THE BRAIN. 



391 



lion. It thus happens that the sensory impulses also 

 cross and reach the opposite side of the brain. Ex- 

 ternal to the nucleus gracilis is the substantia gelati- 

 nosa of Rolando, a continuation of that of the cord. 

 Just external to this is a cross section of nerve 

 fibers, the ascending root of the fifth nerve. The 

 central area of each half of the section shows a large 

 number of scattered nerve fibers interlacing and in 

 cross sections. This area is called the formatio 

 reticularis. Anterior to it, a collection of nerve cells 

 represents the olivary body, median and dorsal to 

 which a second and smaller collection of cells consti- 

 tute the mesial olivary nucleus. The bulk of the 

 anterior portion shows a cross section of the pyra- 

 mids. Some of these fibers may be seen to sweep 

 to the opposite side, thus making the motor decus- 

 sation. In doing so they seem to pass over in large 

 alternate bundles, rather than uniformly, as is the 

 case with the sensory decussation. At the anterior 

 surface on each side of the median fissure and sweep- 

 ing around the pyramids are the superficial arcuate 

 fibers, and also a collection of nerve cells, the arcuate 

 nucleus. 



In cross sections of the open medulla the resem- 

 blance to that of the cord is less distinct. The thin 

 roof of the fourth ventricle is usually broken, leaving 

 a dorsal expanded cleft. The lateral margin or 

 remnant of the roof is called the lingula. Just be- 

 neath the floor and close to the median sulcus are 

 the nerve cells of the twelfth nerve. Lateral but in 

 close proximity to these are the nerve cells of the 

 tenth nerve. The axones from these cells may be 



