THE NERVOUS SYSTEM. 199 



sciousness, and is then known as reflex action. If, how- 

 ever, the impulse travels along the fiber ir, and thence 

 through the fiber ^ to the brain, whence an impulse de- 

 scends through the fiber t', ts, the process is known as volun- 

 tary reaction. 



THE FIBER TRACTS OF THE CENTRAL 

 NERVOUS SYSTEM. 



As before stated, the white matter of each half of the 

 cord is divided by the exit and entrance of the nerve roots 

 into three columns anterior, lateral, and posterior. Each 

 of these columns is subdivided into tracts which have special 

 names and special functions (Fig. 103). 



In the posterior column two tracts are recognized: the 

 fasciculus gracilis, occupying the medial third of the column, 

 and the fasciculus cuneatus, composing the remainder. In 

 the medulla of the cat these two tracts may be distin- 

 guished by the unaided eye (Fig. 96). They are here 

 called the funiculi of Goll and Burdach, or funiculi gracilis 

 and cuneatus. Their fibers are largely, if not entirely, the 

 axones of the ganglion cells on the posterior roots of the 

 spinal nerves. They terminate in the nuclei gracilis and 

 cuneatus, two small masses of nerve cells in the medulla 

 laterad of the fourth ventricle (Fig. 102). That these 

 fibers are processes of the spinal ganglion cells is proved 

 by the fact that they degenerate if the posterior nerve roots 

 are severed close to the cord. In whales, where the pelvic 

 extremities are wanting, the fasciculi gracilis and cuneatus 

 are very small. 



The lateral column is composed of five tracts : the direct 

 cerebellar tract, the antero-lateral descending cerebellar 

 tract, the antero-lateral ascending cerebellar tract or Gow- 

 er's tract, the lateral ground bundle, and the crossed pyra- 

 midal tract. 



