ipo THE LOWER PRIMATES 



it is in contact with the dorsal undillcrciitiatcd ^ray matter surrounding the 

 ventricle and the Sylvian aqueduct, while its lateral extremity appears to 

 rest upon the lateral and dorsal extensions of the reticular formation. The 

 inferior colliculus is c|uite massiAC and presents a marked expansion on the 

 dorsal surface of the mesencephalon. It is separated from the superior collic- 

 ulus by a narrow groove in which the reticular formation underlying the 

 inferior colliculus comes to the surface. The superior colliculus is a massive 

 structure and presents a well-marked surface elevation on the dorsal aspect 

 of the mesencephalon. It also is continuous mesially with the unditlerentiated 

 dorsal gray matter. Laterally it comes into close relation with the inferior 

 brachium and ventrally with the dorsal extensions of the reticular formation. 

 Cephalically the superior colliculus does not make contact with the thalamic 

 nuclei, being separated from them by a dorsal extension of the reticular 

 formation. 



The Central Gray Matter 



In the reconstruction, the sheet of gray matter in the floor of the fourth 

 ventricle seems featureless and is disposed as a ilat, smooth stratum. The 

 underlying structures produce no marked prominences or depressions in its 

 surface. As the midbrain is approached, the lloor of the ventricle decreases 

 in size, the side walls and roof approach each other and thicken materially so 

 that when the aqueduct of Sylvius is formed, the lateral walls and roof 

 present subependymal gray matter fully as bulky as the gray matter of the 

 floor of the aqueduct. Cephalically the central gray matter is continuous with 

 the gray matter underlying the ependymal lining of the third ventricle, 

 forming the mesial group of the thalamic nuclei. The dorsal gray matter is 

 continuous with the epithalamic structures, the ventral gray matter with 

 the hypencephalic region. 



