134 



THE NERVOUS SYSTEM 



responsible for this gradual transformation. Traced rostrally from the spinal 

 cord, the ventral corticospinal tracts are seen to enter the pyramids within the 

 ventral area of the medulla oblongata, that is to say, they enter the medulla 

 without realignment. But the fibers of the lateral corticospinal tracts on enter- 

 ing the medulla swing ventromedially in coarse bundles, which run through 

 the anterior gray columns and cut them off from the gray matter surrounding 

 the central canal (Figs. 93, 95). After crossing the median plane in the decussa- 

 tion of the pyramids these fibers join those of the opposite ventral corticospinal 

 tracts and form the pyramids (Fig. 96). Thus fibers from the lateral funiculus 

 come to lie ventral to the central canal and displace this dorsally; and at the same 

 time a start is made toward breaking up the H -shaped gray figure characteristic 

 of the spinal cord. 



Cerebral hemisphere 



Spinal 

 cord 



Fig. 93. Diagram of the corticospinal tracts. 



Shortly after entering the medulla oblongata the fibers of the posterior funiculi 

 end in nuclear masses which invade the funiculus gracilis and funiculus cuneatus 

 as expansions from the posterior gray columns and central mass of gray sub- 

 stance (Figs. 95, 96). These are known as the nucleus gracilis and nucleus cu- 

 neatus. They cause a considerable increase in the size of the posterior funiculi 

 and a corresponding ventrolateral displacement of the posterior columns of 

 gray matter. The fibers of the posterior funiculi end in these nuclei about cells, 

 the axons of which run ventromedially as the axis-cylinders of internal arcuate 

 fibers. These sweep in broad curves through the gray substance, and decus- 

 sate ventral to the central canal in what is known as the decussation of the medial 

 lemniscus. After crossing the median plane they turn rostrally between the 



