Chapter IV 



RECONSTRUCTION OF THE GRA\^ MATTER IN THE 

 BRAIN STEM OF TARSIUS SPECTRUM 



"^HE reconstruction of the gray matter of tarsius presents a constitu- 

 tion more elemental in character than that encountered in any other 

 species of the primate series. In general contour the reconstruction is 

 distinctly pyriform, beginning with the small dimensions at the most cephahc 

 of the cervical levels and expanding rapidly in the medulla, mamtaining a 

 uniform width in the pons and midbrain, but presenting at the upper portion 

 of the midbrain near its junction with the diencephalon a marked increase in 

 its lateral development. Viewed from the side the reconstruction has a 

 singularly Hat appearance on the dorsal surface which continues upward to 

 the junction of the metencephalon with the mesencephalon. The ventral 

 contour shows a gradual but continued increase in the dorsoventral dimension 

 from the medulla to the isthmus. At the junction of the metencephalon with 

 the mesencephalon, the dorsoventral diameter of the reconstruction approx- 

 imately doubles itself by the sudden appearance of the colliculi which rise 

 dorsally in a precipitous, almost palisade-likc outgrowth from the dorsal sur- 

 face of the neuraxis. The collieuli almost immediately assume their extreme 

 vertical diameter, producing a right angle transition between the caudal 

 limit of the collicular plate and the plane presented by the floor of the fourth 

 ventricle. This space, empty in the reconstruction, is occupied by the mass of 

 the cerebellum which lies in contact with the collicular plates throughout the 

 major portion of its cephalic surface. 



The geniculate bodies make their appearance at a level unusually low in 

 the brain stem, taking on recognizable form on the lateral aspect of the 

 mesencephalon. The mesial geniculate body, developing first at a point 



