CENTRAL NERVOUS SYSTEM. 



665 



electric stimulus could be applied to the cerebral cortex at a point the excita- 

 tion of which caused contraction of certain muscles of the foot. One of these 

 muscles was attached to a lever so that its contraction could be recorded, and 

 a second electrode applied to the skin of the foot overlying the muscle. The 

 discharging efferent cells in the cord were in this case subject to impulses from 

 two directions, one from the cortex and one from the skin of the foot. With 

 a current of given strength stimulation of the cortex alone caused a contrac- 

 tion of the muscle, and stimulation of the skin of the foot alone, a similar 

 contraction. When both were stimulated simultaneously, the extent of the 

 contraction was greater than when either was stimulated alone. If now the 

 strength of the stimulus applied to the skin was so reduced that, alone, it was 

 inefficient, then a stimulus from the cortex, which produced a reaction, as 

 -indicated by the first cortical stimulus in Figure 172 (A, a), put the efferent 





Stimulation of cortex. A a' 



paw. 



FIG. 172. To show the reinforcing influence of stimuli applied to the cerebral cortex and to the skin 

 of the paw, on the movements of the paw of a rabbit (Exner). The arrows indicate the direction in 

 which the curves are to be read. In curve A the cortical stimulus at a causes a movement of the paw. 

 Dermal stimulus, within a second, at & causes a movement of the paw. Cortical stimulus at a' causes a 

 movement of the paw. Dermal stimulus several seconds later at 6' is ineffective. In curve B dermal 

 stimulus at 6 is ineffective. The cortical stimulus at a several seconds later is also ineffective. The 

 dermal stimulus at 6' is ineffective, but if followed within 0.13 second by a cortical stimulus at a' a move- 

 ment of the paw occurs. 



cells in such a condition that the stimulus from the skin (A, b) Figure 172, 

 applied within 0.6 second, produced a second contraction of the muscle, 

 although, alone, the stimulus from the skin had proved inefficient. Here the 

 first efficient stimulus from the cortex had rendered the discharging cell, for a 

 short period of time, more excitable. In the same figure the record shows that 

 if a longer interval, here more than three seconds, be allowed to elapse, then 

 .the second stimulus from the skin remains inefficient. A similar relation be- 

 tween the two incoming impulses is also found to hold, when the stimulus 

 from the skin is made to precede. The curve B, Fig. 172, shows the results 

 when both stimuli are inefficient. In this the stimuli (b and a) produce no 

 effect when given several seconds apart, but when they occur within a short 

 interval (b f and a') in this case 0.13 second a contraction of the muscle 

 follows. These various experiments, taken together, show in a beautiful way 

 that in the cases chosen the two sets of impulses tend to reinforce each other, 

 whether they are efficient or inefficient, and without regard to the order in 

 which they come. 



This relation between the discharging cell and those by way of which it is 

 stimulated can be illustrated in still another way. It was observed by Jeu- 

 drassik l that when a patient was being tested for the height of his knee-kick, 

 1 Deutsches Archiv fiir klinische Medicin, Bd. xxxiii. 



