44 



THE NERVOUS SYSTEM 



angles to the parent stem. The axon 

 terminates in a multitude of fine branches 

 usually at a considerable distance and 

 sometimes as much as a meter from its 

 origin. The origin of the axon from the 

 perikaryon is marked by an expansion 

 known as the cone of origin or im- 

 plantation cone. This cone, like the 

 axon, differs somewhat in structure from 

 the perikaryon. Such long axons as 

 have just been described are character- 

 istic of the cells of Golgi's Type I. 



That not all axons are long and 

 relatively unbranched is seen from Fig. 

 24, which illustrates a cell of Golgi's 

 Type II. The axons of these cells are 

 short, branch repeatedly, and end in the 

 neighborhood of the cell body. 



Another good example is furnished 

 by the primary motor neurons. Figure 

 25 illustrates such a cell from the anterior 

 gray column of the spinal cord. This 

 is a large nerve-cell with many rather 

 long branching dendrites and an axon, 

 which forms the axis-cylinder of a motor 

 nerve-fiber and terminates by forming 

 a motor ending in a muscle. As illus- 

 trated in this figure, long axons tend to 

 acquire myelin sheaths, and those 

 which run in the cerebrospinal nerves 

 are also covered by a nucleated mem- 

 branous sheath the neurilemma. 



Nerve-cells with many processes, 

 such as have just been described, are 

 called multipolar. Examples of unipo- 

 lar and bipolar cells are furnished by the 

 cerebrospinal ganglia (Fig. 40). These cells, which will be described in more 



Fig. 23. A pyramidal cell from the cere- 

 bral cortex of a mouse : a, Dendrites from the 

 base of the cell; b, white substance of the 

 hemisphere into which the axon, e, can be 

 traced; c, collaterals from the first part of the 

 axon; /, apical dendrite; p, its terminal 

 branches near the surface of the cortex. 

 Golgi method. (Cajal.) 



