CHAPTER VII 

 THE CENTRAL NERVOUS SYSTEM 



SECTION I 







THE EVOLUTION AND SIGNIFICANCE OF THE 

 NERVOUS SYSTEM 



EVERY vital phenomenon may be regarded as a reaction con- 

 ditioned by some change in the environment of the animal and 

 adapted to its preservation. In the community of cells forming 

 the whole organism, the defence of any one part must involve the 

 co-operation of the whole community ; no change in a cell of the 

 body can be regarded as a matter of indifference to any of the other 

 cells. For this subordination of the activities of each part to the welfare 

 of the whole, as for the co-operation of all parts in maintaining the 

 welfare of each, a means of communication is necessary between the 

 various cells. For some of the lower functions the channel of com- 

 munication is the blood, which serves as a medium for carrying 

 food material from one part of the body to another, or for the 

 transmission of chemical messengers, which, elaborated by one set 

 of cells, may affect the metabolism of cells in distant parts of the 

 body. This method of correlating different activities would, 

 however, be too slow and clumsy for the quick adaptation of the 

 organism to sudden changes of environment. Such a rapid correlation 

 can be effected only by a propagation of some molecular change from 

 the seat of incidence of the stimulus either to all parts of the body, or 

 to some mechanism controlling all parts of the body. The medium 

 for the propagation of a state of excitation is furnished by the nervous 

 system. We have seen that stimuli of various kinds, involving such 

 various forces as thermal, mechanical, chemical, and electrical energy, 

 are transformed by a muscle or nerve fibre into what we call a state 

 of excitation, which is propagated along the fibres, whether nerve or 

 muscle, at a certain definite rate, its passage in the case of the muscle 

 being followed by a wave of contraction. 



In unicellular animals, such as the amoaba and vorticella, there 

 is no differentiation of any structure which can be regarded as 



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