THE CENTRAL NERVOUS SYSTEM. 63 



which are brought about by impulses originating in the deeper tissues 

 of the body are often called "deep" reflexes. One of the most 

 important is the knee jerk, which consists in contraction of the extensor 



FIG. 15. Reflexes produced by summation ot weak stimuli. (Sherrington. 



Starling's Principles of Physiology. ) 

 A, Reflex contraction of flexor muscles of knee. B, Reflex inhibition of extensor 



muscle. In each case the effect follows the sixth stimulus, the stimuli being applied 



to the central end of the internal saphenous nerve. 



muscles of the thigh in response to a sharp tap on the patellar tendon. 

 The stretching of the extensor muscles by this tap gives rise to a 

 stimulus in sensory structures (muscle-spindles) within the muscle 

 itself; from these structures the impulse passes to the spinal cord 

 along the afferent nerve fibres of the muscle. 



