1348 A MANUAL OF ANATOMY 



it consists of bundles of nerve-fibres, disposed longitudinally. 

 Inferiorly it is somewhat narrow, but it widens superiorly. At 

 the lower border of the pons Varolii it undergoes a slight constric- 

 tion, after which it sinks into the pons. As it traverses the pons 

 its funiculi become separated into several strata, and these are 

 gathered together at the upper border of the pons into the crus 

 cerebri of the corresponding side. 



Inferiorly each pyramid is disposed in two parts — inner and outer. 

 The inner portion represents as a rule the inner three-fourths, and 

 its fibres cross to the opposite side in the lower part of the ventral 

 median fissure of the bulb. Thereafter they sink deeply into the 

 dorsal part of the lateral column of the spinal cord on the side to 

 which they have crossed, where they constitute the crossed pyra- 

 midal tract. The intercrossing of fibres which takes place in the 

 lower part of the ventral median fissure of the bulb is called the 

 decussation of the pyramids, or the motor decussation, and, as 

 stated, it usually involves the fibres of the inner three-fourths of 

 the pyramid. 



The outer portion of the pyramid represents as a rule the outer 

 fourth, and its fibres take no part in the decussation. The path 

 of most of them is downwards into the anterior column of the spinal 

 cord of the same side, where they lie close to the anterior median 

 fissure of the cord and constitute the direct pyramidal tract. A few 

 of them, however, descend into the lateral column of the same side, 

 and constitute the uncrossed lateral pyramidal tract. 



The pyramid of the bulb, therefore, only corresponds topo- 

 graphically with the anterior column of the spinal cord. The 

 direct pyramidal tract of the anterior column of the cord forms the 

 greater part of the outer fourth of the corresponding pyramid of 

 the bulb ; and the crossed pyramidal tract of the lateral column of 

 the cord forms the inner three-fourths of the pyramid of the oppo- 

 site side. The remainder of the anterior column of the cord sinks 

 deeply into the bulb and lies on the dorsal aspect of the pyramid. 



The ventral surface of each pyramid is crossed above the level of 

 the decussation of the pyramids by the anterior superficial arcuate 

 fibres, which emerge from the ventral median fissure and take an 

 arched course outwards and then backwards to the restiform body. 



The sixth cranial nerve emerges close to the lower border of the 

 pons Varolii, immediately external to the pyramid, and in line with 

 the funiculi of the hypoglossal nerve as these emerge from the 

 ventro-lateral sulcus. 



Lateral Area of the Medulla Oblongata. — This superficial area is 

 situated between the ventro-lateral sulcus, with the funiculi of the 

 hypoglossal nerve, and the dorso-lateral sulcus, with the funiculi 

 of the glosso-pharyngeal, pneumogastric, and bulbar part of the 

 spinal accessory nerves. Superiorly it presents an oval eminence, 

 called the olive {oliva B.). Inferiorly it has the appearance of 

 being a prolongation of the lateral column of the spinal cord, but 

 this is not the case. The crossed pyramidal tract of the lateral 



