches. The range stated in the text is somewhat greater, being founded probably on a comparison of the extreme observations of different observers :

30

99

Thigh, by the knee,

Upper and lower extremities of the legs,

Breast,

Back of neck near occiput,

Middle of fore-arm, middle of thigh, middle of the back

of the neck, middle of the back, If the points are placed within the limit of doubleness and gradually separated, the interval that gives doubleness is greater than in the reverse

process.

the thigh, and the middle of the fore-arm, an area of three inches diameter, or between six and seven square inches, is supplied by the filaments of a single unit. On the point of the finger, the units are so multiplied, that each supplies no more than a space whose diameter is the tenth of an inch. Such units would correspond to the entire body of the olfactory or gustatory nerve, for these nerves gives but one undivided impression for the whole area affected; or at most would give two impressions, one for each side.

It is important to observe that the primitive susceptibility to a plurality of distinct points, does not enable us to judge what the real distance of the points is; nor can we tell previous to experience whereabouts on the body the impression is made. Hence in those of the experiments that relate to our sense of the relative interval of the points, as when they pass from a duller to a more sensitive region, there are involved perceptions that we have got at in some other way than through the sense of contact. This other means is the feeling of movement or the muscular sensibility, without which it is impossible to comprehend fully the sensations of Touch.

11. (2.) Sensations of Pressure.-When a contact passes from the soft touch to a certain amount of energy of compression, the character of the sensation is entirely changed. It becomes indifferent as regards pleasure and pain, unless the pressure is on the verge of injuring the parts, when it becomes painful. The nerves of touch are of course affected, but probably not they alone. The compression may extend its influence to the nerves in the deep seated parts, that is, to fibres supplied to muscles, &c.

If the compressed limb is unsupported, its muscles re-act and give the feeling of resistance. If it is supported, as when the hand lies on the table, the effect is one of pressure solely, whether the nerves stimulated are those of the skin alone or of the skin and the interior tissues combined. The sense of pressure is found to have a certain power of discrimination, applicable to determine degrees of weight, hardness, elasticity,

and other properties. The most sensitive parts, as the tips of the fingers, can distinguish 20 oz. from 19.2 oz.; the forearm distinguishes 20 oz. from 18.7 oz. The interval of time affects the discrimination, as we might suppose. The difference between 14, or even 14.5, could be distinguished from 15, within 30 seconds; 4 and 5 could be distinguished within 90 seconds.

The discrimination of pressure does not increase proportionably with the supply of tactile nerves.

12. (IV.) Sensations of Touch involving muscular perceptions. In discussing these, we shall begin with examples that are almost purely muscular, the tactile sensibility being a mere incident of the situation. The feeling of weight is of this description; depending on the sense of muscular exertion, although capable also of being estimated to some extent by the feeling of compression of the skin. On this last point, I add some further illustrations from Messrs. Todd and Bowman. 'Weber performed experiments to ascertain how far we are capable of judging of weight by the mere sense of contact [without muscularity]. He found that when two equal weights, every way similar, are placed on corresponding parts of the skin, we may add to, or subtract from one of them a certain quantity without the person being able to appreciate the change; and that when the parts bearing the weights, as the hands, are inactively resting upon a table, a much greater alteration may be made in the relative amount of the weights without his perceiving it, than when the same parts are allowed free motion. For example, 32 ounces may thus be altered by from 8 to 12, when the hand is motionless and supported; but only by from 1 to 4, when the muscles are in action; and this difference is in spite of the greater surface affected (by the counter pressure against the support) in the former than in the latter case. Weber infers that the measure of weight by the mere touch of the skin is more than doubled by the play of the muscles. We believe this estimate to be rather under than over the mark.'-p. 431.

That the discriminating sensibility of the skin to degrees

of compression may operate in appreciating weight is further confirmed by the following statement. The relative power of different parts to estimate weight corresponds very nearly with their relative capacities of touch. Weber discovered that the lips are better estimators of weight than any other part, as we might have anticipated by their delicate sense of touch and their extreme mobility. The fingers and toes are also very delicate instruments of this description. The palms and soles possess this power in a very remarkable degree, especially over the heads of the metacarpal and metatarsal bones; while the back, occiput, thorax, abdomen, shoulders, arms, and legs, have very little capacity of estimating weight.'-ib. p. 432.

What is said of weight applies to any other form of pressure, force, or resistance. The impetus of a push or a squeeze received on the hand is measured by the muscular exertion induced to meet it, and in some small degree, as above described, by the compression of the skin and other parts at the place of contact.

It must not be supposed that we could derive our original feeling of RESISTANCE, with its reference to the object world, by mere tactile sensibility through pressure. The sense of resistance is primarily the feeling of expended energy. When the notion is once formed, we can remark that the degrees of resistance coincide with degrees of the tactile sensibility to pressure; and hence the passive feeling can suggest the active, and become a criterion of its amount.

The qualities of hardness and softness are appreciated by this combined sensibility; the one means a greater resistance to compression, and the other a less. From the unyielding stone or metal to the mobility of the liquid state, we have all degrees of this property; the entire class of soft, viscous, and fibrous substances lying between. It belongs to many of the manual arts to appreciate minute differences of consistence in the class of soft bodies; the pastry-cook, the builder, the sculptor, &c. In this they are assisted by practice, which improves all sensibilities: but there are great varieties of