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 colxinms (Funiculi laterales) are lateral to the 

 gray colimms 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 

 fascicuh 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 afTerent 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 proprii 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 ppss 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-bu: die 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 intraccmmissuralis 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 He also in the medial part of the ventral column. Posteriorly it diminishes 

 in size and comes to occupy a position next to the ventral median fissure, corresponding to the 

 sulco-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 GoU and of Burdaoh respectively. 



