THE MEDULLA. 99 



They have already been presented as Fihrce arcuatm internee (Figs. 43 and 

 43). This tract, which reaches full deyelopment only in the mammals, 

 is probably already present among fishes. It is a portion of the great central 

 fiber-system from the sensory terminal miclei. After decussation it passes 

 toward the brain in the fillet, and with it are gradually associated all of the 

 crossed bundles from the nuclei of the cranial nerves, — ^the Tractus tecto- 

 nueleares, — together forming the fillet. 



The second important fiber-system belongs to the lateral margin of the 

 medulla. Here lies the Tractus cerebello-spinalis, ventral from the spinal 

 root of the Trigeminus. It arises, also, from the termini of the sensory 

 nerves in the spinal cord. It does not turn toward the fillet, however, but 

 passes anteriorly to the exit of the Acusticus, where it turns dorsally to the 

 cerebellum. On the way it has received reinforcements from the posterior 

 columns, — the Fihrce arcuatce externce, — demonstrated in fishes, amphibians, 

 and birds. The united bundle is called the Corpus restiforme: inferior 

 peduncle of the cerebellum. It has been carefully studied only in mammals; 

 in this class it contains still other elements. Only a few of them are found 

 also in birds and reptiles: e.g., the aeusticus-eerebellar tract and the bundle 

 from the Deiter nucleus to the spinal cord. 



Where an olivary body is demonstrable the fibers run from it to the 

 cerebellum through the inferior peduncle restiforme. 



At the frontal end of the medulla the Tractus tecto-bulbares et spinales 

 turns dorsally to reach the roof of the midbrain. At this point a nucleus 

 is always located within it (Fig. 50), — ^the tegmental nucleus, — which is 

 much subdivided. The tracts to the cerebellum have, at this point, already 

 turned upward; those which pass to and from the cranial nerves are present 

 only in small measure. What remains of the features mentioned is confined 

 to tracts which pass downward from the cerebellum and then to tracts which 

 pass toward the medulla from the mesencephalon and the thalamencephalon. 

 They lie in the ventral divisions, and are not yet, with certainty, to be differ- 

 entiated from the fillet in the lower vertebrates. Besides this, the Fascic- 

 ulus longitudinalis posterior lies dorsally and distributed over the whole 

 breadth of the lateral areas, — the system of commissure cells with short 

 bundles joining the different levels of the medulla with the floor of the mid- 

 brain. 



In mammals — ^but, so far as I know, only in these — still another tract 

 descends from the cerebrum, which remains in part in the frontal ganglia 

 to be described later and in part ends in the nuclei of the cranial nerves 

 with their final processes even in the spinal cord, where we have already 

 learned to know it as Tractus cortico-spinalis. 



We may now close our survey of the structure of the Medulla Ob- 

 longata. Now that it is shown what important nuclei of origin and termina- 



