1356 A MANUAL OF ANATOMY 



beneath the anterior superficial arcuate fibres as they arch out- 

 wards over the pyramid after emerging from the ventral median 

 fissure. Superiorly it lies over the mesial aspect of the pyramid, 

 close to the ventral median fissure. It contains small nerve-cells, 

 in connection with which some of the anterior superficial arcuate 

 fibres terminate, whilst others arise as axons of the cells, and 

 many of them pass over the nucleus without entering it. 



Nucleus Lateralis. — This is a special collection of nerve-cells in 

 that portion of the formatio reticularis grisea which lies on the 

 dorso-lateral aspect of the olive. It is situated deeply between the 

 olive and the substantia gelatinosaof Rolando. 



White Matter of the Medulla Oblongata. — ^The white matter is 

 situated chiefly on the surface. Over the dorsal aspect of the upper 

 or open part of the bulb, however, the grey matter comes to the 

 surface and covers the lower or bulbar half of the floor of the fourth 

 ventricle. The white matter is disposed in tracts or strands which 

 are chiefly longitudinal, but a few run transversely in an arched 

 manner. The tracts are as follows : 



1. Pyramidal tract, or p5n:amid. 



2. Dorsal or direct spino-cerebellar tract. 



3. Ventral spino-cerebeUar tract. 



4. Restiform body. 



5. Funiculus cuneatus. 



6. Funiculus gracilis. 



7. Dorsal or posterior, and ventral or anterior 



longitudinal, bundles. 



8. Superficial arcuate tract. 



9. Deep arcuate tract. 



10. Fillet. 



11. Olivo-cerebellar tract. 



The pyramid of either side, and the decussation of the pyramids, 

 have been already described. It may, however, be again stated 

 that the path of their motor nerve-fibres is downwards into the 

 spinal cord. The fibres comprising about the inner three-quarters 

 of a pyramid, having taken part in the decussation, enter the 

 dorsal portion of the lateral column of the cord on the side to which 

 they have crossed, and here they constitute the deeply placed 

 crossed lateral pyramidal tract. The fibres comprising about the 

 outer quarter of a pyramid take no part in the decussation. Most 

 of them pursue a direct path downwards into the anterior column 

 of the cord on the same side. Having entered the anterior column, 

 they take up a position close to the ventral median fissure and 

 constitute the direct or ventral pyramidal tract {tract of Turck) . A 

 few of them, however, enter the lateral column of the same side, 

 and constitute the uncrossed lateral pyramidal tract. 



The pyramidal tract has descended from the pons Varolii. 



Dorsal or Direct Spino-cerebellar Tract. — This tract extends 

 upwards from the lateral column of the spinal cord. It traverses 



