CHAPTER VII. 

 TRACING OF IMPULSES. 



Having studied the grouping and chaining together of neu- 

 rones, let us now make the knowledge practical by tracing impul- 

 ses through the better known paths formed by these various neu- 

 rone groups. The paths thus formed are of three kinds, namely: 

 I. Efferent, or motor. II. Afferent, or sensory general and spe- 

 cial sense. III. Reflex. 



I. EFFERENT, OR MOTOR PATHS. 



The CEREBRO-SPINAL OR PYRAMIDAL PATHS (Fig. 105) 

 are direct, as they do not pass through the cerebellum. Their im- 

 pulses ultimately run either through the spinal or the cerebral nerves, 

 and are both motor and inhibitory. Hence the increased reflexes 

 and spastic contractions of lateral sclerosis in which these tracts 

 are diseased. 



i. Through the Spinal Nerves (Fig. 105). Starting in the 

 upper three-fourths of the gyrus centralis anterior of the cerebral 

 cortex, motor and inhibitory impulses run down through the corona 

 radiata, the anterior two-thirds of the occipital part of the inter- 

 nal capsule, the middle three-fifths of the basis pedunculi, the 

 anterior longitudinal fibers of the pons, and the pyramid of the 

 medulla oblongata, whence they proceed by the lateral and an- 

 terior pyramidal tracts to the gray crescent, partly in the same side 

 but chiefly in the opposite side of the spinal cord. By the former 

 route, the impulses cross over in the medulla, through the decus- 

 sation of the pyramids, and descend in the lateral column of the 

 spinal cord to the gray substance in the vicinity of the nucleus 

 dorsalis (Clarki), where the path is relayed, and intrinsic neurones 

 carry the impulses forward into the anterior columna; but by the 

 anterior route, they descend in the anterior column of the cord 

 and decussate, in succession, through the white anterior com- 



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