CHAPTER XXV 

 THE NERVOUS SYSTEM (Continued) 



C. THE CONDUCTION PATHS OF THE CENTRAL NERVOUS SYSTEM 



THE functions of the central nervous system, as well as the in- 

 terpretation of its lesions, all center around its capacity for the 

 transmission of nerve impulses, the process of conduction. Hence 

 the importance of tracing with certainty the histological paths by 

 which these impulses are conveyed. 



These paths are divisible into three general classes : 



1. Centrifugal or " motor" tracts. 



2. Centripetal or "sensory" tracts. 



3. Association tracts. 



The centrifugal tracts are so arranged as to conduct nerve im- 

 pulses in a direction from the cerebral cortex toward the peripheral 

 nerve endings ; the centripetal conduct from the periphery toward 

 the cerebral cortex ; while the association paths connect not only 

 the opposite sides of the central organs by commissural fibres, but 

 pass between different levels of the spinal cord and brain and may 

 even unite very remote parts e. g., the nuclei of the midbrain and 

 the ventral horns of the spinal cord. 



A. The Motor Paths 



The centrifugal or motor paths begin in the motor area of the 

 cerebral cortex and include the neuraxes of all those pyramidal 

 nerve cells which occur in the grey matter of this area. These 

 neuraxes penetrate the deeper layers of the cortex and as projection 

 fibres enter the corona radiata of the cerebral medulla. Here, they 

 converge toward the knee of the internal capsule, which they enter 

 in a compact bundle. They then pass beneath the optic thalamus 

 to enter the crusta of the midbrain as the large pyramidal bundles. 



In the midbrain and pons the pyramidal tracts coming from the 

 two cerebral hemispheres converge toward the median line. In 

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