THE ADEQUATE STIMULUS FOR TOUCH. 925 



The first instance in which a variation of the resting currents of a 

 sensory nerve was obtained by application of an adequate stimulus to 

 the end-organs was in the case of the eye. 1 Working with cooled (hyper- 

 sensitive) nerves, Steinach has obtained negative variation of the resting 

 sciatic current by dealing a light blow to the skin of the frog's foot with 

 a graduated lever. Within limits the extent of the negative variation 

 varies directly with the force of the blow applied. 2 



Fontana 3 showed in 1780 that by gradually increased compression a 

 nerve can, without exciting it, be paralysed as a conductor. The first 

 application of the pressure is felt, and then the sensation subsides, if the 

 compression be steady and not too rapidly increased. Similar pressure 

 applied to the skin evokes a sensation which does not so subside, — the 

 sensation continues. The inner stimulus must therefore be tetanic in 

 character. This difference between nerve and end-organ does not, 

 however, hold if the stimulus to the latter is but little above liminal, — 

 then the touch is felt for a moment after contact, and speedily subsides. 4 

 It may be again felt for a moment at the sudden discontinuance of this 

 stimulus ; 5 in this the reaction resembles many reflexes obtainable from 

 the spinal cord, where discontinuance 6 of a prolonged stimulation evokes 

 a renewal or an increase of a reflex reply that had ceased or become 

 feeble. 7 " Touches " evoked by brief stimuli, even of very slight intensity, 

 especially from the region of the nose, may persist for very long, or 

 leave a very protracted after sensation. The prolongation of the sensa- 

 tion is of an itching, tickling quality, and may continue even for minutes. 

 This secondary sensation may be due to a local vascular reflex initiated 

 by the stimuli, for the itching spot can generally be seen to be flushed. 8 



The principle of " adaptation," laid stress upon by Hering and G. E. 

 Miiller, in the visual and muscular senses, plays a part also in tactual. 

 After the skin has been tested by a number of light weights, a heavy 

 weight appears disproportionately heavy ; and light weights dispropor- 

 tionately light, after a series of much heavier. The large areas over 

 which contact of the skin with clothing is habitual become, even under 

 attention, almost insentient of the contact. A plate carrying artificial 

 teeth is for the first few hours almost unbearable to the mouth, and 

 provokes reflexly for a time excessive salivation, etc. In a comparat- 

 ively short period the intensity of the reaction to the stimulus dwindles 

 to an extraordinary extent. A similar adaptation is experienced when 

 eye-glasses, spectacles, etc., are first worn. 



The method of application of the adequate stimulus for touch has been, 

 in the vast majority of instances, the application of pressure. This has been 

 done with weights in a scale-pan ; 9 with a spring balance (the barsesthesio- 

 meter) ; 10 with a pidsating elastic tube, distended by hydrostatic means ; n 

 by a mercurial balance, enabling rapid variations of pressure to be made 



1 "W. Kuhne and J. Steiner, Untersuch. a. d. physiol. Inst. d. Univ. Heidelberg, 1881, 

 Bd. iv. The earlier work by Holmgren and M'Kendrick dealt with the retinal currents. 



2 Arch. f. d. ges. Physiol., Bonn, 1896, Bd. lxiii. S. 495, etc. 



3 "De corpor. animal., etc.," Firenze. 



4 Kiesow, Arch. ital. de biol., Turin, 1896, tomo xxvi. p. 417. 



5 Cf. also Steinach, op. cit., S. 514. 



6 Head, Journ. Physiol., Cambridge and London, 1889, vol. x. p. 279. 



7 Sherrington, Phil. Trans., London, 1896. 8 v. Frey, loc. cit. 



9 Dohrn, "De varia variarum cutis, etc.," Diss., Kiel, 1859; Ztschr. f. rat. Med., 

 1861, Bd. x. S. 339. 



lu Eulenberg, ibid., S. 360 ; Berl. klin. Wchnschr., 1869, Bd. vi. S. 469. 

 11 Goltz, Centralbl.f. d. med. JFissensch., Berlin, 1863, S. 273. 



