502 THE NERVOUS SYSTEM 



The lateral and mesial fillets, superior cerebellar peduncles, 

 pyramidal tracts, and posterior longitudinal fasciculi are continued 

 upward through the mesencephalon. The mesial fillet lies rather 

 more lateral from the median raphe than in the lower levels, and 

 its outer border is blended with the lateral fillet. The superior 

 cerebellar peduncles approach the median line and soon decussate. 

 This decussation occurs in the cephalic end of the midbrain. The 

 pyramidal tracts form the large bundle of longitudinal fibres which 

 compose the columns of the crurae cerebri. The fibres of the pos- 

 terior longitudinal fasciculus on either side of the median line, 

 form a deep longitudinal trough in which rests the grey matter 

 which incloses the aqueduct of Sylvius. In this grey matter sev- 

 eral groups of large motor cells mark the beginning of the nuclei 

 of the third pair of cranial nerves. These nuclei extend cephalad 

 for a considerable distance. The nuclei of the fourth cranial nerves 

 form, on either side, a group of large pigmented cells which indent 

 the dorsal margin of the posterior longitudinal fasciculus. 



Opposite the anterior corpora qiiadrigemina the third nerve 

 makes its exit. At this level the grey matter, which was continued 

 upward from the medulla oblongata and dorsal half of the pons 

 Varolii, is reduced to a comparatively thin cylindrical mass which 

 surrounds the aqueduct of Sylvius. The nucleus of the third 

 cranial nerve, which is embedded in this grey matter, presents a 

 median and a lateral cell group. The median nucleus lies in the 

 median line, in the angle of the trough formed by the posterior 

 longitudinal fasciculi ; the lateral nucleus indents the dorsal sur- 

 face of these bundles. 



In the ventral portion of the organ at this level, and on either 

 side of the median line, is a large oval reddish-grey mass of nerve 

 cells, the red nucleus (nucleus ruber). This nucleus occurs at the 

 level of the decussation of the superior cerebellar peduncles, and 

 in it the fibres of these tracts terminate immediately after their 

 decussation. 



Ventral to the red nucleus, but separated from it by a dark 

 mass of nerve cells, the substantia nigra of Sommering, is the free 

 portion of the crus cerebri, which contains the continuation of the 

 pyramidal tracts. 



The optic tracts at this level pass dorsal ward around the lateral 

 surface of the mesencephalon to reach the medial geniculate bodies 

 in the angle between the optic thalamus and the brain stem. 



Above the border of the pons Varolii the crura cerebri diverge, 



