40 THE LOWER PRIMATES 



the cuneus and the chiva. Other markings on the dorsal surface, as well as 

 upon the lateral surface of the oblongata, are faint and may only with 

 difficulty be identified. 



The Hindbrain. The axial portion of the metencephalon presents 

 upon its dorsal surface the cephahc continuation of the fourth ventricle, 

 bounded upon either side by the middle and superior cerebellar peduncles. 

 The markings on the floor of the fourth ventricle are not easy to discern. 

 The median sulcus is prominent, running from the region just i^elow the obex 

 cephalad to the beginning of the iter. Well-defined striae acusticae cross the 

 floor of the ventricle at the level of the lateral recesses. Their direction from 

 their point of entry toward the midline is almost at right angles to the 

 median sulcus in the floor of the ventricle. Some fibers, however, have an 

 obliquity caudad from their point of entrance toward the midline. In the 

 inferior triangle of the fourth ventricle a shallow depression upon either side 

 of the median sulcus indicates the position of the nucleus hypoglossus, 

 lateral to which is a faintly marked fovea vagi. It is impossible to discover 

 any distinctly demarcated region corresponding to the area postrema or area 

 plumiformis. 



The cephalic triangle of the fourth ventricle is devoid of distinct mark- 

 ings, although immediately above the striae acusticae may be seen a slight 

 elevation, the eminentia abducentis. There is no evidence of a locus 

 coeruleus. 



The Midbrain. The dorsal aspect of the midbrain presents the char- 

 acteristic cfuadrigeminal plate with the median sulcus intersected at right 

 angles by the intercollicular sulcus. At the cephalic extremity of the median 

 sulcus there is a triangular expansion, the fovea pinealis, which lodges the 

 epiphysis cerebri. 



The superior colliculi are somewhat larger than the inferior colliculi, 

 but both collicular elevations are well-defined and prominent tectal features. 



