MECHANICAL STIMULATION. 467 



stances. Hence the permanent impairment of tissue vitality is not a 

 necessary termination to prolonged chemical excitation. 1 



Mechanical stimulation. — Various modes of mechanical excita- 

 tion have been devised. The simplest is that of a rapid compression, 

 such as is caused by dividing the nerve trunk. In order to secure the 

 repetition of the stimulus, du Bois-Reymond 2 employed a small toothed 

 wheel which compressed successive portions of the nerve, and thus always 

 affected a new region. Heidenhain used a hammer, which struck the 

 nerve where it lay upon a vulcanite block, whilst by means of a small 

 cylinder a fresh portion was brought under the vulcanite anvil in the 

 interval between the ascent and descent of the hammer. 3 A similar 

 " tetanomotor " was employed by Wundt ; it was materially modified 

 by Tigerstedt, the hammer being made very light, and the force of 

 the blow rendered capable of adjustment by altering the position of 

 a small load, placed between the fulcrum of the hammer and its head. 4 

 With this apparatus carefully adjusted, it is possible to excite once 

 every few seconds upon the same spot many times in succession. 

 Langendorf employed an entirely different method ; the cut end of 

 the sciatic nerve of the frog was ligatured and attached to the free 

 end of a tuning-fork, which was then adjusted so as to lightly pull 

 the nerve. On setting the tuning-fork in vibration, the nerve was 

 alternately stretched and relaxed with each vibration and thus stimu- 

 lated. 5 The conditions which determine effective mechanical stimulation 

 have been investigated by Uexkull, 6 who states that this is best achieved 

 by an arrangement in which the hammer is rigid but the counter- 

 resistance extremely yielding. This condition is fulfilled when the nerve 

 is rapidly shaken against an elastic pad by a vibrating ivory bar. 

 The apparatus devised by Uexkull, and termed by him the nerve 

 shaker, consists of a thin ivory lever, moving on a central fulcrum, 

 and held in position by an adjustable spring. By means of a rotating 

 toothed wheel one end of this lever is lightly struck in rapid suc- 

 cession, and the whole thus set in vibration. The nerve either rests 

 upon a yielding pad under the further end of the lever, or is twisted 

 round the lever point. The vibration of the lever is communicated to 

 the nerve, and the shatter thus produced excites it. Each shatter of the 

 lever can be registered by means of a small contact, which opens and 

 closes an electrical circuit. Uexkull found that, with this apparatus, 

 the nerve could be excited many times at the same spot, and that thus 

 the injurious effects caused by striking with the hammer could be 

 eliminated. The shatter appears to excite the motor nerve in the same 

 way as the induced current, at least in so far as the time relations of the. 

 resulting muscular contraction are concerned. 



A distinct mode of stimulation is that caused by suddenly lifting a 

 compressing weight; it appears doubtful whether this is in reality a 

 form of mechanical excitation, since it is possible that the release of the 

 compression may cause the nerve to regain its condition of moisture, 

 which during the squeeze has undoubtedly been diminished ; the demar- 

 cation nerve current due to the injury of the squeeze is thus suddenly 



1 Buchncr, Ztschr.f. Biol., 1876, Bd. xii. 



2 E. <lu Bois-Reymond, " Untersuchungeu," 184i), Bd. ii. 



3 Heidenhain, Untersuch. z. Naturl. d. Mensch. u. d. Thieve, 1858, Bd. iv. 



4 Tigerstedt, Bcitr. z, Physiol. Carl Ludwig z. s. 70 Geburtst., Leipzig. 



5 Langendorff, Centralbl.f. d. mrd. Wissenseh., Berlin, 1882, S. 114. 



6 Uexkull, Ztschr.f. Biol., 1895, Bd. xxxi. and xxxii. 



