THE MESENCEPHALON. 



1113 



as they pass upward to the red nuclei, in which a large number of their component fibres end. 

 On each side of the median raphe of the tegmental field and above (behind) the peduncular 

 tract, is seen the posterior longitudinal fasciculus, which here, broader than in the pons, passes 

 close to the ventral side of the nucleus of the trochlear nerve. The attenuated crescentic tract 

 of transverely cut fibres which lies along the lateral margin of the central gray substance, medial 

 to the nucleus of the inferior colliculus, represents the mesencephalic root of the trigeminal nerve. 

 In sections taken slightly below the level of the trochlear nucleus, irregular bundles of obliquely 

 cut fibres mark the dorsally directed course of the fourth-nerve to gain its decussation in the roof 

 of the aqueduct at the lowest limit of the mesencephalon (P'ig. 959). 



Fig. 962. 



•Caudate nucleus - 



Anterior nucleus 



Thalamus 

 Ventral nucleus 



Pulvinar 

 Posterior commissure 



Pineal body ---^ 



Sylvian gray matter 



Corpora quadrigemina 

 Posterior longitudinal 

 fasciculus 



Superior 

 medullary velum 



Cerebellum 



Fourth ventricle 



Fibres of 

 abducent nerve 



Internal capsule 



External 

 medullary lamina 



Lenticular nucleus 



Stratum medullare 



hypothalaniicum 

 Tuber cinereum 



Optic commissure 



Crusta and substantia nigra 

 (latter to left) 



Red nucleus 



Fibres of oculomotor nerve 



Superior cerebellar peduncle 

 Ponto-cerebellar tracts 



Pyramidal tracts 



Mesial fillet 



Inferior olivary nucleus 



Floor of IV ventricl 



(medulla 

 Formatio reticularis 



Internal arcuate fibres 

 Nucleus cuneatus 



Nucleus gracilis 

 Posterior fasciculi 



Sagittal section of brain-stem ; plane of section is somewhat lateral to mid-line. X 



by Professor Spiller. 



Preparation 



As seen in cross-sections passing through the superior quadrigeminal bodies, the details of 

 the tegmentum differ considerably from those at the levels previously stated. The lateral fillet 

 is no longer present as a distinct field, since with the exception of a few strands that are con- 

 tinued into the superior colliculus, its fibres end within the lower colliculus or pass into the 

 inferior brachium. The median fillet now shows ( Fig. 963 ) as a somewhat attenuated crescentic 

 field, lying to the inner side of the obliquely cut inferior brachium, in consequence of many of 

 its fibres having ended within the lower part of the superior colliculus, the more dorsally situated 

 of those remaining being seen within the upper colliculus as the stratum lemnisci. 



