368 



PHYSIOLOGY 



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 superficial 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 



Posterior longitudinal fasciculus 



Substantia gelatinosa Rolandi 

 Spinal root of fifth nerve 

 Nucleus ambiguus 

 Cerebello-olivary fibres 

 Dorsal accessory olivary nucleus 

 Anterior superficial arcuate fibres 

 Fillet 



Mesial accessory olivary nucleus 

 Inferior olivary nucleus 



FIG. 183. 



Pyramid 

 Arcuate nucleus 



Anterior superficial arcuate fibres 



Transverse section through the middle of the olivary region of the human 

 medulla. (CUNNINGHAM.) 



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 corresponding 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 corre- 



