262 



STUDIES IN PHYSIOLOGY 



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neu'ri-um (Greek peri = around + neuron = nerve.) l And 

 finally, each fiber consists of an axis cylinder wrapped up 



in the medullary and 

 primitive sheaths. 

 Now, the essential 

 part of every nerve 

 fiber is its axis cylin- 

 der, for this carries 

 the nerve impulse 

 from the cell body to 

 another cell with 

 which it is connected. 

 Hence a nerve trunk 

 may be compared to 

 a cable composed of 

 separate bundles of 

 telegraph wires, in 

 which each individual 

 wire (corresponding 

 to the axis cylinder 

 of a nerve) is sepa- 

 rated or insulated by 

 its covering from its 

 neighbor (Fig. 122). 



Structure of a Spinal 

 * | f -3 Ganglion. The dor- 

 1? "I, U ' sal root of every 

 & 8 spinal nerve has an 

 i - < jl enlargement, called 

 ^3 a spinal gan'gli-on 

 $ <=$ (Greek ganglion = a 



o swelling). These 



ganglia are very small 

 nerve centers, and consist largely of nerve cells. Many of 



i In much the same way the smaller bundles of muscle are sur- 

 rounded and held together by the connective tissue called perimysium. 





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