46 



THE MEDULLA OBLONGATA AND PONS VAROLII. 



pont'iculus of Arnold. Each pyramid then enters the substance of the pons in one 

 large bundle, which soon breaks up into several bundles, and these may be traced 

 through the pons into the corresponding peduncle of the cerebrum. 



The decussation of the pyramids is the name given to the obliquely crossing 

 bundles of the lateral pyramidal tract which are seen in the anterior median fissure 

 at the lower part of the medulla oblongata. The extent to which the decussation is 

 visible varies considerably in different individuals ; for in some the bundles take a 

 deeper, in others a more superficial course. Further, in some cases a larger share 

 than usual of the longitudinal fibres of the pyramids passes down in the anterior 

 pyramidal tract and a correspondingly smaller share in the lateral pyramidal tract. 

 And since the anterior tract, which in the pyramid is external to the lateral tract, 

 does not cross in the medulla oblongata but merely passes obliquely at its lower end 

 to attain the side of the anterior median fissure, the decussation in these cases is of 

 less extent. On the other hand, in rare cases, the whole of the fibres of the pyramid 

 may cross over at the lower part of the bulb and become lateral pyramidal tract, 

 in which case the anterior or uncrossed tract is wanting in the cord, and the 

 bulbar decussation is very well marked. All transitions are found between these 

 two conditions (see p. 24). 



The pons Varolii forms a prominence marked by transverse fibres above and in 

 front of the medulla oblongata, and between the lateral hemispheres of the 



a\ 



Fig. 38. TRANSVERSE SECTION THROUGH LOWER PART OF PONS VAROLII. (After Stilling and Schwalbe.) 

 The description of this figure is given on p. 61. 



cerebellum (fig. 38) ; at the sides its transverse fibres are gathered together into 

 a compact mass, which passes into the cerebellum, and is named the middle cms 

 or middle peduncle of the cerebellum. On its ventral surface the pons has a shallow 

 median groove along which the basilar artery runs, and it is perforated by small 

 branches of the artery. The groove is in some measure due to the circumstance 

 that the pyramids of the medulla oblongata are continued up through the pons with 

 a slightly divergent course, and thus produce a prominence on either side of the 

 middle line, covered, however, by the superficial transverse fibres. 



