THE STRUCTURE OF THE BRAIN STEM 389 



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. 



(l).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. They 

 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 is to be seen in 

 all sections through the brain stem below the level of the oculo-motor nucleus. 

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

 wards. Most of the fibres take origin in the cells of Deiters' nucleus and of 

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

 certain cells in the sensory nucleus of the fifth nerve. The fibres traced 

 upwards can be seen to send collaterals to end in the various parts of the 

 nuclei of the third, fourth, and sixth nerves. Lower down it becomes con- 

 tinuous with the anterior basis bundle of the spinal cord and merges in the 

 internuncial fibres which serve to connect the various levels of the cord. 

 Some of the fibres, which are descending, are derived from a small nucleus, 

 the so-called nucleus of the posterior longitudinal bundle, which is found in 

 the grey matter at the side of the posterior part of the third ventricle. This 

 bundle also receives fibres from the superior olivary body. It is one of the 

 earliest to undergo myelination in the foetus (cp. also Fig. 205, p. 407). 



(3) The VESTIBULO-SPINAL TRACT takes origin for the most part in the 

 cells of Deiters' nucleus. The fibres pass down in the anterior part of the 

 spinal cord and terminate in the anterior horns. ' They are sometimes known 

 as the antero-lateral descending tract. It is probably through this tract 

 that the cerebellum is able to affect indirectly the activity of the motor 

 mechanisms of the cord. 



Two other descending tracts which are important in the lower vertebrates 

 are insignificant in man. These are the thalamo-spinal tract, consisting of 

 descending fibres derived from the optic thalamus, and the tecto-spinal tract, 

 containing fibres derived from the roof of the mid-brain. In the mid- and 

 hind-brain these fibres run in the tegmentum. In the cord they are found 

 in the anterior columns. The olivo-spinal tract, which is supposed to 

 originate in the olivary body, forms a small tract in the cervical region near 

 the surface, opposite the lateral angle of the anterior horn. 



