224 



THE NERVOUS SYSTEM 



Motor nerve cell 

 of spinal cord 

 ritic branches 



s cylinder 

 Collateral branch 



Medullary sheath. 



but a means of protecting and strengthening the central 

 nerve fibres. In all nerves, the fibres form the central 

 line, or axis, of the cylindrical sheaths and are there- 

 fore known as axis cylinders. They reach the cells of 

 the various tissues of the body by branching into fine 



short filaments, one of 

 which ordinarily passes 

 into each cell. The fila- 

 ments in turn split up 

 into such fine and delicate 

 branches that it is difficult 

 to distinguish them from 

 the cell structure itself, 

 even with the most power- 

 ful microscope. Each axis 

 cylinder controls by means 

 of its branches the activity 

 of a small group of cells. 



Course of a motor nerve. 

 If we select the axis cyl- 

 inder of a small group 

 of cells in a muscle and 

 trace its course inward, 

 we find that it passes along 

 through its fatty sheath 

 in complete isolation from 

 the many accompanying 

 axis cylinders in their 

 sheaths, which are packed 

 with it in the nerve. It 

 finds its way along the 

 nerve of which it is a part up to the point where the 

 spinal nerve forms two roots to enter the spinal cord. 

 It then passes through the ventral root into the ventral 



Muscle 



FIG. 121 . Diagram of a motor nerve 

 cell from the anterior horn of the 

 spinal cord, and its branches con- 

 nected with a muscle (highly magni- 

 fied except as to length). 



