82 THE LOWER PRIMATES 



is supported not only by the deep pontile layer, but also rests upon the two 

 buttresses which form laterally and mesially in the pontile gray matter. It 

 is dense and heavy in lemur and is connected mesially to its fellow of the 

 opposite side by means of the interpeduncular gray matter which appears to 

 be quite undiflerentiatcd. This interpeduncular gray matter is continuous 

 with the structures forming the hypencephalon. In the lateral portion of the 

 substantia nigra there develops a discrete nucleus containing many tangled 

 nerve fibers which pass dorsally into the tegmentum. The substantia nigra 

 gradually attenuates and disappears. It is not continuous with any definite 

 gray mass in the thalamic region of the brain. 



The Nucleus Ruber 



The reconstruction of this nuclear mass, so predominant in sections of 

 the mesencephalon in man and the higher primates, is relatively insignificant 

 in size. The nucleus is located in the most cephalic portion of the mesenceph- 

 alon extending into the reticular formation of the diencephalon. It is 

 relatively indistinct and poorly demarcated from the surrounding tissue by 

 the encapsulating fibers of the superior cerebellar peduncle. 



The Central Gray Matter 



In the upper regions of the spinal cord this nuclear mass is roughly 

 quadrilateral, receiving the bases of the two dorsal gray columns and ventro- 

 laterally the two ventral gray columns. Passing upward, the central gray 

 matter receives the bases of the developing nuclei of Goll and Burdach and 

 then gives ofl" dorsally a narrow, tongue-like extension which passes along the 

 dorsal median septum. As the caudal extremity of the fourth ventricle is 

 approached, the entire central gray matter migrates dorsally, carrying 

 the central canal with it, and as the dorsal gray columns begin to diverge, the 

 quadrilateral body of gray matter gradually flattens dorsoventrally and 



