538 



THE NERVOUS SYSTEM 



There is a great increase of the reticular formation around the central canal, 

 and the lateral approaches the anterior cornu. Then at the ventral or anterior 

 aspect the decussation of the pyramids begins. By this crossing over of the 

 fibers, the tip of the gray anterior cornu is cut off from the rest of the gray mat- 

 ter. The central canal is pushed farther toward the posterior surface, first of 

 all by the decussation of the anterior pyramids just mentioned, and later on, 

 i.e., above, by another decussation of more dorsal fibers. These fibers of 

 the second decussation as they cross form a median raphe and also help to 

 break up the remaining gray matter into what is called a reticular jormation. 



a.m.f. 



FIG. 378. Anterior or Dorsal Section of the Medulla Oblongata in the Region of the Superior 

 Pyramidal Decussation. a.m.f., Anterior median fissure; f.a., superficial arciform fibers emerg- 

 ing from the fissure; py, pyramid; n.ar., nuclei of arciform fibers; f.a., deep arciform becom- 

 ing superficial; o, lower end of olivary nucleus; n.l., nucleus lateralis; f.r., formatio reticularis; 

 /.a. 2 , arciform fibers proceeding from the formatio reticularis; g., substantia gelatinosa of Rolando; 

 a. V., ascending root of fifth nerve; n.c., nucleus cuneatus; n.c.', external cuneate nucleus: n.g., 

 nucleus gracilis; f.g., funiculus gracilis; p.m.f., posterior median fissure; c c., central canal surround- 

 ed by gray matter, in which are n.XL, nucleus of the spinal accessory, and n.XII., nucleus of the 

 hypoglossal; s.d., superior pyramidal decussation. (Modified from Schwalbe.) 



These fibers arise from the nuclei of the fasciculus gracilis and fasciculus 

 cuneatus of either side, and they are looked upon as a sensory decussation. 



There are to be made out various masses of cells in addition to the reticu- 

 lar formation, viz., the nuclei of the fasciculus gracilis and fasciculus cuneatus, 

 figure 379, n.g. and n.c. 



The olivary bodies extend forward almost to the level of the pons. They 

 consist of cells and fibers. The cellular matter consists of a plicated thinnish 

 layer of small nerve-cells, folded upon itself in the form of a loop, with the ends 

 turned inward and slightly dorsal, figure 379, o. The gray loop is filled with 

 and covered by white fibers. 



Internal to the olivary body on either side are two small masses of gray 



