SPINAL CORD. 191 



anterior cornu of the gray matter of the opposite side of the 

 cord. (Fig. 22.) 



Through this decussation each half of the cerebrum governs the 

 muscle movements of the opposite side of the body. 



The fibers composing the crossed and the direct pyramidal tracts 

 are therefore the channels by which the volitional nerve impulses 

 are conducted from the motor area of the cortex to the multipolar 

 cells in the anterior cornua of the gray matter of the spinal cord, 

 and by them and their related nerves transmitted t6 the muscles. 



Spino-encephalic, or Sensor Conduction. The nerve impulses 

 that are brought to the spinal cord by the afferent spinal nerve- 

 fibers are transmitted by afferent paths in the cord for the most 

 part to the cortex of the cerebrum where they are translated into 

 conscious sensations. These paths are therefore termed sensor. 

 The sensor tract passes through the cord, the medulla oblongata, the 

 pons Varolii, the superior portion of the crus cerebri, the posterior 

 third of the posterior limb of the internal capsule, to sensor percep- 

 tive areas in the cerebral cortex. The sensor pathway decussates 

 at all levels of the spinal cord and medulla, and therefore the sensi- 

 bility of each side of the body is associated with the opposite side 

 of the brain. 



The paths for the nerve impulses that give rise to different sensa- 

 tions have been variously located by different observers. The path- 

 way for the impulses that give rise to the sensations of temperature 

 has been located in the gray matter ; the pathway for the impulses 

 that give rise to the sensation of pain has been located in Gower's 

 tract ; the pathway for tactile impressions has been located in the 

 posterior columns. 



Properties of the Spinal Cord. Irritation applied directly to the 

 anterolateral white columns produces muscular movements, but no 

 pain ; they are, therefore, excitable, but insensible. 



The surface of the posterior columns is not sensitive to direct 

 irritation, except near the origin of the posterior roots. The sensi- 

 bility is due, however, not to its own proper fibers, but to the fibers 

 of the posterior root, which traverse it. 



Division of the anterolateral columns abolishes all power of vol- 

 untary movement in the lower extremities. 



Division of the posterior column impairs the power of muscular 

 coordination, such as is witnessed in locomotor ataxia. 



