866 THE NERVE SYSTEM 



I. The Pyramids (pyramides medullae oblongatae) constitute the oblongatal 

 portion of the direct cerebrospinal efferent tracts conveying (voluntary) motor 

 impulses from the precentral cortex, through the internal capsule, crusta, and 

 ventral pons to descend in the crossed and direct pyramidal tracts to the efferent 

 (motor) cell groups in the ventral horns of the spinal gray. In their external 

 appearance in the medulla oblongata they are moderately constricted at their 

 pontile ends, appear to become somewhat expanded, to again taper as they pass, 

 partly into the ventral columns of the cord, partly, by decussation, into the lateral 

 columns. The pyramids are separated from each other by the ventral (or ventro- 

 median) fissure except where this is more or less completely obliterated by the 

 decussating bundles. Each pyramid is bounded laterally by a slight furrow, the 

 ventro lateral or pyramido-olivary groove, in which arise the hvpoglossal nerve 

 roots and which separates the pyramid from the olive. The pontile end of each 

 pyramid is frequently traversed by a band of arched fibres (fibrae arcuatae ectales) ; 

 ponticulus of Arnold (not the ponticulus of Henle), the ectal arcuate fibres. 



The decussation of the pyramids (decussatio pyramidum) is a term given to the 

 obliquely intercrossing bundles seen at the oblongata-myelon transition. The 

 extent to which this decussation occurs and the degree of its visibility varies in 

 different individuals. While in most cases the majority (90 per cent.) of the fibres 

 cross the median line in this decussation to continue as the crossed or lateral 

 pyramidal tract, it is sometimes observed that a larger share of the fibres pass 

 into the direct or uncrossed pyramidal tract with a corresponding reduction of the 

 crossed tract. Occasionally the decussating bundles are so deeply situated in the 

 ventral fissure as not to be visible. 



II. The Lateral Area of the medulla oblongata is continuous with that of the 

 spinal cord, and is bounded by the dorsolateral and ventrolateral fissures. It 

 is composed of the ventrolateral spinocerebellar tract, or tract of Gowers (fasciculus 

 anterolateralis super ficialis), the ventrolateral ground bundle (fasciculus proprius 

 anterolateralift) , and the direct spinocerebellar tract (fasciculus cerebellospinalis 

 [FlechsigJ), while it is invaded from above by the crossed pyramidal tract. The 

 olive is interpolated in the cephalic part of this area. 



The olive (oliva; olivary body) is a prominent, elongated oval mass bulging from 

 the cephalic part of the lateral area of the medulla oblongata, bounded by shallow 

 grooves, of which one, for the hypoglossal nerve roots (ventrolateral fissure) 

 separates it from the pyramid, while the other, containing the nerve fascicles of 

 the vagus, glossopharyngeal and spinal accessory nerves, separates the olive from 

 the restiform body. From the pons it is separated by a shallow groove in which 

 a band of arched fibres is sometimes seen. Numerous white fibres (external or 

 ectal arcuate fibres) emerging from the ventral fissure and traversing the pyramid 

 loop across the lower parts of the olive to enter the restiform body. The olive 

 is formed by the olivary nucleus, embedded in a thin layer of white matter. 



The olive is about 12 mm. (^ inch) in length and 5 mm. (^ inch) in breadth. 



III. Dorsal Area. (a) The funiculus gracilis is the direct continuation of the 

 tract of the same name in the spinal cord. It is a narrow white band placed along 

 the dorsomedian fissure, and separated from the funiculus cuneatus by the dorso- 

 paramedian furrow (sulcus intermedius posterior). At the caudal apex of the rhom- 

 boidal fossa (fourth ventricle) each funiculus gracilis diverges from the median 

 plane, presenting at this point a club-like enlargement, the clava. The promi- 

 nence of the funiculus gracilis (and clava) is due to the gray nucleus funiculi 

 gracilis beneath the surface. 



(6) The funiculus cuneatus is the direct continuation of the tract of the same 

 name in the spinal cord. It enlarges as it ascends, exhibiting a slight eminence 

 or enlargement, the cuneate tubercle (tuberculum cinereum), which is marked only 

 in the medulla oblongata of young individuals, and is due to the nucleus funiculi 

 cuneati beneath the surface. 



