CHAPTER XIX 

 THE NERVE SYSTEM 



CEREBRO-SPINAL AND SYMPATHETIC DIVISIONS, 

 NERVE TISSUES AND THE SPINAL CORD 



The preceding chapters have been devoted to the study of 

 many organs grouped into systems for different purposes. Of 

 some we can say that their functions are exercised consciously 

 and under voluntary control; others are so exercised to a partial 

 extent only ; as the muscles of the extremities and those of respira- 

 tion. Still others are absolutely beyond our control as the heart, 

 the stomach and intestine, and others. We are, therefore, pre- 

 pared to find in the nerve centers of the body, a wonderful plan 

 for providing nerve force that shall stimulate the activities of 

 these widely differing organs, and at the same time bring them 

 into one harmonious whole. 



The body functions are classified as voluntary and involuntary, 

 so the nerve system is arranged in two divisions belonging 

 respectively to voluntary and involuntary processes, the first 

 being called the cerebro- spinal division; the second, the sympathetic 

 division. 



NERVE TISSUES 



The foundation cells of which nerve tissues are composed are 

 microscopic in size and called neurons. A neuron consists of a 

 nucleated cell body, an axon, and terminal divisions. 



The cell body has short branches called dendrites, one of which 

 (sometimes two) grows longer to form the axon or axis cylinder 

 which becomes a nerve fiber. 



Note. The term nerve cell is often used to signify the cell body of a 

 neuron. 



When the axon is invested with a sheath, or medulla, it is a 

 medullated nerve fiber, and such are found in voluntary muscles 

 and all sensitive parts of the body. Axons without sheaths are 



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