MOTOR PATHS IN THE CORD 243 



main body of gray matter and does not reach the surface of 

 the cord. Its fibers are likewise lost in the medulla ob- 

 longata. (d) The' column of Burdach (postero-external 

 column) is situated posteriorly in a location corresponding 

 to the anterior fundamental fasciculus anteriorly external 

 to the column of -Goll and internal to the posterior cornu. 

 Its fibers reach the cerebellum through the inferior pe- 

 duncles, having passed through the restiform bodies. 



Functions of the Columns. Remarks already made touch- 

 ing the direction of degeneration in the separate columns 

 throw some light upon the physiological function of the 

 fibers in each. 



Motor impulses pass downward from the brain through 

 certain fibers to the cells of the anterior cornua of gray mat- 

 ter in the cord, and are sent thence through the spinal nerves 

 to the muscles. The paths in the cord conveying these im- 

 pulses are found to be the columns of Turck and the crossed 

 pyramidal tracts, and these are the only parts of the cord 

 known so to act. Impulses to the upper segment of the cord 

 may be conveyed by either of these columns, but impulses to 

 the lower segment must follow the crossed pyramidal tract, 

 since the column of Turck ceases to exist in the dorsal re- 

 gion. Only some 3-7 per cent, of motor fibers from the cor- 

 tex are thought to enter the columns of Turck. The others 

 decussate in the medulla and enter the crossed pyramidal 

 tracts. In any case motor impulses originating in the brain 

 and so conveyed are manifested on the side opposite their 

 cerebral origin, since the fibers in both these tracts decussate 

 in passing downward. It is a well known pathological fact 

 that the lesions of motor areas in the brain, or section of one 

 lateral half of the cord, are followed by paralysis on the side 

 opposite the lesion. 



Following a motor fiber (A, Fig. 77) through the anterior 

 root of a spinal nerve, it is found to originate from one of 

 the large multipolar cells (3) in the anterior cornu of gray 

 matter. Around these anterior horn cells (i, 2, 3, 4) arbor- 



