THE STRUCTURE OF THE BRAIN STEM 



certain cells in the sensory nucleus of the fifth nerve. Tin- 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. 408). 



(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. 



