82 HUMAN PHYSIOLOGY 



imparted is known as the motor end-plate. It is the junc- 

 tion of a nerve fiber with a muscle fiber. 



From what has been said it will appear that every 

 muscle fiber is reached by a nerve fiber. These nerve 

 fibers, individually too small to be visible, come to a 

 given muscle in its motor nerve which contains so many 

 of them that it is a cord of some size. We must not con- 

 clude that a muscle having 100,000 fibers will receive an 

 equal number of nerve fibers from without. The nerve 

 fibers branch freely before uniting with the contractile 

 units and the result is that from 10 to 100 muscle fibers 

 may be governed through a single fiber such as exists in 

 the nerve outside the muscle. If this were not so, 

 nerves would be much larger than they are in reality. 



Nerve Fibers. A nerve fiber is more slender than a 

 muscle fiber and it is likely to be a great deal longer. 



FIG. 19. Two nerve fibers shown diagrammatically especially to em- 

 phasize the continuity of the axon. A detail, further enlarged, to suggest 

 fibrils in the axon. 



Moreover it is a compound affair and cannot be regarded 

 as a modified cell. At first glance and ever after it 

 suggests an insulated wire. There is a central core, the 

 axon, and there is no doubt that here, as with the wire, 

 it is this internal core which is the essential conductor. 

 Around the axon there is a sheath of fatty substance. 

 External to this again there is an extremely thin outer 

 sheath, the neurilemma. The axon is continuous through- 

 out the longest fiber but the fatty sheath is a jointed 

 structure with interruptions of which there are about 

 twenty-five to the inch. These interruptions are known 

 , as Nodes of Ranvier. 



