Volume 1
The physiological anatomy and physiology of man / by Robert Bentley Todd and William Bowman.
- Date:
- 1845-1856
Licence: Public Domain Mark
Credit: The physiological anatomy and physiology of man / by Robert Bentley Todd and William Bowman. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![tinguislied. That our power of varying the force of contact adds much to the delicacy of touch, is evident from this : that a plane surface may be made to seem concave, by drawing it over the passive tip of the finger of a person whose eyes are covered, pro- vided it be pressed at first strongly, then lightly, then strongly again ; or it may be made to seem convex by reversing these gra- dations of pressure. Hut, if the individual himself is the regulator of the pressure, the deception vanishes. We may obtain some knowledge of the irregularities of surfaces, and the shape of objects, by simply bringing the taetile organ into contact with them ; but much more by moving it over them with attention. Thus, too, the infinite diversities of texture may be made distinguishable by the education of tact, combined with that of the muscular sense. It is related of Saunderson, the blind professor of mathematics at Cam- bridge, that he could distinguish a spurious from a genuine medal, when the deception had imposed upon connoisseurs; and the case of the blind man, referred to by ltudolphi, who was able to dis- tinguish between woollen cloths of different colours, of course by some slight variety in their texture, is rendered credible by many well-attested examples of a parallel kind. Our power of appreciating the iceifjht of bodies, as well as resist- ances in genera], depends on those of estimating, separately and in concert, both pressure on the tactile organ and the amount of con- tractile energy acting in the muscles. Weber performed experi- ments to ascertain how far we are capable of judging of weight by the mere sense of contact. 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 lieing 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 ac- tion : and this difference is in spite of the greater surface affected (by the counter pressure against the support) in the former than iu tin1 latter case. Weber infers that the measure of weight bv 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. I be relative power of different parts to estimate weight corre-](https://iiif.wellcomecollection.org/image/b28043327_0001_0453.jp2/full/800%2C/0/default.jpg)
