THE NERVOUS SYSTEM 113 



of which in turn joins a departing motor or secretoi-y 

 fiber, the peripheral termination of which is in the muscle 

 or gland which is to receive the stimulus to activity. 

 Moreover there are ascending branches connected with 

 similar cells which put the entire arc in communication 

 with the brain, and radiating branches which form con- 

 nections with other sensory and motor centers, so that all 

 may be under the control of the higher centers and all 

 coordinated with each other. 



The conducting fibers for descending impulses effer- 

 ent cross at the lower part of the medulla and form the 

 anterior pillars or columns of the cord. The ascending 

 impulses are carried in the posterior columns and cross 

 at various levels as they ascend, though many cross in 

 the lower part of the medulla forming the posterior or 

 sensory, decAissation. Other afferent fibers are con- 

 nected with lateral tracts of the cord, near the entrance 

 of the posterior roots of the spinal nerves, and are car- 

 ried through the restiform bodies of the medulla into 

 the cerebellum on the same side. Apparently the paths 

 in the cord along which tactile impressions are carried 

 are not the same as those traveled by pain and temper- 

 ature sensations, as indicated by a disease in which there 

 is, in affected regions, loss of the power of feeling pain 

 or detecting differences in temperature, while the pres- 

 sure sense is not affected ; but these paths are not clearly 

 defined. 



As the motor fibers all decussate (cross to the oppo- 

 site side) near the junction of the medulla and cord, and 

 many of the sensory fibers are crossing all the way from 

 the entrance of the sensory roots upward, injury to one- 

 half of the cord will produce total motor paralysis of 

 the same side and partial sensory paralysis of the same 

 side, but all sensation is not abolished below the injury. 



