136 THE NERVOUS SYSTEM 



disappearance of these fibers and nuclei there ceases to be any nervous sub- 

 stance dorsal to the central canal, and this, which has been displaced dorsally 

 by the accumulation of the corticospinal fibers and those of the lemniscus ven- 

 tral to it, opens out as the floor of the fourth ventricle (Fig. 97). 



The outline of the gray matter in the most caudal portions of the medulla 

 oblongata closely resembles that of the spinal cord. The anterior columns are 

 first cut off by the decussation of the pyramids (Fig. 95). Then the posterior 

 columns are displaced ventrolaterally due to the increased size of the posterior 

 funiculi and the disappearance of the lateral corticospinal tracts from their 

 ventral aspects. This rotation of the posterior column causes the apex of 

 that column with its spinal tract and nucleus of the trigeminal nerve, which are 

 continuous with the fasciculus dorsolateralis and substantia gelatinosa of the 

 spinal cord (Fig. 94), to lie almost directly lateral ward from the central canal 

 (Fig. 96). The shape of the gray figure is still further altered by the develop- 

 ment of special nuclear masses, many of which are very conspicuous. These 

 include the nucleus gracilis, nucleus cuneatus, inferior olivary nucleus, and the 

 nuclei of the cranial nerves. The greater part of the gray substance now becomes 

 broken up by nerve-fibers crossing in every direction, but especially by the 

 internal arcuate fibers. This mixture of gray and white matter is known as the 

 reticular substance. The central gray matter is pushed dorsad first by the pyra- 

 mids and later by the medial lemniscus until it finally spreads out to form a thin 

 gray covering for the floor of the fourth ventricle. 



The Pyramids and Their Decussation. We have had occasion repeatedly 

 to refer to the crossing of the lateral corticospinal tracts in this and preceding 

 chapters, but there remain some details to be presented. The pyramids are 

 large, somewhat rounded fascicles of longitudinal fibers, which lie on either side 

 of the anterior median fissure 01 the medulla oblongata (Fig. 86). The constit- 

 uent fibers take origin from the large pyramidal cells of the anterior central 

 gyrus or motor cerebral cortex. The decussation of the pyramids or motor 

 decussation occurs near the caudal extremity of the medulla oblongata (Fig. 

 93). Approximately the medial three-fourths of the corticospinal tract passes 

 through the decussation into the lateral funiculus of the opposite side of the 

 spinal cord, as the lateral corticospinal tract (fasciculus cerebrospinalis lateralis 

 or lateral pyramidal tract); while the lateral one-fourth is continued without 

 crossing into the ventral funiculus of the same side as the ventral corticospinal 

 tract (fasciculus cerebrospinalis anterior or anterior pyramidal tract Figs. 

 94, 95, 96, 98). The decussating fibers are grouped into relatively large bundles 



