768 NERVOUS SYSTEM OF THE HORSE 
ventral columns (Funiculi ventrales) are situated on either side between the median 
fissure and the ventral gray columns. They are connected above the fissure by 
the white commissure. The lateral columns (Funiculi laterales) are lateral to the 
gray columns on either side; their limits are indicated superficially by the dorso- 
lateral groove and the emergence of the ventral root-fibers. The intermediate 
groove (where present) indicates a subdivision of the dorsal column into two fasci- 
culi or tracts; the medial of these is the fasciculus gracilis; the lateral one is the 
fasciculus cuneatus.! 
The amounts of gray and white matter vary greatly in different parts of the 
cord both absolutely and relatively. In cross-section the absolute areas of both are 
greatest in the enlargements. The relative area of gray matter is smallest in the 
thoracic region (except at its anterior end), and increases from the lumbar enlarge- 
ment backward. 
Investigations have shown that in man the columns of white matter are subdivided into 
fasciculi or tracts, which constitute definite conducting paths of greater or less length. Our 
knowledge of the tracts in the domesticated animals is very limited, and it is quite unsafe to make 
inferences from the arrangement in man. As evidence of this it may be noted that the ventral 
cerebrospinal or direct pyramidal tract of man cannot be recognized as such. 
The dorsal white columns consist essentially of two sets of axones. The afferent or sensory 
axones which come from the cells of the spinal ganglia enter as the dorsal roots of the spinal nerves 
and divide into two branches in the vicinity of the dorsal gray column. The anterior branches 
form the direct sensory path to the brain and extend in the fasciculus cuneatus and fasciculus 
gracilis or corresponding tracts to nuclei in the medulla oblongata. The posterior branches 
extend backward for varying distances and give off numerous collaterals to cells of the gray column, 
thus forming part of the mechanism for the mediation of reflex action. Some collaterals cross 
in the white commissure to the opposite side. Many of these fibers are collected in the comma- 
shaped tract between the fasciculus gracilis and cuneatus. The second set of axones arises from 
the smaller cells of the gray column. They enter the white matter, divide into anterior and 
posterior branches, forming the fasciculi proprit or ground bundles of the cord. Some branches 
cross to the opposite side. The function of this set of axones is chiefly to associate various levels 
of the cord. 
The lateral columns contain some axones of the dorsal nerve-roots, which (in man) are 
grouped in the marginal tract of Lissauer, situated just dorsal to the apex of the dorsal horn. 
The cerebellospinal fasciculus (direct cerebellar tract of Flechsig) extends along the periphery 
of the lateral column. It contains the axones of the cells of the nucleus dorsalis (Clarke’s column), 
which proceed to the medulla oblongata and enter the cerebellum by the restiform body. The 
rubrospinal tract (of Monakow) appears to take the place of the lateral cerebrospinal fasciculus or 
crossed pyramidal tract of man. It lies at the medial side of the cerebellospinal fasciculus. Its 
fibers arise in the nucleus ruber of the mid-brain, cross the median plane (decussation of Forel), 
and pass backward in the tegmentum and medulla oblongata to the lateral column of the cord. 
It is a path for motor impulses coming from the cerebral cortex and the cerebellum. The lateral 
fasciculus proprius or ground-bundle is deeply situated at the side of the gray columns. The 
bulk of its fibers are axones of cells of the dorsal column which divide into anterior and posterior 
branches. They are intersegmental paths which associate different levels of the gray matter of 
the cord. The significance of the remaining fibers is not yet known. 
The ventral white columns do not contain a ventral cerebrospinal or direct pyramidal tract, 
as in man. There is a small tract (Fasciculus intracommissuralis ventralis) dorsal to the white 
commissure, Which separates it from the rest of the ventral column. It extends to the middle of 
the thoracic region, It consists of intersegmental fibers, and contains in the anterior part of 
the cervical cord in the sheep and goat both crossed and direct pyramidal fibers. The descending 
cerebellospinal fasciculus extends ‘from the cerebellum to the lumbar region. In the cervical 
region it occupies a semilunar area which reaches almost to the surface ventro-laterally. Scattered 
fibers belonging to it lie also in the medial part of the ventral column. Posteriorly it diminishes 
in size and comes to occupy a position next io the ventral median fissure, corresponding to the 
suleo-marginal fasciculus of man (Dexler) 
THE BRAIN 
The brain or encephalon is the part of the central nervous system that is 
situated in the cranial cavity. It is the enlarged and highly modified cephalic 
part of the primitive neural tube. It conforms in great part in size and shape to 
the cavity in which it lies. Its average weight without the dura mater is about 
1 These are also known as the columns of Goll and of Burdach respectively. 
