DISTEIBtlTION OP NETJEA 409 



hold true for all classes. In connection with the third 

 law of action it may be noted that whenever there is a 

 passage of impulse from one neuron to another there is at 

 this point a delay in the transmission, and sometimes 

 several impulses must reach a cell before it can overcome 

 this resistance between dendrites and axon contacts and 

 discharge into the next cell. We may compare this delay 

 to the effect of placing a small barrier in the path of a 

 stream. The effect is not to stop the stream, but to check 

 it until the volume of water is great enough to push aside 

 the barrier. The normal rate of nerve discharge is about 

 ten impulses per second. 



It win be noted finally that the above laws give no 

 explanation of the manner in which nerve impulses origi- 

 nate. The manner in which externally stimulated im- 

 pulses originate will be considered in our study of the 

 sensory end organs. The manner in which impulses 

 originate in the cell body is still unknown. It is believed, 

 however, that just as oxidation of fuel and metabolism 

 account for the contraction of muscle protoplasm, so these 

 nerve impulses set up in the nerve cells result from certain 

 chemical changes in the composition of the cell proto- 

 plasm. The fact that nerve cells require food and become 

 fatigued by overwork justifies this belief, but the exact 

 nature of the metabolic changes involved is unknown. 



Distribution of neura. — In the central nervous system, 

 the cell bodies of the neura which originate there are 

 found mainly in the gray matter of the cord and in the 

 cortex of the brain. The axones of these cells may 

 extend far beyond the limits of this system, or again they 

 may be confined within the central system. The spinal 

 p,nd cranial nerves represent bundles of axones which 



