2I 4 



OUR BODIES AND HOW WE LIVE 



FIG. 133. Diagram of a 

 Neuron, or Nerve Unit. 



Showing a motor cell with its 

 long, unbranched process 

 (with two little lateral off- 

 shoots), with motor endings 

 in striated, voluntary mus- 

 cular tissue. 



288. Structure of the Nerves. If 

 we take a small piece of a nerve from 

 a dead rabbit or frog, and with needles 

 separate it lengthwise on a glass 

 slide, we find it can be pulled apart 

 into bundles of silky threads. If 

 these threads are frayed out as finely 

 as possible, the high power of the 

 microscope reveals still smaller 

 threads or fibers. It would take 

 about four thousand average-sized 

 nerve fibers to cover an inch when 

 placed side by side. 



The nerve fibers bound together 

 in cords of various sizes form the 

 nerves. 



289. General Arrangement of the 

 Nervous System. The nervous sys- 

 tem consists of two great sets of 

 nerves and nerve centers which are 

 intimately related, and yet for con- 

 venience may be studied apart. 



These are the cerebro-spinal system 

 and the sympathetic system. 



The cerebro-spinal, or central nerv- 

 ous, system consists of the brain and 

 the spinal cord, together with the nerves 

 which branch off from each. 



The sympathetic system consists 

 chiefly of a double chain of ganglia, 

 or knots of nerve cells, lying at 

 the sides and in front of the spinal 



