H^'LOBATES HOOLOCK, THE GIBBON 439 



where the large space of the foLirth \entricle is reduced as it approaches the 

 aqueduct of Sylvius. The central gray matter (Ccn) surrounds the ventricle 

 whose roof is formed by the superior medullary velum in which the fibers of 

 the trochlear nerve (N4 ) undergo complete decussation. These libers subse- 

 quently emerge m relation with the dorsal aspect of the isthmus. In the 

 central portion of the central gray matter (Cen) are the dense bundles 

 constituting the fasciculus longitudinalis posterior (PL) and the fasciculus 

 predorsalis (PD). Lateral to the central gray matter are the fibers con- 

 stituting the superior cerebellar peduncle (Sep) preparatory to their 

 inward dellection toward their decussation in the midbrain. Ventral to these 

 libers on the circumference of the axis is a mass of medullated axons forming 

 the lateral fillet (Lf). At the boundary line between the tegmentum and 

 basis are the transversely disposed bundles of the mesial lillet ( Mf), while 

 the general arrangement of the pons Varolii in its three characteristic 

 layers appears ventral to this boundary. The pons here contains the stratum 

 superficiale, the stratum complexum and the stratum profundum lying 

 immediately ventral to the tegmental portion of the axis. 



LEVEL OF THE INFERIOR COLLICULUS (fIG. 2o8) 



Here the cross section shows further modification by the appearance of 

 the two dorsal elevations constituting the primordial stations for the auditory 

 pathway. These colliculi (IC) form a prominent elevation in the quadri- 

 geminal plate of the midbrain and show a degree of stratification which harks 

 back to then" highest specialization in the lower vertebrates. Entering into 

 the inferior colliculus are the collected bundles of the lateral fillet (Lf) 

 which serves to convey impulses from the lower relay stations in the pathway 

 of hearing. The central gray matter ( Cen) completely surrounds the aque- 

 duct (jf Sylvius. In its ventromesial pcjrtion is the caudal extremity of the 



