752 SPECIAL SENSES. 



are on record. The blind have been known to become proficients in conchology and 

 botany, guided simply by the sense of touch. It is related of a blind botanist, that he 

 was able to distinguish ordinary plants by the fingers and by the tip of the tongue. It 

 is well known that the blind learn to read with perfect facility, by passing the fingers 

 over raised letters but little larger than the letters in an ordinary folio Bible. Eudolphi 

 cites the remarkable faculty acquired by Baczko, of distinguishing the colors of fabrics by 

 the sense of touch alone. 



An exceedingly ingenious and accurate method of determining the relative delicacy 

 of the tactile sensibility of different portions of the cutaneous surface was devised a 

 number of years ago (1829) by E. H. Weber, whose researches upon this subject, which 

 have been repeatedly confirmed by other observers, are still the most careful and reliable 

 on record. This method consists in the application to the skin, of two fine but blunt 

 points, separated from each other by a known distance. The individual experimented 

 upon should be blindfolded, and the points applied to the skin simultaneously. By care- 

 fully adjusting the distance between the points, a limit will be reached where the two 

 impressions upon the surface are appreciated as one ; i. e., by gradually approximating 

 them, the subject will suddenly feel both points as one, when, an instant before, with the 

 points a little farther removed from each other, he distinctly felt two impressions. This 

 gives a very accurate measure of the delicacy of the tactile as distinguished from the 

 general sensibility of different parts, and it has lately been found a most important guide 

 in the investigation of diseases of the nervous system attended with partial anaesthesia 

 of the surface. Of course, the instrument used may be very simple (a pair of ordinary 

 dividers will answer), but it is convenient to have some ready means of ascertaining the 

 distances between the points. An instrument, consisting simply of a pair of dividers, 

 with a graduated bar giving a measure of the separation of the points, is the best, as it 

 combines simplicity, convenience of use, and portability. This instrument is called the 

 sesthesiometer. 



The experiments of Weber were made upon his own person, and, of course, they do not 

 show the variations that may occur in different individuals in health, a point of consider- 

 able importance in estimating the extent of anaesthesia in disease. His observations also 

 showed some slight variations with the direction of the line of the two points, but these 

 are not important. Valentin repeated the experiments of Weber, and, in addition, took 

 the maximum, minimum, and mean, in six persons. Aside from these observations, the 

 repetition of Weber's experiments has done little more than confirm the original facts. 

 The table upon the next page, taken from the article on "Touch " by Dr. W. B. Carpen- 

 ter in the Cyclopedia, of Anatomy and Physiology, London, 1849-1852, vol. iv., part ii., 

 p. 1169, gives the results obtained by Weber and by Valentin. 



If we note the distribution of the tactile corpuscles in connection with this table, it 

 will be seen that the sense of touch is most acute in those situations in which the cor- 

 puscles are most abundant. In the space of about one-fiftieth of a square inch on the 

 palmar surface of the third phalanx of the index-finger, Meissner counted the greatest 

 number of corpuscles, viz., one hundred and eight. In this situation, the tactile sensi- 

 bility is more acute than in any other part of the skin, the mean distance indicated by 

 the sesthesiometer being 0'603 of a line. In the same space on the second phalanx, forty 

 corpuscles were counted, the sesthesiometer marking 1 -558 line, this part ranking next in 

 tactile sensibility after the red surface of the lips. We can readily understand how the 

 tactile corpuscles, embedded in the amorphous substance of the cutaneous papillae, might 

 increase the power of appreciation of delicate impressions by presenting hard surfaces 

 against which the delicate nerve-filaments can be pressed. 



As regards those portions of the general cutaneous surface in which no tactile corpus- 

 cles have been demonstrated, it is not easy to connect the variations in the tactile sen- 

 sibility with the nervous distribution, as we know little or nothing of the comparative 

 richness of the terminal nervous filaments in these situations. 



