JOHNSTON ON HIND BRAIN OF ACIPENSER. 115 



6. The cells of the tuberculum acusticum send their dendrites to the 

 base of the medulla. 



7. There is a secondary tract from the acusticum which joins the 

 spinal Vth. 



8. The sensory Vllth, IXth, and Xth nerves (exclusive of lateral 

 line and spinal Vth constituents) enter a common center, the Lobos vagi. 



9. The secondary vagus tract divides into ascending and descending 

 bundles. The ascending bundle ends in the Rindenknoten as described 

 by others. The descending bundle extends into the cord. 



10. Cells of the II type are found in the Lobus vagi, the acusticum, 

 and in both layers of the cerebellum. 



11. A remarkable cell of the II type found in the valvula has 

 dendrites similar to those of the Purkinje cells and a very coarse neurite 

 with peculiar club-like thickenings. 



12. Meynert's bundles have two sets of fibres, one of which after 

 decussating ends in a nucleus dorsal to the ansiform commissure and 

 bordering on the central cavit}* at the posterior end of the base of the 

 mid-brain. The other, composed of fine fibres, probably ends, after par- 

 tial decussation, in the granular layer of the cerebellum. 



13. The Corpus interpedunculare is probably a nucleus of secondary 

 importance in connection with the bundles of Meynert. 



B. Theoretical conclusions. 



11. The structure of the sensory nerve centers in the medulla indi- 

 cates that the cranial sensory nerves are arranged in two quite distinct 

 complexes. One of these consists of the nerves supph'ing structures of 

 ectodermal origin, the Vth, Vlllth, and lateral line nerves. The other 

 consists of the nerves which supply structures of entodermal origin, the 

 Vllth, IXth, and Xth nerves. 



15. The sensory Vth, Vlllth, and lateral line nerves alone are homol- 

 ogous with the sensory roots of the spinal nerves. 



16. The tuberculum acusticum and the cerebellum are the repre- 

 sentatives in the hind brain of the dorsal horns of the cord. 



17. There is in Acipenser a spinal Vlllth tract which is probably 

 homologous with that in man. 



18. The sensory Vllth, IXth, and Xth nerves are not homologous 

 with any nerves in the trunk region. 



11). The Lobus vagi has no homologue, or only a rudimentary homo- 

 logue, in the spinal cord of the adult. 



20. The sensory roots of the cranial nerves can not be considered as 

 serially homologous with (the dorsal roots of) the spinal nerves in 

 determining the segmentation of the brain or head. The motor roots 

 alone are directty comparable to (the ventral roots of) the spinal nerves. 



21. The peculiar character of the Purkinje cell dendrites seems to be 

 due to their physiological relation with the very fine fibres of the molec- 

 ular laver of the cerebellum. 



