THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 



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exclusively with that of the mammal, and more particularly with that of man. 

 In man we are so accustomed to think of the nervous system as the organ and 

 agent of the mind that its true physiologic position is often forgotten. In this 

 introductory chapter we have attempted to show that the primary function of 

 the nervous system is to receive stimuli arising from changes in the environment 

 or within the organism, and to transmit these to effectors which bring about 

 the adjustments necessary for life. Biologically speaking, the nervous system 

 is not to be regarded as an intelligence bureau, which gathers information for 



CO- 



Fig. 5. Peripheral sensory neurons of various animals: A, Oligochaetic worms (Lumbricus); 

 B, polychaetic worms (Nereis); C, molluscs (Limax); D, vertebrates. The figure illustrates the 

 gradual change in the position of the sensory cells in the phylogenetic series: e, Epithelial cells of 

 sensory surface; c, cuticula; sz, cell-body of peripheral sensory neuron; rm, rete Malpighii of epi- 

 dermis; sn, axon; co, central nervous system. (Barker, Retzius.) 



a sovereign mind, enthroned within the brain, nor yet as a chief executive officer 

 to carry out that sovereign's decrees. Sensory impulses from many sources 

 reach the brain, where they pass back and forth through a multitude of asso- 

 ciation paths, augmenting or inhibiting each other before they finally break 

 through into motor paths. Previous experience of the individual, having left 

 its trace in the organization of the central nervous system, alters the character 

 of the present reactions. It is in connection with the neural activity involved 

 in these complex associational processes that consciousness appears shall I 

 say as a by-product? at least as a parallel phenomenon. 



