622 THE HIGHER ANTHROPOIDS 



lateral to this is a triangular depression which corresponds to the underlying 

 dorsal nucleus of the vagus. 



The \entricle as it opens becomes more cxtensi\e from side to side. 

 In the mid- ventricular portion the markings become more evident, showing 

 the outline of such underlying nuclear masses as the vestibular and cochlear 

 complexes. Abo\'c the mid-point of the ventricle as it begins to narrow to\\'ard 

 theaqueduct of Sylvius a marked median eminence appears on both sides of the 

 midline, representing the eminentia abducentis and the subventricular course 

 of the seventh cranial nerve. 



Above the lateral recess, the walls of the ventricle rapidly approach each 

 other and the formation of the acjueduct of Sylvius results m the develop- 

 ment of a massive central gra}- matter encircling the ventricular cavity. 

 In the isthmus the nucleus of the fourth cranial nerve appears m this ventral 

 prolongation of the central gray matter. \\ ith but little distinction between 

 the nucleus trochlears and the nucleus oculomotorius above it, the latter 

 nucleus occupies this same ventral prolongation. As the diencephalon is 

 approached this prolongation becomes more extensive until it is continuous 

 with the interpeduncular gray matter, thus forming a complete griseal 

 lamina between the midline and the mesial surface of the red nucleus. 



The central gray matter continues upward to the junction of the 

 mesencephalon and the diencephalon, where it becomes continuous with the 

 subependymal gray matter of the third ventricle. It merges with the ill- 

 defined mesial group of the diencephalic nuclei, the epithalamic structures 

 and the hypencephalic region. 



