1168 



TOUCH. 



hot or cold fluid or solid body is for the time 

 completely lost, a feeling of pain alone being 

 experienced, which is the same whether the 

 body be hot or cold. This, too, he found to 

 be the case, when, instead of applying the heat 

 or cold to the peripheral extremities of the 

 nerve, he acted on its trunk. For this ex- 

 periment the ulnar nerve was selected, as its 

 trunk, at the elbow, lies immediately beneath 

 the surface. After immersing the elbow in 

 a mixture of ice and water for about six- 

 teen seconds, Professor Weber observed that 

 a peculiar painful sensation was experienced 

 along the under side of the fore-arm, the 

 wrist, the little finger, and the inner side of 

 the ring-finger. This pain had no resemblance 

 to that of cold. On continuing the immersion, 

 the pain increased considerably, and eventu- 

 ally became almost intolerable ; then it gra- 

 dually diminished, and the middle and ring- 

 fingers became numb, as if " asleep," had no 

 longer the power of distinguishing between 

 heat and cold, and could only imperfectly per- 

 ceive the contact and pressure of bodies. * 



The exercise of the sense of touch jnay be 

 first considered under its simplest mode, 

 namely, that in which the object is simply ap- 

 plied to the tactile organ ; and in this we 

 have specially to consider the power of Tactile 

 Discrimination, and the Sense of Temperature. 



Tactile Discrimination. A very ingenious 

 method was devised by Professor Weber"}", for 

 determining the relative power of tactile dis- 

 crimination in different parts of the skin, which 

 is by no means accordant with their general 

 sensibility. His mode of ascertaining this, was 

 to touch the surface with the points of a pair 

 of compasses, guarded by bits of cork or seal- 

 ing-wax ; the eyes being closed at the same 

 time, the legs of the compasses were approxi- 

 mated to each other; until they were brought 

 so near that the points could be no longer felt 

 to be distinct from each other. The smallest 

 distance at which this can be perceived (en- 

 titled "the limit of confusion," by Dr. Graves), 

 is found to differ remarkably on different 

 parts of the cutaneous surface ; and the com- 

 parison of these diversities affords us the 

 means of estimating, not their relative tactile 

 sensibility (for this it cannot measure), 

 but their relative discriminating power. The 

 figures in the first column of the following 

 TABLE represent these distances, as determined 

 by Professor Weber on his own person, stated 

 in Paris lines. The inquiry has been more re- 

 cently pursued by Professor Valentin j, whose 

 results on the whole correspond very closely 

 with those of Weber. He found, however, a 

 considerable extent of individual variation; 

 some persons being able to distinguish the 

 points at half, or even one- third of the dis- 

 tances required by others. In the following 

 table, the second column expresses the maxima 



* Muller's Archiv. 1847, p. 342. 



f De Pulsu, Respiratione, Auditu, et Tactu, An- 

 notationes Anatomicae et Physiologic*. Auct. H. E. 

 Weber. Lipsiaj, 1834. 



t Lebrbuch cler Physiologic des Menschen, Band 

 ii. S.566. 



of the "limit of confusion," the third column 

 the minima, and the fourth column the mean of 

 all the observations made by Professor Valen- 

 tin ; it will be observed that his maxima cor- 

 respond almost exactly with the measure- 

 ments of Professor Weber on his own person. 

 In the fifth and sixth columns are shown the 

 relative acuteness and relative obtuseness of 

 the discriminating sense in different parts, 

 calculated from the mean results ; " the limit 

 of confusion " (0'483) at the tip of the tongue 

 being taken as I'OOO. Thus the co-efficient 

 of the acuteness of the discriminating sense in 

 the palm of the hand, calculated by this 

 standard, is CM 26 ; that of its obtuseness is 

 7 '930. The co-efficient of acuteness for the 

 crown of the head, is 0*050 ; that of obtuse- 

 ness, for the same part, is 19'827. The co- 

 efficient of acuteness for the middle of the 

 dorsal spine is 0'020 ; that of its obtuseness 

 is 50-086. Or, in other words, its acuteness 

 is only T g^ths that of the tip of the tongue ; 

 its obtuseness fifty times as great. 



Similar experiments, with the like results, 

 have been made by M. H. Belfield-Lefevre*; 

 and from all these, the following general pro- 

 positions may be laid down : 1. On almost 

 any part of the integument, the interval 

 between the two points is more clearly dis- 

 tinguished, when the line which joins them is 

 transverse (i. e. perpendicular to the axis of the 

 body or member), than when it is longitudinal, 

 or parallel to that axis. According to Weber, 

 however, the tips of the fingers and of the 

 tongue constitute an exception to this rule; the 

 discriminating power being greatest in them, 

 when the line joining the points of the com- 

 passes is longitudinally directed. 2. When 

 two points, applied simultaneously to any 

 part of the integument, are clearly distin- 

 guished, the distance which separates them 

 seems to be greater, in proportion to the acute- 

 ness of the discriminating sense in the part of 

 the surface which is the subject of examina- 

 tion. Thus, as Weber remarks, if the points 

 of the compasses, set at a distance of two or 

 three lines, be applied to the cheek just before 

 the ear, and be then moved gradually towards 

 the angle of the mouth, the points will seem 

 to recede from one another, in consequence of 

 the increase of the discriminating sense in the 

 parts to which they are applied. 3. When 

 the two points are successively brought into 

 contact with the skin, they seem to be at a 

 greater distance from each other than if they 

 are simultaneously applied ; and, in general, 

 the distance will seem greater in proportion to 

 the interval between their application. 4. Two 

 points applied on different sides of the median 

 line, seem more remote from one another 

 than two points equally distant, but applied on 

 one and the same side* of the median line ; in 

 other words, the power of discrimination is 

 greater when the two points are applied on 

 the two sides of the median line, than when 

 they are both applied on the same side. 5. 

 If two parts of the tegumentary surface be 



* Eecherches sur la Nature, la Distribution, et 

 1'Organs du Sens Tactile. Paris, 1837. 



