THE CEREBRUM 



161 



of gray matter devoted to motor coordination, such as the black 

 substance (substantia nigra) and the red nucleus (nucleus ruber, 

 see p. 189). 



The diencephalon (betweenbrain or thalamencephalon) in 

 early embryonic development is a transverse region of the simple 

 neural tube (Fig. 48, p. 117) surrounding the third ventricle. In 



Tectum mesencephali 

 -"Commissure tecti 

 Nuc. and tr. mesen. V 

 Tr. spino-tectalia 

 . thalamo-olivaris 

 lemniscufl 



Tractus optic 

 Brachium quad 



Dora. tegm. 

 decuss. 



Vent. tegm. 

 decuss. 

 Tr. rubro- 

 spinalia 



Tr. cortico- 

 bulb. lat. 



Tr. mam.- 

 pedunc. 



Tr. cortico 

 bulb. med. 



Fig. 75. Diagrammatic cross-section through the midbrain at the level 

 of the superior colliculus (cf. Fig. 71), to illustrate the arrangement of the 

 chief conduction pathways: Aq., Aqueduct of Sylvius; m., medial part of 

 motor nucleus of oculomotor nerve; n.III, oculomotor nerve; nuc.III, 

 motor nucleus of oculomotor nerve; Tr.mam.-pedunc., tractus mamillo- 

 peduncularis. The fibers of the dorsal tegmental decussation (Dors. 

 tegm.decuss., also known as the fountain decussation of Meynert) arise 

 from the roof of the midbrain (tectum opticum) and immediately after 

 crossing the median plane descend toward the spinal cord, where they form 

 part of the tractus tecto-spinalis (Fig. 59, p. 130). The fibers of the ventral 

 tegmental decussation (Vent. tegm. decuss., also known as Forel's decussa- 

 tion) in a similar way arise from the nucleus ruber and enter the opposite 

 tractus rubro-spinalis. 



the adult human brain, however, it is entirely concealed by other 

 parts. The posterior part of it is visible from the side in the 

 dissection shown in Fig. 45 (p. 114), its medial surface in Fig. 

 52 (p. 119), and its dorsal surface is exposed in the dissection, 

 Fig. 76 (see also Fig. 77). This part of the brain is devoted 

 11 



