830 THE CENTRAL NERVOUS SYSTEM 



plied to the tendon. If this voluntary effort coincides in time with the 

 tapping of the tendon, the knee-jerk will be found much augmented; but 

 if the two events do not accurately coincide, we may find instead that 

 the knee-jerk is diminished ; that is to say, we may have positive fol- 

 lowed by negative reinforcement. The most usual way of having the 

 patient make this voluntary effort is to ask him to lock the fingers of his 

 two hands together and then at a given signal try to pull the locked 

 arms apart. 



Similar reinforcement may also be produced by the application of a 

 strong sensory stimulus in some distant part of the nervous system, as, 

 for example, by pulling the hair or pinching the ear. Accurate work 

 on the time relationship between the reinforcing act and the tap on the 

 tendon has shown that the knee-jerk is most marked when the tap ac- 

 curately corresponds with the voluntary effort or sensory stimulation. 

 It then quickly declines and an inhibitory influence appears in about 

 0.3 to 0.6 of a second, immediately -after which it becomes pronounced 

 again, gradually fading off to be no longer evident in about 1.7 of a 

 second ; that is, no change from the normal will be found in the knee-jerk 

 in about 1.5 of a second after the reinforcing act (Lombard 8 ). 



Many explanations have been offered of the mechanism involved in 

 this reinforcement. The most commonly accepted is that it is due to 

 the overflow of impulses from other parts of the nervous system, par- 

 ticularly the cerebrum, upon the reflex arc concerned in the knee-jerk. 

 During voluntary effort the cerebral impulses discharged down the spinal 

 cord pass not only to the neuron for which they are intended, but ir- 

 radiate or spread to other, even far distant, neurons, thus adding their 

 effect to that of the afferent impulse entering the cord locally. The suc- 

 ceeding inhibition may be assumed to be due to successive induction (see 

 page 824). It is difficult to offer direct experimental proof in support of 

 the explanation, but indirect evidence is furnished, in so far at least as 

 the augmentation is concerned, by the results of the experiments which 

 we have already described concerning the integration of allied reflexes 

 (page 822). To these might be added the well-known fact that the simul- 

 taneous application of two subliminal stimuli, one to the cerebral cortex 

 and the other to the skin of the corresponding body area, may call forth 

 a contraction of certain groups of muscles. 



AFFERENT SPINAL PATHWAYS 



The nature of the impulses transmitted by the various afferent path- 

 ways in the spinal cord. We have seen that the sensory impulses travel- 

 ing from the periphery to the spinal cord group themselves into three 



