PAPIO CYNOCEPHALUS 303 



portions of the animal. This influx is certainly much inferior in vohmie to that 

 of such forms as are possessed of definitely prehensile tails, like mycetes, spider 

 monkeys and woolly monkeys. The tail in baboon, when developed to any 

 extent, has its importance as a steering or balancing organ, a function much 

 less exacting than that requiring the discriminative elements of sensibiHty 

 possessed by a prehensile organ. 



Further cephalad, the dorsal surface of the axis presents the meten- 

 cephalon and the cephaHc continuation of the fourth ventricle, which is 

 bounded upon either side by high elevations caused by the middle and supe- 

 rior cerebellar peduncles. In the floor of the fourth ventricle the markings 

 are fairly conspicuous. The median sulcus extends from the obex to the 

 beginning of the iter and is well dehned. It is crossed at about its middle 

 by marked striae acusticae entering the floor of the ventricle at the level of 

 the lateral recess. The inferior triangle of the fourth ventricle contains a 

 depression upon either side of the median sulcus indicating the position of 

 the nucleus hypoglossus, lateral to which the fovea vagi appears in plain 

 outline. It is diflicult to discern any specialization corresponding to the 

 area postrema or the area plumiformis. In the cephalic triangle of the 

 fourth ventricle the markings are less distinct, although immediately above 

 the striae acusticae may be seen a slight elevation, the eminentia abducentis. 

 There is no region in this portion of the ventricle which may be identified 

 as the locus coeruleus. The elevations of the vestibular area are fairly 

 well marked. 



The midbrain along its dorsal aspect presents a characteristic quad- 

 rigeminal plate with the median sulcus intersected at right angles by the 

 intercollicular sulcus. At the cephalic extremity of the median sulcus is 

 the triangular depression, the fovea pinealis, in which is lodged the epiphysis. 

 The quadrigeminal plates are well developed, the superior colliculi being 



