346 THE INTERMEDIATE PRIMATES 



The Central Gray Matter 



The central gray matter in the baboon is first found as a transversely 

 arranged mass of gray matter which laterally receives the base of the cervix 

 of the dorsal horn. Dorsally the central gray matter presents the formation 

 of tile nucleus of the cohimn of GoII which has ah"eady been described as 

 narrow laminae of graj' matter in the mesial dorsal white column, while 

 the lateral extension of the gray matter forms the dorsal horns. From the 

 point of confluence of the dorsal horn with the central gray matter of the 

 reticular formation, the nucleus of the column of Burdach arises. As the stem 

 is followed upward, the lateral trend of these gray masses becomes evident, 

 the substantia gelatinosa trigemini passing outward into the lateral meridian 

 of the cord, the nucleus of the column of Goli passing laterally and con- 

 forming to the opening of the lower extremity of the fourth ventricle, while 

 between the nucleus of the column of Goll and the substantia gelatinosa 

 trigemini appears the nucleus of the column of Burdach. 



The body of the central gray matter rapidly flattens out into a narrow 

 band of gray matter which forms the lloor of the fourth ventricle. This 

 floor is practically featureless, being, as in the other forms, almost smooth 

 in the lower half of the ventricle. It gradually increases in its lateral extent 

 until the level of the lateral recess is reached, from which point it then begins 

 to contract. 



The floor of the upper half of the fourth ventricle presents the medial 

 elevation corresponding to the development of the nuclear mass forming 

 the nucleus of the sixth cranial nerve, the nucleus abducentis. The walls 

 of the upper half of the ventricle rapidly approach each other and at 

 the junction of the metencephalon and the mesencephalon, the central 

 gray matter again constitutes the relatively heavy wall surrounding the 

 aqueduct of Sylvius. Ventrally this central gray matter is continued for- 

 ward into a relatively long tongue of gray matter which passes forward in 



