REFLEX ACTIONS. 155 



■effect began to appear when the reinforcing act (hand-squeeze) 

 preceded the blow by an interval of from 0.22 to 0.6 sec, and the 

 maximum inhibiting effect was obtained at an interval of from 

 0.6 to 0.9 sec. Beyond this point the effect became less noticeable, 

 and at an interval of 1.7 to 2.5 sec. the reinforcing act had no 

 influence at all upon the jerk. These relations are shown in the 

 accompanying curve (Fig. 70). These authors confirmed also the 

 fact that a sensory stimulus, such as a gentle blast of air on the 

 conjunctiva or the knee, may reinforce the jerk. The physio- 

 logical explanation of the reinforcement, negative and positive, is 

 a matter of inference only, but the view usually held is that it 

 is due to "overflow." That is, many facts, such as strychnin 

 tetanus, indicate that the neuromuscular machinery of the entire 

 central nervous system is more or less directly connected and that 

 functional activity at one part may influence the irritability of the 

 remainder, either in the direction of reinforcement or inhibi- 

 tion. We may conceive, therefore, that when the hands are 

 .squeezed, the motor impulses sent down from the cortex of the 

 brain to the upper portion of the cord overflow to some extent, 

 sufficient at least to alter the irritability of the other motor neurons 

 in the cord. Experimental stimulation of the cortex has given 

 similar results. Exner* found that when the motor center for the 

 foot in the cortex of a rabbit was stimulated, the stimulation, even 

 if too weak to be effective itself, caused an increase in the contraction 

 "brought about reflexly by a simultaneous stimulation of the skin 

 of the paw, and furthermore if these stimuli were so reduced in 

 strength that each was ineffective, then when applied together a 

 •contraction was obtained. In this case an ineffective stimulus 

 from the cortex reaching the spinal cord increased the irritability 

 of the motor centers there so that a simultaneous reflex stimulus 

 from the foot, ineffective in itself, became effective. 



Is the Knee-jerk a Reflex? — The most interesting question 

 in this connection is whether the jerk is a true reflex act or is due 

 to a direct mechanical stimulation of the muscle. Opinions have 

 •been divided upon this point. Those who believe that the jerk is a 

 reflex lay emphasis upon the undoubted fact that the integrity of 

 the reflex arc is absolutely essential to the response. The quad- 

 riceps receives its motor and sensory fibers through the femoral 

 -nerve, and pathological lesions upon man as well as direct 

 experimental investigation upon monkeys prove that if either the 

 posterior or anterior roots of the third and fourth lumbar spinal 

 nerves are destroyed the knee-jerk disappears entirely. The oppo- 

 nents of the reflex view explain this fact by the theory that in 

 ©rder for the quadriceps to respond it must be in a condition 

 * Exner, "Archiv f. die gesammte Physiologie," 27, 412, 1882. 



