THE NERVOUS SYSTEM 247 



makes an older member superior to a younger one in his 

 powers of adjustment and maintenance. This is, of course, 

 exemplified in the fullest degree by human beings. The 

 prolonged period of growth and progress must indicate 

 a plasticity in the arrangements of the nervous elements 

 that is hardly ever entirely outlived. 



Descartes in the seventeenth century grasped the two 

 properties with his usual keenness. He pictured the reflex 

 process with quaint symbolism, but with essential correct- 

 ness. He likened a path of afferent transmission to a cord 

 that is pulled. A valve is opened in the central organ and 

 a fluid released to flow back along the nerve. Reaching 

 a group of muscles this fluid wakes them to action. Des- 

 cartes had used different analogies for the afferent and the 

 efferent impulses, making one a mechanical twitch and the 

 other a spurt of liquid; we now believe them to be nearly 

 or quite identical in character. The same writer recog- 

 nized the ability of the higher nervous systems to retain 

 impressions, and likened the registration of a memory to 

 the imprint of a seal upon wax. 



Physiologically, the particular mark of the highly de- 

 veloped nervous axis is this capacity for recording indi- 

 vidual and not merely racial experience. Anatomically, 

 the advance in organization is signalized by an increasing 

 predominance of the brain over the cord and of the 

 "f ore-brain" over the parts behind. A frog will carry out 

 many complex reactions when the entire brain has been 

 destroyed. That is to say that in the frog the cord is ade- 

 quate for a great deal of reflex action. In a human para- 

 lytic whose injury has removed the lower half of the spinal 

 cord from functional union with the brain, the reflexes 

 which can be obtained from the lower extremities are few 

 and slight. In other words, the cord in man has become 

 less important as an organ for correlating afferent with 

 efferent impulses and more important as the largest of all 

 nerves, the highway to and from the brain. We shall now 

 set forth in some detail certain of the adaptive changes 

 which are constantly occurring through the .mediation of 



