INTENSITY OF THE STIMULUS. 829 



that for obtaining contraction by excitation from the corresponding 

 efferent root ; especially is the difference marked for very brief stimuli. 

 The application of 2 grins, to an area of 1 sq. mm. of the plantar skin of 

 the spinal frog usually evokes a reflex response ; this represents for the 

 mechanical stimulus the threshold value efficient to evoke from that 

 region a reflex reaction. This high threshold is probably determined by 

 the " neuron-threshold " (Goldscheider a ) at synapsis between afferent 

 and efferent root cells ; though it has been found actually lower for the 

 reflex movement than for galvanometric effect led off from afferent nerve. 2 

 But the efficiency of the stimulus, of course, much depends on its sudden- 

 ness of application. A very gradually increased pressure will require 

 values far above what is otherwise liminal. 3 It is said that the spinal frog 

 may be crushed without evincing a movement, if the crushing force be 

 applied to it very gradually. Similarly, the application of warmth or 

 cold to the frog's skin, if very gradual, 4 excites no reflex reaction. If 

 the temperature of water in which one limb of the spinal frog is 

 immersed, be raised not more quickly than ^1^ C. per second, no reflex 

 ensues, even though the warming be carried up to the production of 

 rigor caloris. Foster 5 has, however, shown that a fallacy in this 

 observation consists in neglecting the warming of the circulating 

 blood which passes through the limb and so into the general circulation. 

 The warmed blood much depresses the spinal reflex activity of the frog. 

 When one toe only is immersed in the gradually warmed water, reflex 

 movement ensues at a temperature of 35 C. 



The strength of stimulus exerts some influence on the extent and 

 duration of the reflex movement excited ; these tend to increase with 

 increase of the stimulus. But there is no such regular relation between 

 intensity of stimulus and intensity of reply as obtains in indirect 

 excitation of skeletal muscle through its motor nerve. " The reflex arc, 

 as judged by the contraction of its muscles, behaves somewhat like 

 cardiac muscle in responding its approximate best or not at all." 6 In 

 other words, internal conditions affect the amount of response much 

 more than does mere intensity of external stimulus. This is no doubt 

 the secret of the effectiveness of repetition of slight stimuli and of 

 summation. If in the spinal mammal an external stimulus of an 

 intensity that is usually efficient fails to excite a reflex, to increase 

 its intensity is usually not of much avail. 



Eosenthal 7 has established the fact that there is a distinct relation between 

 the intensity of the external stimulus and the latent time of spinal reaction, 

 the latter becoming much shorter as the former is increased. Wundt 8 has 

 pointed out that strychnin still further limits the degree to which intensity of 

 the external stimulus influences the intensity of the contraction in response ; 

 the evoked contraction tends to be always maximal, provided a certain period 

 has elapsed since the preceding contraction. 9 During this period the contrac- 



1 " Die Bedeutung der Reize," Leipzig, 1898. 



' 2 E. Steinach, Arch. f. d. ges. Physiol., Bonn, 1895, Bd. Ixiii. 



3 C. Fratscher, Jenaische Ztschr.f. Naturw., Jena, 1875, Bd. ix. S. 156. 



4 A. Heinzmann, Arch. f. d. ges. Physiol., Bonn, 1872, Bd. vi. S. 222. 



5 "Studies from the Physiol. Lab. of the Univ. of Cambridge," 1873, vol. i. p. 36. 

 See also Tarchanow, Bull. Acad. imp. d. sc. de St. P&ersbourg, 1871, tome xvi. p. 226 ; 

 Rudnow, Journ. dly a normal, i patol. gistologii, St. Petersburg, 1872, vol. v. p. 238. 



6 Sherrington, Phil. Trans., London, 1897. 



7 Abhandl. d. k. Akad. d. Wissensch., Berlin, 1881. 



8 " Untersuch. z. Mechanik. d. Nervencent.," 1876, Bd. i. Ahth. 2, S. 70. 



9 G. L. Watson, Arch. f. Physiol., Leipzig, 1882, S. 49 ; and Journ. Physiol., Cambridge 

 and London, 1881, vol. iii. p. 301. 





