696 THE HIGHER ANTHROPOIDS 



to pass inti) llu' nic'sciUTi:)halic tc<z;iiu'iituni. D()rsall\- it is concaxc and sepa- 

 rated from the reticular formation bv liber bundles of the tegmentum. 

 Cephalieally it extends upward to the junction !)et\veen ttie mesenceijhalon 

 and diencephalon and seems to be continuous with the zona incerta of the 

 interbrain. 



The Nucleus Ruber 



The nucleus ruber appears in the lower portion of the mesencephalic 

 reticular formation as a rounded nuclear mass, separated from the reticular 

 formation by an encapsulation deri\ed trom the superior cerebellar ]:)tclun- 

 cle. It is almost spherical in outline and rapidly attains its maximum diam- 

 eter, occupying the greater portion of the mesencephalic tegmentum. Passing 

 its level of greatest transverse diameter, the nucleus diminishes and disap- 

 pears as the mesencephalon begins to assume the characteristics of the dien- 

 cephalic portion of the brain stem. 



The Central Gray M.\tter 



The central gray matter in the high cervical sections of the spinal cord 

 resembles closely that of the human brain stem, being quadrilateral and 

 meeting at its dorsolateral angles the dorsal cornua and at the ventrolateral 

 angles the ventral gray columns. It gradualls' is drawn dorsally as has been 

 found in the lower forms, giving origin to the dorsal sensory nuclei. As the 

 oblongata is approached the usual narrow |)rolongation ot gray matter passes 

 dorsally around the mesial surfaces of tht' two tracts of the column of Coll, 

 seeming, as it were, to draw the central gray matter backward toward the 

 dorsal surface of the spinal cord. In this way the central gray matter is llat- 

 tened out and forms the floor of the fourth ventricle. As the lloor opens out- 

 ward the h\ poglossal eminenct' situated close to its median sulcus and m tin- 

 most caudal part of the inferior \entricular triangle becomes apparent. 



