CHAPTER V 

 ADJUSTMENT THE NERVOUS SYSTEM 



IN its simplest aspect, the central nervous system may be said to 

 be concerned with the adjustment of the organism to external 

 changes. We have seen how these outer changes are enabled, by 

 appropriate receptors, to impress themselves and how the appro- 

 priate responses are made by muscular movements. We have now 

 to try to understand something of the way in which the connec- 

 tion between them is made. In the physiological discussion of the 

 functions of the nervous system, we are not concerned with the 

 fact that the activity of the highest parts of the brain is associated 

 with what we call the mind, with conscious knowledge of their 

 activity. By the " highest " parts of the brain we mean those parts 

 which are the most removed in anatomical relationship to incom- 

 ing stimuli from that which we know to be the seat of the 

 simplest reflex movements and to be devoid of consciousness, 

 namely, the spinal cord (E., p. 215). It may be remarked here that 

 the parts in question, the cerebral hemispheres, are developed in 

 relation more especially to what we have recognised as the distance- 

 receptors, the eye and the ear ; and it may be noted that these are 

 the receptors, together with the hand, chiefly concerned with the 

 development of speech and the use of written language, without 

 which intellectual growth would have been impossible. It need 

 scarcely be said that in animals of high mental development, a 

 large number of processes and much lapse of time may intervene 

 between the reception of a message and the execution of the 

 response appropriate to it. 



When an impulse arrives in the nerve centre along a fibre from 

 a receptor organ, what happens to it? We find, by histological 

 examination (E., p. 216), that the fibre divides, and that its 

 branches are connected to a cell containing a nucleus, usually to 

 fibres proceeding from the cell.- In the simplest conceivable case, 

 this cell is the " motor centre" of some particular muscle That 

 is, the nerve fibre given off by it passes to a muscle, and when set 

 into action causes contraction of that muscle. This is the most 

 elementary form of "reflex action," and is rarely met with. It may 

 be represented by the parts H and n of the diagram in Fig. 6. 



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