420 THE INTERMEDIATE PRIMATES 



acusticae, a large elevation appears in the floor marking the vestibular area. 

 This fact accords well with the exquisitely arboreal locomotion of the gibbon 

 and reveals one of the most important regulating mechanisms which makes 

 possible its almost bird-like passage through the forest. The median fissure 

 becomes somewhat deeper as the caudal (jrihce of the Sylvian aqueduct is 

 approached, while the superior cerebellar peduncles, formmg the lateral 

 boundaries of the ventricle, converge toward the acjueduct. Immediately 

 above this orifice of the aqueduct the trochlear nerve emerges from the brain 

 stem after decussation in the superior medullary velum. At this level appear 

 the characteristic developments in the roof of the midbrain, the collicular 

 eminences. Of these, as in other primates, the inierior colliculi are the smaller. 

 However, the disparity m size between these two sets of colliculi in gibbon 

 is not so marked as in the macacus or baboon. This modification appears to 

 be clue to the fact that the superior colliculus has lost somewhat in promi- 

 nence as compared with the other two intermediate primates. The correctness 

 of this observation gains support from the more complex development of 

 the occipital lobe in gibbon when compared with the baboon or macacus, 

 thus implying a still further delegation of ^•isual function to the occipital 

 lobe than in either of the other primates mentioned. 



The Pons Varolii. Upon the ventral surface of the axis the pons 

 Varolii appears as a fairly prominent transverse structure, separated from 

 the oblongata by a well-defined bulbopontile sulcus. It is also separated at 

 its cephalic extremity from the cerebral peduncles by the pedunculo-pontile 

 sulcus. The pons in the gibbon gives the impression of a greater degree of 

 prominence than in either the baboon or macacus. The inference based upon 

 the pontile appearance accredits the gibbon with the possession of skilled 

 movements more complex than either the baboon or the macacus. 



The Ventral Surface of the Brain Stem. The brain steni upon its 

 ventral surface terminates cephalically by the typical divergence of the two 



