SPINAL NERVES. 119 



degeneration. The nutrition of the anterior root is governed by nerve cells 

 in the gray matter of the cord ; for if these cells undergo atrophy, or if the 

 nerve be divided, it undergoes degeneration outward. 



COURSE OF THE ANTERIOR AND POSTERIOR ROOTS. 



The Anterior Roots pass through the anterior columns, horizontally, 

 in straight and distinct bundles, and enter the anterior cornuae, where they 

 diverge in four directions, (i) Many become connected with the prolon- 

 gations of the multipolar nerve cells. (2) Others leave the gray matter, 

 pass through the anterior white commissure, and enter the anterior columns 

 of the opposite side. (3) A considerable number enter the lateral columns 

 of the same side, through which they pass to the medulla oblongata, where 

 they decussate and finally terminate in the corptts striatum of the opposite 

 side. (4) Others traverse the gray matter horizontally, and come into 

 relation with the posterior roots. 



The Posterior Roots enter the posterior horns of the gray matter (l) 

 through the substantia gelatinosa, (2) through the posterior columns; of 

 the former, some bend upward and downward, and become connected 

 with the anterior cornuae ; others pass through the posterior commissure to 

 the opposite side; of the latter, fibres pass into the gray matter, to the 

 posterior vesicular columns, passing obliquely through the posterior white 

 columns upward and downward for some distance, and enter the gray 

 matter at different heights. 



Decussation of Motor and Sensory Fibres. The Motor fibres, 

 which conduct volitional impulses from the brain outward to the anterior 

 cornuae, arise in the motor centres of the cerebrum ; they then pass down- 

 ward through the corona radiata, the internal capsule, the inferior portions 

 of the crura cerebri, the pons Varolii, to the medulla oblongata, where the 

 motor tract of each side divides into two portions, viz : I . The larger, 

 containing 91 to 97 per cent, of the fibres, which decussates at the lower 

 border of the medulla and passes down in the lateral column of the oppo- 

 site side, and constitutes the crossed pyramidal tract. 2. The smaller, 

 containing 3 to 9 per cent, of the fibres, does not decussate, but passes down 

 the anterior column of the same side, and constitutes the direct pyramidal 

 tract, or the column of Tiirck. Some of the motor fibres of these two 

 tracts, after entering the anterior cornuae of the gray matter, become con- 

 nected with the large multipolar nerve cells, while others pass directly into 

 the anterior roots. Through this decussation each half of the brain governs 

 the muscular movements of the opposite side of the body. 



