84 NERVOUS SYSTEM OF VERTEBRATES. 



may pass out over several nerves and give rise to coordinated 

 contractions of many muscles. In this case we may speak of a 

 complex reflex. A large number of the ordinary actions of the 

 body are carried out in this way without the intervention of the brain. 

 A large part of the brain also, even in man, may take part in such 

 reflexes without voluntary effort or effect in consciousness. When 

 the brain is involved, however, even in the frog or other lower animal, 

 the actions are likely to become so long continued, so complex 

 and varied that they are difficult to analyze into their simple 

 constituents and to describe as reflexes. Nevertheless in all these 

 cases essentially the same events are taking place. Receptive 

 cells when stimulated transmit impulses which, after passing 

 through a larger or smaller number of central cells, are sent out 

 by excitatory cells to provoke contraction of muscles. If we 

 speak of the cells through which an impulse is transmitted as the 

 path of the impulse, the path in this case instead of being simple 

 and direct is more complex, and the movements resulting may 

 be much more complex and the means by which the end is attained 

 more indirect than when the spinal cord alone is involved. 



It is evident that there is no limit to the extent to which this 

 conception of the reflex activity may be carried. So far as the 

 nervous activities are concerned they are always of the same 

 general type as that represented in the reflex. The term reflex 

 is properly used only when a stimulus, through the medium of 

 the nervous system, provokes a responsive action in the organism. 

 But, given a nerve impulse of any kind, aroused in any way, 

 the series of events in the nervous system is similar to that of a 

 reflex act. The impulse may pass from cell to cell, the motor 

 response may be inhibited, and the impulse traveling into the 

 sensory areas of the cerebral cortex, a sensation may result. This 

 may mark the beginning of a new series of events within the 

 cerebral cortex, associations, memory, thought processes. In 

 all cases we are dealing with the origination of nerve impulses 

 and their transmission over definite paths which may be anatom- 

 ically studied. It is these anatomical pathways, themselves 

 determined by experience, hereditary and individual, which 

 determine the course taken by nerve impulses and the responses 



