826 THE SPINAL CORD. 



stimulus, to withdraw by a pathic (Pfliiger). It is curious that the 

 sense organs of both skin and muscle seem to evoke reflexly the same 

 movements one as other. Irritation of the skin of the foot evokes 

 flexion at knee and hip; so also does stretching the muscular sense 

 apparatus in the pedal muscles and ligaments. Certain reflexes are, 

 however, peculiar to skin stimuli ; such are the " scratch " reflex and the 

 " shake " reflex in the spinal dog. These are elicitable by rubbing the 

 skin of the trunk, especially of shoulder, back, and flank. They are 

 rhythmic alternating movements. 



Mechanical stimuli applied to the skin were those earliest employed in the 

 study of reflex actions. Not only the application of the mechanical stimulus 

 but the cessation of its application is reflexigenous. 1 Of chemical stimuli the 

 application of acidulated water to the skin of the frog was methodised by 

 Tiirck, 2 and his plan has been used in many subsequent researches. The 

 fluid is used by bringing it up to the foot or by bits of soaked paper of 

 measured area allowed to rest on the skin a definite time, the skin being then 

 washed with fresh water. The threshold value for chemical and for thermal 

 stimuli applied to the skin of the spinal frog varies greatly in different experi- 

 ments. The constant current both at opening and closure can excite reflexes 

 under appropriate conditions. 3 Pfliiger's law regarding the production of the 

 muscular twitch through the motor nerve by galvanism, is found to apply Avhen 

 examined in reflex preparations (frog). 4 The reflex movement evoked by the 

 break of the current is apt to be peculiarly prolonged. Single induction 

 shocks can also excite reflexes, 5 but to do so must be relatively very strong. 

 Faradisation easily excites reflexes. Both with faradic and with mechanical 

 stimuli, and both when the stimulation is of cutaneous and of muscular sense 

 structures, it is very usually found that the discontinuance of a prolonged 

 stimulation, especially when that stimulation is itself becoming or has become 

 inefficient, provokes a fresh outburst 6 of discharge from the spinal centres. 

 After stretching a muscle by its tendon steadily and then suddenly relaxing its 

 strain, a fresh reflex movement results ; this is a phenomenon closely similar 

 to that noted by Head 7 in studying the effects of the vagus nerve on the 

 activity of the respiratory centre for the diaphragm. 



The spinal animal owes much of its interest as a field for study of reflex 

 action to the combination it offers of a relatively homogeneous sensi- 

 facient surface connected with a large and varied machinery of 

 expression, its musculature. The spinal transection above cuts off 

 all influences of light, sound, taste, smell, from that machinery, and 

 in large measure the very visceral organs. The projicient senses 

 are gone. Tactile and thermal stimuli are well-nigh the only 

 phenomena of the purely external world remaining to it, and of the 

 " material me " the chief remaining phenomena are those adequate for 

 the so-called muscular sense, and in some part the visceral. The quality 

 of the stimulus, as determined by the particular avenue of sense through 

 which it impinges on the central organ, would influence, one might 

 think, the character of the reaction. The spinal frog, when its skin is 



1 Sherrington, Phil. Trans., London, 1897. 



2 Z/schr. d. lc.-h. Gesellsch. d. Aerzte zu Wicn, 1851, Bd. vii. S. 189. 



3 Setschenow, "Ueber d. elektrische u. chemische Eeizung d. sens. Riickenm. d. 

 Frosches," Graz, 1868; Fick, Arch. f. d. ges. Physiol., Bonn, 1870, Bd. iii. S. 326; 

 Tarchanow, Bull. Acad. imp. d. sc. de St. Petersbourg, 1871, tome xvi. p. 65. 



4 Hallsten, Arch.f. Physiol., Leipzig, 1885, S. 188. 



5 Setschenow, op. cit.; Fick, op. cit. 



6 Sherrington, Phil. Trans., Loudon, 1897. 



7 Journ. Physiol., Cambridge and London, 1889, vol. x. 



