THE STRUCTURE OF THE BRAIN STEM 417 



Deiters' nucleus with the cerebellum. The re'stiform body is thus 

 made up of the following fibres (Fig. 185) : 



(1) The direct or posterior cerebellar tract, derived from the cells 

 of Clarke's column on the same side of the cord. 



(2) The posterior superficial arcuate fibres, derived from the 

 gracile and cuneate nuclei of the same side. 



(3) The anterior superficial arcuate fibres, from the gracile and 

 cuneate nuclei of the opposite side. 



(4) The cerebello- olivary fibres. 



(5) The vestibulo- cerebellar fibres. 



A section through the pons shows the fourth ventricle widely 

 dilated, with a floor formed of grey matter as in the medulla. The 

 chief difference in the appearance of the section is due to the great 

 masses of transverse fibres which pass into the pons by the lateral 

 peduncles of the cerebellum, cross by the median raphe, and turn either 

 upwards or downwards on the opposite side or end in connection 

 with the nerve- cells which are scattered throughout the white fibres. 

 The pyramids can still be seen as thick longitudinal bundles on each 

 side in the midst of the transverse fibres. They are considerably 

 larger than in the medulla and become larger as we trace them up 

 towards the mid-brain, owing to the presence of a number of fibres 

 which are derived from the cortex cerebri and end in the grey matter 

 of the pons. The tract of the fillet lies on each side of the middle 

 line dorsally to the transverse fibres. A little to the outside of the 

 fillet is seen a special mass of grey matter, known as the superior olive. 

 The nervous mass lying behind the transverse fibres of the pons, 

 between them and the grey matter of the floor of the fourth ventricle, 

 is known as the for mat io reticularis. It is divided into a lateral and 

 mesial part by the fibres of the hypoglossal nerve. In the lateral 

 portions there is a considerable quantity of grey matter, which can be 

 regarded as continuous with the grey matter of the lateral horns of the 

 cord. The ' lateral nucleus ' is simply a condensed part of this grey 

 matter, lying between the olive and the gelatinous substance of Rolando. 

 The mesial part of the formatio reticularis is almost free of nerve- 

 cells. The reticular appearance of this part of the pons is due to 

 the intersection of fibres which run longitudinally and transversely. 

 The transverse fibres are a continuation of the deep arcuate fibres. 

 The longitudinal fibres in the outer part of the formatio reticularis are 

 the representative of the lateral columns of the cord after the removal 

 of the direct cerebellar and the crossed pyramidal tracts. They 

 include therefore the antero-lateral ascending tract (tract of Gowers) 

 and a number of other fibres corresponding to the lateral basis bundle 

 in the cord. In the mesial part of the formatio reticularis the longi- 

 tudinal tracts are the tract of the fillet and the posterior longitudinal 



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