THE NERVOUS SYSTEM 49 



of circulation, of respiration, of propagation, all coor- 

 dinated and controlled by a correspondingly increased 

 mass of nervous tissue which finds its highest 

 development in a brain, wherein each kind of impulse 

 forms its own pattern, the stimulation of which results 

 in a specific reaction of the organism, these resultant 

 reactions varying from the simplest motor response 

 to the most complex process of "reason." 



The age-long process by which evolution develops 

 complex from simple mechanisms is strikingly illus- 

 trated by the embryology of the individual. In the 

 development of the adult from the ovum there is repro- 

 duced in brief the evolution of the higher organism 

 from the single cell. In the early human embryo 

 appears the first rudiment of a nervous system which 

 is of the same form as that found in larval Astidians, - 

 namely, a series of ganglion cells which coordinate re- 

 flexes for separate segments of the body. Together 

 with the simultaneous development of the whole 

 embryo there develops a spinal tube like that which is 

 found in the Amphioxus, the lowest of chor dates. 

 At a later period, a brain develops by the enlargement 

 of the anterior extremity of this spinal cord. At first 

 this brain is of simple structure, like the brain of the 

 lowest forms of fish. Gradually this simple brain 

 develops into the more highly differentiated human 

 brain passing consecutively through stages which 

 in general represent the brains of the lower animal 

 types through which man was evolved. (Fig. 4.) 



The evidence strongly supports the belief that it 



is by no quality exclusive to itself and alien to other 

 3rotoplasm that the nervous system performs its special 

 



