THE MESENCEPHALON 499 



junctivum or superior peduncle of the cerebellum, in a convergent 

 cephalad course, at the same time approaching the median line to 

 decussate in the mesencephalon with its fellow of the opposite 

 side. On the ventro-mesial aspect of the superior peduncle, near 

 its origin, is the superior end of the restiform body or inferior 

 peduncle of the cerebellum, which here approaches its termination 

 in the vermis cerebelli. 



The roof of the fourth ventricle at this level is formed by the 

 vermis of the cerebellum. On its ventral surface and near the 

 median line is a small group of cells, the nucleus fastigius. Dorsal 

 to this nucleus are the convolutions of the superior vermis, while 

 farther lateral is the dentate nucleus of the cerebellum, in section 

 a ribbon-like mass of grey matter whose general appearance closely 

 simulates that of the inferior olivary body. The dentate nucleus 

 is embedded within the central white matter of the cerebellum. 



At the mid-level of the pons Yarolii the fifth nerve penetrates 

 the organ from its lateral surface and passes inward to the teg- 

 mentum. On the ventro-mesial side of the trigeminal fibres is its 

 chief motor nucleus, a small group of motor cells ; on its dorso- 

 lateral side is the larger, triangular, sensory nucleus of the trigem- 

 inus, whose apex extends downward through the pons, and beyond 

 which the spinal root of the fifth nerve is continued into the 

 medulla oblongata. 



In the upper part of the pons the superior cerebellar peduncles 

 have passed to a more ventral plane, and now lie in the lateral wall 

 of the much narrowed fourth ventricle. Mesial to the peduncles, 

 in relation to the ventro-lateral angle of the ventricle, is a small 

 bundle of scattered nerve fibres, the descending cerebral root of 

 the fifth nerve. The ventral margin of the grey matter on the 

 inner side of this root contains a group of pigmented nerve cells 

 which, as the substantia ferruginea or locus cceruleus, extends 

 cerebralward into the midbrain. 



The central portion of the pontal tegmentum in its upper part 

 still consists of the f ormatio reticularis, which is continued upward 

 through the isthmus rhombencephali and midbrain. 



THE MESENCEPHALON 



Entering the mesencephalon, at the isthmus rhombencephali, 

 the decussation of the fourth nerve can be seen forming the roof 

 of the aqueduct of Sylvius, which canal represents the cephalic 

 continuation of the fourth ventricle and occupies the axis of the 



