CHAPTER VII 

 THE CENTRAL NERVOUS SYSTEM 



SECTION I 



THE EVOLUTION AND SIGNIFICANCE OF THE 

 NERVOUS SYSTEM 



EVERY vital phenomenon maybe regarded as a reaction conditioned 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 communication 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, 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 amceba and vorticella, there is no 

 differentiation of any structure which can be regarded as peculiarly nervous. 

 A stimulus applied to any part of the amoeba may evoke responsive activity 

 in all other parts. A slight touch applied to any point on a vorticella will 

 cause an excitation which is rapidly propagated to the stalk, causing this to 



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