166 BIOLOGY: GENERAL AND MEDICAL 



the brain of a batrachian, though it cannot be followed 

 by any regeneration, does not prevent the animal from 

 living until it starves to death, but the destruction of the 

 brain of a mammal is commonly followed by almost in- 

 stant death. As it becomes one of the most complex of 

 organs, it becomes one of the most indispensable. 



The complexity of structure depends upon a vast in- 

 crease in the number of nerve cells, the separation of 

 many of these into groups possessing specific functions, 

 and the formation of connecting paths or tracts, through 

 which the communicating neuraxes pass in orderly ar- 

 rangement from cell-group to cell-group. The resulting 

 localization of cerebral function in these cell-groups and 

 the intercommunication of the many cells in the groups 

 with one another and with the cells in other groups 

 through neuraxes and dendrites affords an explanation of 

 the greater plasticity of behavior in higher animals as con- 

 trasted with the lower. 



Thus, if a worm, with a very simple nervous system 

 composed of little more than simple reflex circuits, be 

 touched, the sensory receptors in its cuticle transmit to 

 the controlling center an impulse that is passed on to 

 several muscle fibres, which contract, causing the worm 

 to wriggle. In all probability no greater response is pos- 

 sible because the simplicity of the nervous communication 

 makes no provision for it. 



If the general structural complexity of an animal en- 

 ables it to perform more complicated movements, the 

 nervous system usually shows a corresponding complexity, 

 with the result that the reflexes involve a more complex 

 and coordinated response. This is well exemplified by 

 the following experiment upon a frog. The head of the 

 animal is cut off or its brain entirely destroyed. A small 

 piece of blotting-paper moistened with acetic acid is 

 placed upon the inner surface of one hind leg which is 

 held to prevent the simple reflex of withdrawal. In 

 many cases, after a brief interval, the animal brushes the 

 paper away with the opposite foot. Here is a very com- 



