LEMUR MONGOZ 45 



to the ventral gray column are the scattered bundles of the pyramidal tract 

 ( Py) as they begin to assemble in the position occupied by them in 

 their descent through the oblongata and into the spinal cord. On the periphery 

 of" the section is a narrow band of medullary substance, the circumferential 

 zone, which contains the two large ascending spinocerebellar tracts 

 (Gow, Fie). Mesial to this zone and bordering upon the gray matter 

 of the ventral gra\- column is the intermediate zone which in its ventral 

 portion contains the two descending Deiterso-spinal tracts (DT) and, in 

 its lateral portion, the spinothalamic and rubrospinal tracts (Spt, Rst). 



LEVEL OF THE CAUDAL EXTREMITY OF THE INFERIOR OLIVE (fIG. 23) 



At the level of the caudal extremity of the inferior olive the general 

 appearance of the section has changed considerably. The chief altera- 

 tion is due to the presence in the ventral field of a new collection of 

 gray matter. This is the inferior olivary nucleus (10) which lies im- 

 mediately dorsolateral to the liber bundles constituting the pyramid 

 (Py). Dorsal to the olive is a smaller mass, the dorsal accessory olive. 

 Situated in front of the central gray matter (Cen) are fibers (Fai) 

 arising in the dorsal nuclear masses, which pass inward and forward 

 to the midline where they undergo decussation. At this point they turn 

 upward and constitute the mesial fillet. These decussating axons are 

 internal arcuate fibers. Most of them take origin in the nucleus of Goll 

 (NG) and contribute to the formation of the lowest portion of the 

 mesial fillet which here represents the lower extremity and tail. The pyramid 

 (Py) appears as a compact bundle occupying the most ventromesial 

 portion of the section. Its transverse and anteroposterior diameters give it 

 the proportion of about i to 1 1 to the entire section. This ratio conveys 

 some impression as to the relative importance of the pyramidal system in the 



