236 THE LOWER PRIMATES 



as the ventral gray columns diminish and linally the latter disappear by 

 merging into the former at the level of the inferior ohvary nucleus. 



The reticular formation furnishes the same matrix for the ascending and 

 descending fiber tracts found in the two preceding forms. It is surrounded on 

 all sides, except the dorsal, by the ascending and descending fiber bundles of 

 the stem. It is connected laterally with the mesial surface of the substantia 

 gelatinosa trigemini and dorsaliy with the bases of the nuclei of Goll and 

 Burdach. Embedded in it ventromesially is the dorsal lamina of the inferior 

 ohvary nucleus together with the dorsal accessory olivary nucleus. As the 

 reticular formation proceeds upward in the stem it gradually increases 

 in size until it assumes considerable proportions. The nucleus lateralis of the 

 reticular formation and the superior olive are developed to a somewhat 

 greater extent than in lemur and marmoset. In the region of the midbrain 

 the pronounced nuclear condensation of the reticular formation appears in 

 the form of the nucleus ruber which is developed to a somewhat greater 

 extent in this form than in either of the preceding types. In the dorsal aspect 

 of the reticular formation are successively embedded the nuclei of the mesial 

 somatic motor cell column, namely, the hypoglossus nucleus in the region of 

 the medulla, the nucleus abducentis in the midpontile region, the nucleus 

 trochlearis and the nucleus oculomotorius in the mesencephalic segment. 

 In the dorsolateral angle, between the subependymal gray matter and 

 the cephalic extremity of the nucleus of Burdach, appears a condensation in 

 the reticular formation which gives rise to the vestibular complex. The 

 nucleus of Deiters separates the subependymal gray matter and the nucleus 

 of Burdach, reaching its maximum diameter at a level somewhat above the 

 midventricular level of the stem. At this point the triangular nucleus of 

 Schwalbe appears, and continues upward in its characteristic position. 

 The nucleus of von Bechterew lies lateral and dorsal to the rest of the 

 vestibular complex in the lateral wall of the fourth ventricle. The reticular 



