REFLEXES 105 



The reflex principle is recognized in the brain as well 

 as in the cord and, in our own case, reflexes that are 

 purely regulated by the cord are not usual. 



Association Units. A diagram modified to suggest 

 the path followed by impulses when the brain figures 

 in reflex action should show more than two orders of units. 

 Thus far we have pictured the afferent element as bring- 

 ing it's influence to bear directly upon the efferent. It 

 is not likely, even in the cord, that this simple relation- 

 ship is the common one. Intermediate nerve cells are 

 ordinarily concerned in the transmission. If this is 

 so, then, as a rule, more than one order of synapses 

 will lie in the path. The intermediate cells, which are 

 not dearly afferent or efferent, have been called ad- 

 justors or association units. As more and more of these 

 are introduced we have more tendency toward varia- 

 tion in reflex action and we find that we are moving 

 from a lower to a higher type of nervous system. 



Reflex Time. We can measure with great accuracy 

 the time that elapses between the giving of a stimulus, 

 such as an electric shock, and the beginning of a reflex 

 movement in response. The trial is often made for 

 the wink. The shock is applied to the eyelid and the 

 twitch that follows makes a written record. By means 

 of the tuning fork the interval can be estimated. It 

 is likely to be about 0.05 second. Many reflexes are 

 slower, few, if any, are faster than this. The course 

 run by the nerve-impulses from the eyelid to the lower 

 part of the brain and back to the muscles is a short one; 

 most of the time seems to be consumed at the synapses 

 and in the nerve cells rather than in transit. 



If the wink following an electric shock is not in- 

 voluntary but made "on purpose," as we say, the delay 

 is something like three times as great as with the pure 

 reflex say 0.15 second. It is then what we call a 

 voluntary reaction time. The impulses must have gone 

 over much longer and more interrupted routes. 



