THE ORIGIN AND SIGNIFICANCE OF THE PRLMITIVE 

 NERVOUS SYSTEM. 



By G. H. PARKER. 

 {Read April 21, 1911.) 



Linnaeus defined a plant as an organized, living, but non-sentient 

 body and an animal as an organized, living, and sentient body. 

 Although no modern biologist would attempt to support the conten- 

 tion that animals are sentient and plants are not, the distinction 

 drawn by Linnaeus is not without a certain foundation in truth, for 

 sentience in its full development and as Linnaeus probably under- 

 stood it, is the exclusive and supreme possession of the higher ani- 

 mals. That these animals possess intelligence as contrasted with 

 all other natural bodies is a statement to which few naturalists will 

 offer any serious objection. The seat of this intelligence is the ner- 

 vous system and, though the integrity of the other systems of organs 

 is essential in most cases to the well-being of the animal body, the 

 fact that the totality of activities that makes up the mental life of 

 human beings as well as that of other animals, is absolutely depen- 

 dent upon the nervous system, is evidence sufficient of the paramount 

 importance of these organs. It is, therefore, not without interest 

 to inquire into the origin of this system of organs and to trace the 

 early steps by which it passed from a position of initial obscurity to 

 one of the highest significance. 



The nervous system of the higher animals, though enormously 

 complex in its organization, is composed of relatively simple ele- 

 ments, the neurones, arranged upon a comparatively uniform plan. 

 This plan is well exemplified in the spinal cord of the vertebrates. 

 In this organ the sensory neurones, whose cell-bodies lie in the 

 dorsal ganglia, extend from the integument through the dorsal roots 

 to the gray matter of the cord. Motor neurones, whose cell-bodies 

 are situated within the gray matter of the cord, reach from this 

 region to the muscle-fibers which they control. These two classes 



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