368 PHYSIOLOGY 



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- 

 sponding 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 dor sally to the fillet is to be seen another 

 well-marked bundle of longitudinal fibres, known as the posterior longi- 

 tudinal 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 fourth ventricle is covered in by the cerebellum, 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 posterior or direct cerebellar tract. In the region 

 of the dorsal nuclei however, it receives accession of fibres from the gracile 

 and cuneate nuclei of the same side and, through the superficial arcuate 

 fibres, from the nuclei of the opposite side, and thus passes as a thick white 

 bundle into the cerebellum. Among these arcuate fibres are also a number 

 derived from the olivary body of the opposite side, known as the cerebello- 

 olivary fibres. On its way it is joined by a smaller bundle, the ' internal 

 restiform body,' which conveys fibres from the vestibular division of the 

 eighth nerve and also serves to connect Deiters' nucleus with the cerebellum. 

 The reMiionn body is thus made up of the following fibres (Fig. 184) : 



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

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



Cl) 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. 



\ Motion through the |nns sho\\s the fourth ventricle widely dilated, 

 wit h a floor formed of grey matter as in the medulla. The chief difference 

 in the appearance of the section is due to the ^rcat masses of transverse 

 fibres which pa- int.. the poiis by the lateral peduncles of the cerebellum, 

 cross by the median raphc. and turn either upwards ,>r downwards on the 



