148 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



functional activity at one part will influence the irritability of the 

 remainder either in the direction of reinforcement (Bahnung) or 

 inhibition. 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 are 

 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 anterior 

 crural 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 

 order for the quadriceps to respond it must be in a condition 

 of tonus. This tonus depends upon the reflex arc, the sensory 

 impulses from the muscle serving to keep it in that condition 

 of subdued contraction known as tone. On this view destruc- 

 tion of the reflex arc renders the muscle less irritable, so that it 

 will not respond by a contraction to the sudden mechanical exten- 

 sion or pull caused by the blow on the tendon. The adherents of 

 this view lay emphasis upon two facts: First, the knee-jerk is a 

 simple contraction, and not a tetanus, and, generally speaking, 

 the motor centers of the cord discharge a series of impulses when 

 stimulated. Second, the time for the jerk that is, the interval 

 between the stimulus and the response is too short for a reflex. 

 The determination of this time has been attempted by many ob- 

 servers for the purpose of deciding the controversy, but unfor- 

 tunately the results have been lacking in uniformity, varying, in 

 * Exner, " Archiv f. die gesammte Physiologic/' 27, 412, 1882. 



