STRUCTURE AND FUNCTIONS OF THE CEREBRUM 151 



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 

 knowledge, which can be gained only through the receptor chan- 

 nels 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. 



The neurons of the nervous system generally act, in the main, 

 as conductors pure and simple. When they are stimulated nerve 

 impulses are aroused; these spread over them and escape by 

 those synapses whose resistance is not too high ; thus other neurons 

 are involved and so the impulses advance to a motor termination. 



The cortical neurons of the cerebrum owe their dominant po- 

 sition in the nervous system chiefly to a peculiar ability which 

 they possess of "holding up" impulses which come to them, re- 

 taining them indefinitely, and giving them out again in the future. 

 This storing of impulses constitutes memory. The "reflex" 

 animal, because he is deprived of this property, must always re- 

 spond immediately to adequate stimulation; the intact animal 

 may respond immediately or may retain the stimulus as a memory 

 to modify his future activities. Since the intact animal has 

 within his cerebrum a store of impulses "held in leash," he may 

 at any time become active through the liberation of some of them, 

 without immediate external stimulation. 



