SKELETAL MUSCLE AND THE NERVOUS SYSTEM 89 



conception that they act as transmitters. But it is 

 possible that they reinforce the energy which they send 

 forward. One may very crudely liken a muscle to a 

 torpedo or mine and its nerve-supply to the electric 

 wires provided for ignition. The nerve cells then figure 

 as batteries to furnish the current. Batteries are 

 gradually exhausted in the production of currents while 

 wires are hardly affected by carrying them. It has 

 been said that nerve fibers scarcely give evidence of 

 fatigue. Nerve cells are believed to suffer some im- 

 pairment when long in use. 



If impulses proceed from nerve cells along motor 

 fibers to the end-plates in skeletal muscle and, upon 

 their arrival, start the contractile process we may be 

 led next to ask: What significance have the dendrites? 

 It is held that they are receptive in their nature, that 

 through them the nerve cells are wrought upon by stimuli. 

 The ordinary motor cell has many dendrites and a 

 single axon many avenues of approach and only one 

 channel of expression. One is reminded of the teaching 

 familiar to childhood that we have each two eyes, two 

 ears, and only one mouth. A nerve cell with its axon 

 and dendrites constitutes a neuron. 



Nutritive Functions of Nerve Cells. Whether or 

 not the nerve cells reinforce the impulses which they 

 transmit they have another function which we shall do 

 well to emphasize at this time. They are responsible 

 for the maintenance in normal condition of the axons 

 which spring from them and this is true no matter how 

 long these axons may be. It is a general truth that 

 parts of cells separated from the nucleated portion do 

 not long survive. If we regard a neuron as a cell, the 

 impossibility of preserving the axon when it is cut off 

 from the cell-body appears merely to be a special cas^e 

 of this dependence. 



When a nerve is severed a degeneration of the fibers 

 follows and it is found to accord with the following rule. 

 Those portions of the fibers left in connection with their 



