REGIONAL ANALYSIS 47 



lar neuropil and continue from the nucleus in two strands (figs. 83, 

 84). The ventral interpedunculo-bulbar tract descends beyond the 

 nucleus for an undetermined distance in the lip of the ventral fissure. 

 The dorsal tract descends dorsally of the fasciculus longitudinalis 

 medialis and turns laterally to end in wide arborizations in the 

 tegmentum as far back as the IX nerve roots. Associated with these 

 dorsal fibers are interpedunculo-tegmental fibers, which end in the 

 neuropil of the isthmic tegmentum. The dorsal and interpedunculo- 

 tegmental fibers are regarded as comparable with the isthmic and 

 bulbar parts of the mammalian f. longitudinalis dorsalis of Schiitz. 



The physiological problems suggested by this peculiar structure 

 are puzzling. In the light of such scanty experimental evidence as we 

 possess, I have ventured to suggest that the interpeduncular complex 

 provides both activating and inhibitory components of reflex and 

 general integrative activities, the actual patterns of which are else- 

 where determined. 



Topographically, this nucleus lies in the motor zone, but its func- 

 tions clearly are of intermediate-zone type. It is present in all verte- 

 brates at the anteroventral border of the isthmus, that is, at the 

 boundary between cerebrum and rhombencephalon. Most of its 

 afferent fibers come from the cerebrum, and evidently it serves chiefly 

 as an intermediary adjustor between the sensory and intermediate 

 zones of higher levels and the motor zone of the rhombic brain (for 

 details see chap. xiv). 



At this point in our analysis we. cross the boundary between 

 rhombencephalon and cerebrum. The radical differences in structure 

 and physiological properties of these two chief parts of the brain are 

 masked and in large measure overruled, especially in higher animals, 

 by ascending and descending connectives, of which the interpeduncu- 

 lar system is a typical illustration. 



7. TECTUM AND PRETECTAL NUCLEUS 



The tectum and the pretectal nucleus, as sectors of the sensory 

 field, together with the dorsal thalamus, form a physiological unit 

 within which all exteroceptive sensory systems are integrated in the 

 interest of cerebral control of all lower sensori-motor systems in- 

 volved in the operation of the skeletal musculature. This unit is 

 intimately related with the cerebral peduncle and ventral thalamus. 

 In the most primitive vertebrates and in early embryonic stages of 

 all vertebrates, these structures might appropriately be united as a 

 middle subdivision of the brain, which serves as the dominant center 



