REGIONAL ANALYSIS 45 



cally. The auricle, accordingly, is here regarded as the common 

 primordium of several structures which in higher animals are di- 

 versely specialized for different functions. The most notable of these 

 are the superior or pontile nucleus of the trigeminus and the floccular 

 part of the flocculonodular lobe of the cerebellum. The primordia of 

 these structures are clearly evident, and the history of their further 

 differentiation in higher animals has been written. 



Efferent fibers of tractus cerebello-tegmentalis leave all parts of the 

 cerebellar complex for the underlying gray; and one fascicle of 

 these — the brachium conjunctivum — passes forward to a ventral 

 decussation and distributes its fibers to the isthmic tegmentum of 

 both sides (figs. 10, 71). No primordium of the nucleus ruber or of the 

 inferior olive has been recognized. 



This primitive cerebellum exhibits the typical vertebrate pattern 

 in very instructive form, with localization of the vestibular system 

 laterally and the other systems medially. It is an appendage added 

 to the basic sensori-motor systems; it supplements them, not as an 

 aid in determining the pattern of performance, but to insure efficient 

 execution. In species in which it is greatly enlarged, it contains 

 enormous reserves of potential nervous energy, which is released dur- 

 ing motor activity to reinforce and stabilize the operation of the 

 effectors. For additional details see chapters x and xii. 



5. ISTHMUS 



The isthmus is unusually large in urodeles and is clearly circum- 

 scribed from surrounding parts. Dorsally it is small, containing in 

 and near the superior medullary velum a special segment of the 

 mesencephalic V nucleus and probably other peripheral connections 

 through the nerves of the chorioid plexuses and meninges. Below this 

 there is the superior visceral-gustatory nucleus (figs. 2B, 8, 23, 34). 

 The nucleus isthmi, which is large in the frog, is here undifferentiated. 

 The ventral part of the isthmus is very large, containing the nucleus 

 of the IV nerve and a mass of tegmental cells. This isthmic tegmen- 

 tum is interpolated between the primary sensori-motor systems of the 

 medulla oblongata and midbrain, and it serves as an intermediary 

 between them. There is a large central nucleus of small cells which 

 receives fibers from a wide variety of correlation centers of inter- 

 mediate-zone type. These enter by all the dorsal tegmental fascicles 

 and by several other paths (figs. 16, 17, 21). This nucleus is enveloped 

 by a group of larger cells, which is continuous posteriorly with similar 

 tegmental cells of the region of the trigeminus (figs. 29, 30, 91). The 



