THE BRAIN. 389 



the lateral margins, and represent nerve fibers car- 

 rying impulses from the eighth cranial nerve. 



On the lateral surface of the medulla a prominent 

 oval elevation appears called the olivary body, 

 which represents a crescent collection of nerve cells. 

 Just dorsal to the olivary body is the continuation 

 of the dorsal groove of the cord, and it is from this 

 groove that fibers of the ninth, tenth, and eleventh 

 cranial nerves emerge. Near the anterior extremity, 

 at the lower margin of the pons, is the superficial 

 origin of the seventh and eighth nerves. The origin 

 of these nerves corresponds to the entrance into the 

 cord of the posterior root of the spinal nerves. Just 

 median or ventral to the olive is a groove that cor- 

 responds to the points of exit of the anterior root of 

 the spinal nerves. From this groove the fibers of 

 the twelfth cranial nerve escape. 



The ventral region of the medulla presents a 

 median fissure, the continuation of the anterior 

 fissure of the cord. The upper end of this fissure 

 forms a pit at the lower margin of the pons, called 

 the foramen cecum. Just lateral to this cecum, and 

 curving around the pons, is the superficial origin of 

 the sixth pair of cranial nerves. On each side of the 

 median fissure is a longitudinal ridge called the pyr- 

 amids which represents the fibers of both the crossed 

 and the direct pyramidal tract. Near the lower ex- 

 tremity of the medulla the fissure seems partly 

 obliterated by ridges recrossing. These represent 

 pyramidal fibers crossing to form the crossed pyr- 

 amidal tract, and constitute, therefore, the motor 

 decussation. Just below the olive curved striae may 



