PROGRESS OF PSYCHOLOGY. 463 



Minute anatomy has made it possible to map out 

 many of the possible routes in the spinal cord and 

 brain which was no long time ago an un-mapped 

 country. But it is like a country in which, though 

 the roads are known, no passenger has ever been 

 seen, and where the possibilities of short-cuts across 

 the fields are endless. ^' One thing is quite certain, 

 namely, that the routes which are most frequently 

 used are the most open, and therefore the most easily 

 traversed." Measurements of the time taken by 

 nervous impulses in travelling from part to part of 

 the body make this clear. 



It is usual to call the possible path of a sensory 

 stimulus from, let us say, the finger to the spinal 

 or basal brain ganglia, and of a resulting motor 

 stimulus from the ganglion via motor nerve fibres 

 to the muscles, a complete arc. And what we have to 

 conceive of is that part of the impulse may be in many 

 cases diverted from the short arc and ascend to the 

 brain-cortex, there provoking impulses which de- 

 scending fibres carry back to the short arc. It is 

 in some such way that reflex actions may be strength- 

 ened or restrained by the control of the higher nerve 

 centres. 



The familiar " knee-jerk " is a good example of 

 a pure reflex, occurring in sleep, in the hypnotic 

 state, in unconsciousness, — not much of an action, 

 indeed, but enough to link us back physiologically to 

 the jelly-fish with its pulsating disc. From this 

 simple reflex, with consciousness at zero as far as it 

 is concerned, we can make a long inclined plane on 

 which are arranged more complex reflexes, compound 

 reflexes, reflexes which are apt to arouse conscious- 

 ness, and reflexes which are very liable to be in- 

 fluenced by conscious control. 



