THE STRUCTURE OF THE BRAIN STEM 388 



matter of both sides of the cord and passing in the lateral columns through 

 the lateral part of the medulla and pons, and finally attaining the superior 

 vermis through the superior cerebellar peduncles. 



(3) The posterior columns, ending chiefly in the homolateral posterior 

 column nuclei. From these nuclei, though the great mass of fibres passes 

 into the fillet, a certain number from the nuclei of both sides join the resti- 

 form body to pass into the middle lobe of the cerebellum. 



In the medulla these afferent tracts of the cerebellum are joined by the 

 following sets of fibres : 



1. The olivo-cerebellar. 



2. The vestibulo-cerebellar. 



3. A few fibres from the chief sensory nuclei, including those of the vago 

 glossopharyngeal nerves. 



All these fibres terminate in the cortex, chiefly of the middle lobe. From 

 the cortex of this lobe fibres pass to the central and roof nuclei of the cere- 

 bellum, namely, the nucleus dentatus, the nucleus emboliformis, the nucleus 

 fastigii, and the nucleus globosus. The efferent tracts of the cerebellum 

 start from these central nuclei, no fibres which originate in the cortex of the 

 cerebellum apparently leaving the precincts of this organ. Some of these 

 efferent fibres of the cerebellum will be better described with the descending 

 tracts of the brain stem. Of those which take an ascending direction, the 

 great bulk are contained in the superior cerebellar peduncles. These origin- 

 ate for the most part in the dentate nucleus and the nuclei emboliformis and 

 globosus. As the superior peduncles run forwards they sink below the 

 posterior corpora quadrigemina, and in the tegmentum, below the Sylvian 

 iter, decussate with the tract of the opposite side to pass to the red nucleus. 

 In the red nucleus many of the fibres end, some, however, passing through the 

 nucleus together with fibres derived from the cells of the red nucleus itself 

 to end in the thalamus and in the grey matter of the subthalamic region. 



DESCENDING TRACTS 



The chief descending tracts having their origin in the brain stem are the 

 rubro-spinal bundle or bundle of Monakow, the complex system of fibres 

 known as the posterior longitudinal bundle, and the vestibulo-spinal fibres 

 from the upper part of the medulla. 



(1) The RUBRO-SPINAL FIBRES originate in the red nucleus. They cross 

 the median line and run down, at first in the tegmentum and later in the 

 lateral column of the, medulla oblongata and cord. In their passage they 

 communicate with the various motor nuclei of the cranial nerves. 



can be traced to all segments of the cord, where they terminate in connection 

 with the anterior horn- cells. 



(2) The POSTERIOR LONGITUDINAL BUNDLE. This bundle it 



all sections through the brain stem below the level of the oculo-moto 

 It consists of fibres, some of which pass upwards, while others pass 

 wards. Most of the fibres take origin in the cells of Deiters' nucl( 

 the reticular formation of the pons, medulla, and mid-brain, as wel 



