180 THE HUMAN BODY 



pyramidal neuron to the muscle. It is doubtful whether any 

 cortical reflex arcs are actually composed of as few neurons as 

 five; probably the simplest ones contain several additional associa- 

 tion neurons within the cerebrum. 



Cortical Reflexes Compared with Spinal Reflexes. As an ex- 

 ample of a simple spinal reflex was cited the involuntary with- 

 drawal of the hand from accidental contact with a hot body. 

 To illustrate 'a simple cortical reflex suppose that my finger rests 

 upon the terminals of an apparatus for generating electric shocks; 

 I am told that when I feel the shock I must withdraw my hand. 

 The shock may be so feeble as to be barely perceptible. Under 

 such circumstances the withdrawal must be voluntary and the re- 

 sponse, therefore, must involve the cerebrum. The chief objective 

 difference between voluntary withdrawal of the hand in response 

 to feeble stimulation, and its involuntary retraction in response 

 to strongly painful stimulation is that the former reaction requires 

 a noticeabty longer time than does the latter. The only simple 

 reflex whose time has been satisfactorily measured in man is 

 the winking reflex; this requires about 0.06 second for its com- 

 pletion. The quickest cortical reflexes take about 0.15 second. 

 This difference in time is much greater than can be accounted for 

 by supposing the cortical reflex to involve a greater length of 

 nerve-fibers, and therefore must be due to the fact that the cor- 

 tical reflex involves a greater number of neurons and consequently 

 more synapses to be crossed. 



An additional difference which we recognize subjectively be- 

 tween spinal and cortical reflexes is that while the former are 

 involuntary and unconscious, the latter are voluntary responses 

 to stimuli consciously perceived. This difference will be discussed 

 more fully in a later paragraph, when the meaning of the terms 

 " voluntary" and "consciously" shall have been considered. 



Memory. We have seen that the primary function of the 

 cerebrum is to .introduce remote considerations as determining 

 factors in the responses of the organisms. We have seen also that 

 in order to do this the cerebrum must have an equipment of knowl- 

 edge, which can be gained only through the receptor channels of 

 the Body. The information which reaches the brain, to be of 

 service, must be retained there until needed, and must be held in 

 such a way as to be available when required. 



