CHAPTER XVII. 

 THE NERVOUS SYSTEM. 



BY OLIVER S. STRONG. 



GENERAL CONSIDERATIONS. 



There are certain features of the nervous system in general and particularly 

 of the vertebrate nervous system, the comprehension of which makes the 

 processes of development of the nervous system in man more intelligible. 

 First, the nervous systems of the lower Vertebrates are in many respects 

 simpler than those of higher forms and their variations throw light upon the 

 causes which determine neural structures. Second, as the nervous systems of 

 all Vertebrates develop from the same germ plasm, there are resemblances 

 between certain features of both the embryonic and adult systems of lower 

 vertebrates and certain developmental stages in the higher. Certain struc- 

 tures met with in lower adult forms may be regarded as representing stages 

 of arrested development although specialized and aberrant in many respects 

 of structures found in higher forms. Vestigial structures in the developing 

 nervous systems of higher forms may be regarded as recurring developmental 

 necessities in the attainment of the adult form. 



Stated in the most general terms, coordination of bodily activities in response 

 to both external and internal conditions is the biological significance of the 

 nervous system. This implies a transmission of some form of change from one 

 part to another or, in other words, conduction. This functional necessity is 

 shown structurally in the elongated form of the histological elements of the 

 nervous system. That such changes habitually pass along each element or 

 neurone in some one direction seems to find a natural structural expression in 

 the receptive body and dendrites of the neurone, and in its long transmitting 

 axone. 



It is also evident that coordination can only be performed by a transmission 

 of a change from some given structure either back to that structure or to some 

 other structure to cause a responsive change. We thus have not only in the 

 vertebrate, but at a very early stage in the invertebrate nervous system, a dif- 

 ferentiation into afferent and efferent components, the two together usually 

 being termed the peripheral nervous system. The histological elements of these 

 components are the afferent and efferent peripheral neurones. All structures 

 which are so affected as to transmit the change to the afferent peripheral neu- 



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