652 NERVOUS SYSTEM OF THE HORSE 



groove (where present) indicates a subdivision of the dorsal column into two fasci- 

 culi or tracts; the inner of these is the fasciculus gracilis (GoU's column); the outer, 

 the fasciculus cuneatus (Burdach's column). 



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 a.xones. 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 (ascending) 

 branches form the direct sensory path to the l:)rain and extend in the fasciculus cuneatus and 

 fasciculus gracilis or corresponcUng tracts to nuclei in the medulla oblongata. The posterior 

 (descending) 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 ol)longata 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 inner side of the cersbellospinal 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-liundle is deeply situated at the side of the gray columns. The 

 bulk of its fibers are axones of cells of the posterior cornu 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) above 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 lioth crossed and direct pyramidal fibers. The descending 

 cerebellospinal fasciculus extends from the cerebellum to the luml)ar 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 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 rather closely in size and shape to 

 the cavity in which it lies. 



Its average weight without the dura mater is about 23 ounces (ca. 650 gm.), 

 and forms about y of 1 per cent, of the body-weight. 



It is desirable to examine the general external configuration of the brain before 

 studying its various parts in detail.' 



When divested of its membranes and vessels (Figs. 499, 500), its ventral 



' The description given here is intended to present the chief facts in regard to the brain as 

 they maj' be studied in the dissecting room. The vessels and membranes wliich must be ex- 

 amined first have been described. 



