CHAPTER XVI. 



THE NERVOUS SYSTEM. 



THE nervous system is the seat and apparatus of sensation, 

 consciousness, coordination, and volition, and its elements are 

 distributed throughout all parts of the body. Nervous tissues 

 are of epiblastic origin and constitute a class of tissues sui 

 generis. 



Division : The nervous system can for practical convenience 

 be divided into the nerve-centres, nerve-trunks, and distal 

 nerve-terminals ; and the tissue-elements corresponding to 

 these respectively are nerve-cells, nerve-fibres, and nerve- 

 terminals. 



The cells and centres are concerned with the distribution 

 and storage of nervous energy; the fibres conduct nervous im- 

 pulses; and the terminals are connected with the transfor- 

 mation of nervous force into other forms of energy, and vice 

 versa. 



According to present prevailing conceptions, the structural 

 and functional units of the nervous system are elements called 

 neurons, and the subject is here presented from this stand- 

 point. A neuron is a single nerve-cell with all its parts and 

 processes. The nerve-fibres are regarded as greatly elongated 

 processes of nerve-cells, so that a neuron would include the 

 body of the nerve-cell, its nearby processes, the nerve-fibre 

 connected with it, perhaps extending a long distance to the 

 periphery of the body, and the peripheral terminations of the 

 nerve-fibre. A neuron therefore simply represents a single 

 complete nerve-cell with all its parts. 



Nerve -cells. 



Nerve-cells (or ganglion-cells) : In the full sense of the term 

 " cell," a nerve-cell is the same as a neuron ; but as ordinarily 



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