440 PHYSIOLOGY 



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 continuous with 

 the anterior basis bundle of the spinal cord and merges in the inter- 

 nuncial 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. 461). 



(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 indirectly 

 affect 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. 



