710 THE NERVOUS SYSTEM. 



a large mesial and a smaller lateral one. The fibres of the former are directly 

 continuous with those of the crossed pyramidal tract of the opposite side of the 

 cord by means of the decussation already referred to. This decussation is more 

 commonly spoken of as the decussation of the pyramids. The fibres of the lateral 

 bundle are directly continuous with those of the direct pyramidal tract of the 

 same side of the cord. This tract, it will be remembered, in the cord is next to 

 the anterior median fissure. Hence in the pyramid it is displaced laterally by 

 the passage upward, next to the median fissure, of the crossed pyramidal tract 

 after its decussation with its fellow of the opposite side. Each pyramid, close to 

 the pons, is often crossed by a fairly-well marked band of fibres, the ponticulus 

 of Arnold. The fibres of the pyramid are continued directly upward into the 

 pons Varolii. 



THE LATERAL SURFACE OF THE MEDULLA. Each of these surfaces, as 

 already stated, is separated from the corresponding half of the ventral and dorsal 

 surface respectively by the ventro-lateral and dorso-lateral groove. The entire 

 upper half of this surface is occupied by a well-marked olive-shaped prominence, 

 the olive or olivary body. The lower half, below the olive, is often spoken of as 

 the " lateral tract " of the medulla. It is not raised up from the general surface, 

 as is the olive, and consists of white fibres derived from the antero-lateral ground 

 bundle and antero-lateral ascending and descending cerebellar tracts of the cord. 

 These fibres pass upward, some going beneath the olive (the major part), while 

 others proceed over its surface, thus forming part of its structure, and still 

 others are found in the grooves on each side of the olive. The fibres in the 

 grooves may be considered as coming from the cerebellar tracts (ascending and 

 descending), while those on the surface, and those which dip under or beneath the 

 olive, are direct prolongations of the antero-lateral ground bundle. The further 

 destination of all these fibres will be noted later on. 



The Olive or Olivary Body. This has just been partially described. It is 

 made up of the white fibres above mentioned, and also of a nucleus of gray 

 matter in its substance, the projection of which really causes the prominence 

 itself, or the olive. This nucleus is the olivary nucleus or dentate nucleus of the 

 olivary body. It will be further considered below. The upper end of the olive 

 reaches nearly to the pons, only a short but deep, transversely directed groove 

 intervening. This small groove really connects the upper ends of the dorso- 

 lateral and ventro-lateral grooves, between which the olive is placed, and which 

 are here nearer together than their lower portions, owing to the forward tend- 

 ency of the former. Between the olive and pyramid (ventro-lateral groove) 

 emerge the fibres of the hypoglossal nerve. The olive is equal in breadth to the 

 pyramid, is a little broader above than below, and is about half an inch in 

 length. Numerous white fibres (superficial arciform fibres} are seen winding 

 across the lower half of the pyramid and the olivary body to enter the restiform 

 body (see below). 



THE DORSAL SURFACE OF THE MEDULLA. The lower half of this surface is 

 divided in two by the posterior median fissure continued upward from the cord. 

 Each of these halves of the lower half of the dorsal surface of the medulla is sep- 

 arated from the so-called lateral tract or area by the inferior portion of the dorso- 

 lateral groove, and receives the upward prolongation of the direct cerebellar tract 

 of the cord, as already mentioned. 



Situated in and forming parts of this same portion of the dorsal surface of 

 the medulla are three other columns or tracts of white matter, besides the one 

 just mentioned. These columns are known as funiculi, and are placed side by 

 side, separated by slight grooves, between the direct cerebellar tract laterally 

 and the posterior median fissure mesially. The one next to the direct cerebellar 

 tract is the funiculus of Rolando, adjoining which is the funiculus cuneatus, and 

 the innermost, next to the fissure, is the funiculus gracilis. 



The upper half of the dorsal surface of the medulla is considerably wider 

 than the lower, this increase in width being progressive from below upward. Its 



