THE STRUCTURE OF THE BRAIN STEM 415 



pyramids in front and the tubercle of Rolando behind. This is the 

 olivary body, and has on its inner and dorsal sides two little grey 

 masses which are the accessory olivary bodies. The other feature is 

 the new relay of sensory fibres which start from the dorsal nuclei, the 

 nuclei gracilis and cuneatus. These fibres run outwards and forwards 

 from the nuclei right round the medulla. Some fibres pass into the 

 restiform body of the same side. A larger number, forming the super- 

 ficial arcuate fibres, pass superficially to the olive to join the restiform 

 body of the opposite side, while others, the deep arcuate fibres, pass 

 deeply to the olives, and crossing in the median raphe turn upwards 

 in the broken mass of grey and white matter which lies between the 

 olives and the superficial grey matter of the fourth ventricle. This 

 decussation, which is known as the ' decussation of the fillet ' or the 

 sensory decussation, takes place immediately above the level of 

 the decussation of the pyramids. In its upward course it forms a 

 conspicuous strand of fibres, lying close to the mesial plane and 

 separated from its fellow of the opposite side simply by the median 

 raphe. To this collection of fibres is given the name of the fillet or 

 lemniscus. It is perhaps the most important of the afferent tracts 

 of the brain stem, receiving as it does continuations of the posterior 

 columns of the cord as well as contributions from the various sensory 

 cranial nerves. It may be traced forwards as far as the thalamus and 

 subthalamic region, where its fibres terminate. The region corre- 

 sponding to the anterior column of the spinal cord is thus invaded in 

 the medulla by two great longitudinal tracts of fibres, namely, the 

 pyramids and the tracts of the fillet. The region corresponding to the 

 anterior basis bundle, i.e. that part of the anterior columns occupied 

 chiefly by intra-spinal fibres, is thus pushed further backwards and 

 finally comes to lie immediately beneath the grey matter of the floor 

 of the fourth ventricle. Immediately dorsally to the fillet is to be 

 seen another well-marked bundle of longitudinal fibres, known as the 

 posterior longitudinal bundle. These fibres, which serve to connect the 

 nuclei of many of the cranial nerves, can be regarded as analogous to 

 the constituent fibres of the anterior basis bundle in the cord, and can 

 in fact be traced into this part of the anterior columns in the first 

 and second cervical segments of the cord. 



The anterior half of the fourth ventricle is covered in by the cere- 

 bellum, which is attached to the axial part of the brain by three 

 peduncles, the inferior peduncles or restiform bodies, the lateral 

 peduncles, which form the great mass of transverse fibres known as 

 the pons Varolii, and the superior peduncles, which run forward to the 

 posterior corpora quadrigemina. The restiform bodies can be regarded 

 as the direct continuation forwards of the lateral columns of the cord, 

 minus the pyramidal tracts, the chief remaining tract therefore being the 



