SENSE OF TOUCH. 729 



times the lower part of the leg moves through the greatest arc. In the arm 

 the increase of sensitiveness from the shoulder to the tips of the fingers is 

 as 1 : 24; in the leg as 1 : 8^, which may be accounted for by the greater 

 rapidity required in the movements of the arm as compared with those of 

 the leg. 



595. As already stated ( 590), the only idea communicated to our minds 

 by the sense of Touch, when exercised in its simplest form, is that of Resist- 

 ance; and it is by the various degrees of resistance which the sensory surface 

 encounters, of which we partly judge by the muscular sense ( 5.'>8), that we 

 estimate the hardness or softness of the body against which we press. It is 

 only when either the sensory surface or the substance touched is made to 

 change its place in regard to the other, that we obtain the additional notion 

 of extension or space; this also being derived from the combination of the 

 muscular with the tactile sense. By the impression made upon the papillre, 

 during the movement of the tactile organ over the body which is being ex- 

 amined, the roughness, smoothness, or other peculiar characters of the surface 

 of the latter are estimated. Our knowledge of form, however, is a very com- 

 plex process, requiring not merely the exercise of the sense of touch, but also 

 great attention to the muscular sensation?. It is chiefly, as formerly re- 

 marked, in the variety of movements of which the hand of Man is capable, 

 that it is superior to that of any other animal ; and it cannot be doubted 

 that the sense of Touch thus employed, affords us a very important means 

 of acquiring information in regard to the external world, and especially of 

 correcting many vague and fallacious notions which we should derive from 

 the sense of Sight, if used alone. On the other hand, it must be confessed 

 that our knowledge would have a very limited range, if this sense were the 

 only medium through which we could acquire ideas. It is probably on the 

 sensations communicated through the Touch, that the idea of the material 

 world, as something external to ourselves, chiefly rests; but this idea is by 

 no means a logical deduction from our experience of these sensations, being 

 rather an instinctive or intuitive perception directly excited by them. 



596. Various experimenters 1 have endeavored to determine the accuracy 

 with which the Skin can appreciate impressions of weight or pressure when, 

 unassisted by the " muscular sense," that is when the part experimented dn 

 is well supported, as when the hand is laid upon a table. Aubert and 

 Kammler found that on the face and dorsal surface of the upper extremity, 

 the pressure of a portion of elder-pith, presenting a surface of nearly one- 

 third of an inch square, and weighing only ^d of a grain (2 mgrm.), could 

 be distinguished ; whilst the tips of the fingers required a weight, presenting 

 the same superficies, of id of a grain or more, and the toes as much as 8 

 grains, before any sensation of pressure was felt. The presence of the minute 

 hairs on the face was, however, found materially to influence its sensibility, 

 since when these were shaved off it was much diminished. According to 

 Weber, differences in the amount of pressure are more accurately distin- 

 guished if they are applied successively to the same point, than if they are 

 estimated coetaueously. The interval, however, must not be more than a 

 few seconds, unless the difference be considerable. Thus when 4 ounces and 

 5 ounces were 'successively applied, an accurate judgment of the difference 

 might be made after 90 seconds ; but the difference between 14.5 and 15 

 ounces could not be accurately determined after 40 seconds. If the bare 

 sense of pressure were assisted by the muscular sense, as in cases when the 

 hand Avas unsupported, much finer discrimination was always displayed. 



1 Aubert and Kammler, Moleschott's Untersuch., Bd. v, p. 145; Dohrn, Zeits. f. Eat. 

 Med., 1860. 



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