RECONSTRUCTION OF LEMUR iMONGOZ 83 



the central canal emerges dorsally into the fourth ventricle, thus disposing the 

 central gray matter as two lateral halves connected across the midhne. This 

 lateral displacement continues until at last the gray matter becomes flattened 

 out in an ahiiost transverse plane as the floor of the fourth ventricle. From 

 the edges of the lateral walls (;f the fourth ventricle the ependymal lining 

 continues across the roof of the fourth ventricle which caudaliy is formed by 

 the inferior medullary velum. In this locality the roof is invaginated by the 

 chorioidal plexuses of the fourth ventricle and here the layer of central gray 

 matter, which at best is extremely attenuated on the roof of the fourth 

 ventricle, practically ceases to exist. 



In the midpontile portion of the fourth ventricle the cerebellum enters 

 into the roof of the ventricle and cephalad to the cerebellum the superior 

 medullary velum contains the superior cerebellar peduncles. Approaching 

 the upper portion of the metencephalon the lateral walls gradually become 

 convergent until they meet just caudad to the inferior colliculus. At this 

 point the fourth cranial nerves decussate in this reconstituted roof of the 

 ventricular cavity. The lateral walls of the fourth ventricle are formed from 

 below upward by the nuclei of the columns of Goll and Burdach, the nuclei 

 of Deiters and Schwalbe and the inferior cerebellar peduncle. The lateral 

 walls fail at the lateral recesses but appear again as the middle cerebellar 

 peduncle. Cephalically the superior cerebellar peduncles lie in the rapidly 

 converging lateral walls of the cephalic portion of the fourth ventricle. 



The floor of the fourth ventricle is comparatively smooth and presents 

 but little modelling. The underlying nuclear masses and hber tracts produce 

 scarcely any impression on the floor which is stretched evenly from side to 

 side and from below upward. It is in general diamond-shaped, beginning 

 from the opening of the central canal caudaliy, spreading to its greatest width 

 at the level of the lateral recesses and then rapidly narrowing as the aqueduct 

 of Sylvius is approached. 



