898 



THE NERVE SYSTEM 



lamina quadrigemina dorsad from the tegmental zone. Its shape on trans- 

 section varies at different levels, being T-shaped caudad, oval or quadrangular 

 along its middle, and triangular cephalad. It is lined by the ependyma (columnar 

 ciliated epithelium) and surrounded by the central aqueduct gray. The central 

 gray is separated dorsally from the corpora quadrigemina by the stratum lemnisci; 



NUCLEUS OF 



OCULOMOTOR 



NERVE 



INF. BRACHIUM 



MEDIAL LONGI- 

 TUDINAL FASCIC- 

 ULUS 



OCULOMOTOR 

 NERVE 



FIG. 660. Trans-section of the mid-brain through the level of the superior quadrigeminal body. 



ventrad near the median plane lie the medial longitudinal bundles. Within the 

 gray substance lie certain well-defined cell clusters, the nuclei of origin of the 

 oculomotor and trochlear nerves and the mesencephalic root of the trigeminal 

 nerve. These will be described in detail later. 



The substantia nigra or intercalatum is a crescentic layer of deeply pigmented 

 gray substance interpolated between the crusta and the tegmentum. Mesad it 

 nearly touches its fellow of the opposite side, being separated by the rudimentary 



MESENCEPHALIC ROOT 



OF TRIGEMINAL NERVE 



NUCLEUS OF 

 TROCHLEAR NERVE 

 LATERAL LEMNISCUS 

 MEDIAL LONGITUD- 

 INAL FASCICULUS 



DECUSSATION OF 

 CEREBELLAR FED' 

 UNCLES 



MEDIAL 

 LEMNISCUS 



FIG. 661. Trans-section of the mid-brain through the level of the inferior quadrigeminal body. 



ganglionic gray (the posterior perforated substance or postperforatum) in the inter- 

 crural space. Its ventral face sends numerous ramifying prolongations among 

 the fasciculi of the crusta. It extends from the cephalic border of the pons to 

 the subthalamic region, while its lateral edge reaches the surface along the lateral 

 sulcus. Its cells are medium-sized, multipolar, their bodies approaching the 



