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 s to the brain, whence an impulse de- 
scends through the fiber ?’, 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- 
muadal tract. 
