■2-2-2 



AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



the discharging cell for a short period of time more excitable. In the same 

 figure the record shows that ii'a longer interval — here more than three seconds — 

 be allowed to elapse, then the second stimulus from the skin remains ineffi- 

 cient. A similar relation between the two incoming impulses is also found 

 to hold when the stimulus from the skin is made to precede. The curve B, 

 Fig. 94, shows the results when both stimuli are inefficient. In this the 

 stimuli [l> and a) produce no effect when given several seconds apart, but when 

 they occur within a short interval (/>' and a') — in this case 0.13 of a 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 which it is 

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

 Jendrassik ' that when a patient was being tested for the height of his knee- 

 kick, ;i voluntary muscular contraction, or an extra sensory stimulus, occur- 



Moremi-iit of paw. 



Stimtdatioti of cortex. ifl! 



" b' " paw. 



/.'.-.'.'(' .':■■• seconds. 



rlHHrArlrHriru-\i 



Fig. 94.— 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 .1 the cortical stimulus at a causes a movement of the paw. 

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

 movement of the paw. Dermal stimulus several seconds later at V is ineffective. In curve li dermal 

 stimulus :it b is ineffective. The cortical stimulus at a several seconds later is also ineffective. The 

 dermal stimulus at 1/ is ineffective, but if followed within 0.13 second by a cortical stimulus at a' a move- 

 ment of the paw occurs. 



ring about the same time that the tendon was struck, had the effect of 

 increasing the height of the kick. This relation was studied in detail by 

 Bowditch and Warren, 2 and they were able with great exactness to measure 

 the interval between the contraction of the muscle used for reinforcement 

 and the time at which the tendon was struck. The curve shown in Fig. 95 

 represents the results of these experiments. It indicates that, up to 0.4 of a 

 second, the closer together these two stimuli occur the greater the reinforce- 

 ment. At an interval of 0.4 of a second no effect is produced by the muscu- 

 lar contraction. Increasing the interval only very slightly has, however, the 

 effect of greatly diminishing the height of the knee-kick — i. e., decreasing 

 the strength of the discharge of the efferent cells — and this effect is not lost 

 until the interval is increased to 1.7 seconds, when the voluntary muscular 

 contraction ceases to modify the response. A given efferent cell is thus 

 modified in its discharge according to the several stimuli that act upon it. 



1 Dentsches Archir fiir klinische Medizin, Bd. xxxiii. 

 1 Journal of Physiology, 1890, vol. xi. 



