NERVOUS SYSTEM. 



45 



fissures. In the later studies each of these columns has been 

 subdivided (Fig. 46). 



During the increase in size of the cord, the cells produce pro- 

 toplasmic outgrowths (axis cylinders, p. 11), some of which run 

 forwards and backwards in the ventral and lateral columns, 

 while others pass outwards from the cord as the motor roots of 



Dorsal 

 Root 



FIG. 46. Diagrammatic section of spinal cord, after von Lenhossek. Ant. 

 pyr., anterior pyramid tract; apr, anteroposterior reflex fibre; c, collaterals enter- 

 ing the gray substance ; dp, collateral of the lateral pyramidal tract; GC, Golgi's 

 commissural cell ; GP, Golgi's cell of posterior horn ; Let, lateral cerebellar tract ; 

 111, motor cell of anterior horn ; ptc, posterior tract cell. 



the spinal nerves, to be described a moment later. From this 

 it will be seen that the white matter is composed of nerve 

 fibres, the gray matter of nerve cells. Later the dendrites 

 association fibres are formed, while with increase in size blood- 

 vessels and supporting-tissue press into the cord. 



In its early stages the spinal cord shows a marked segmen- 

 tation or repetition of parts one after the other. This metamer- 



