THE CENTRAL NERVOUS SYSTEM. 



59 



nerves and by the latent period of the effector organ. When this is 

 deducted from the total reflex time, a period remains, known a,s_reduced 

 rejlegjAwe, which is occupied by the passage of the impulse through 

 the neurons in the central nervous system. Its duration varies with 

 the complexity of the reflex action, and is taken up mainly in the 

 passage of the impulse across the synapse between the axon of one 

 neuron and the dendrites of the next neuron, the synapse offering a 

 certain resistance to this passage. 



It has been found in the frog, for example, that when one leg moves 

 in response to a stimulus applied to that leg, the reduced reflex time is 

 rather less than one-hundredth of a second. The reduced reflex time 



FIG. 12. Diagram of a reflex arc. 

 a, receptor ; 6, intermediate neuron ; c. muscle fibre. 



for reflex winking of the eye in man has been estimated at about 

 one-twentieth of a second. 



The time required for the production of a reflex benonf]^ less with 

 increasing strength of the stimulus, and becomes longer when the 

 spinal cord is fatigued or is under the influence of drugs, such as 

 chloroform. Increase in the strength of the stimulus may also increase 

 the extent of the reflex response. If the strength of stimulus necessary 

 to elicit a particular reflex is determined, and the experiment is 

 repeated with a stronger stimulus, there may be (1) an increase in the 

 strength of the original response, and (2) additional muscles may also 

 be thrown into contraction. In other words, with a stronger stimulus 

 there is a spread of the excitation in the grey matter of the spinal cord, 

 whereby additional reflex arcs become involved ; this spreading of the 

 excitation is known as irradiation. 



When, for example, a harmful (nociceptive) stimulus, such as a 

 sharp prick, is applied to the sole of the foot in the spinal dog, the leg 

 is flexed and withdrawn from the stimulus. When the strength of 



