400 'Journal of Comparative Neurology and Psychology. 



transverse segment of the neural tube, but as a dorsal structure 

 which reaches down into the lateral walls and floor of the older 

 brain stem like a girdle. The metencephalon should, therefore, 

 include the cerebellum, pons, corpus restiforme, brachium con- 

 junctivum and some of the nuclei in immediate contact with these 

 structures. It should not include the longitudinal conduction 

 paths in the brain floor above the pons, nor the nuclei of the cranial 

 nerves of the same region. 



The term medulla oblongata should be applied to that part of 

 the rhombencephalon lying between the spinal cord and the isth- 

 mus, exclusive of the parts here enumerated as belonging to the 

 metencephalon. That portion 'of the medulla oblongata lying 

 caudad of the VIII nerves and their chief primary nuclei may be 

 called the myelencephalon (which is practically the usage of the 

 BNA), thus limiting this latter term to the region of the typical 

 gill bearing segments in the true fishes. If a distinctive name is 

 required for the preauditory part of the medulla oblongata coordi- 

 nate with myelencephalon for the postauditory part, the term pars 

 facialis medulla; may be suggested. The region so designated 

 includes that portion of the brain stem (exclusive of the meten- 

 cephalon as I here define it) comprised approximately within the 

 metencephalon and isthmus rhombencephali of the BNA, a region 

 which receives the cerebral nerves of the skin and muscles of the 

 face and facial skeleton. 



The medulla oblongata, as here defined, extends sufficiently 

 far forward to include the roots and chief nuclei of the V, VI and 

 IV cerebral nerves and the superior secondary gustatory nucleus 

 (nucleus visceralis cerebelli, Johnston). It is bounded rostrad 

 by a constriction, the isthmus, which marks the adult position 

 of the groove between the embryonic second and third cerebral 

 vesicles. 



The subdivision of the isthmus region is very difl&cult. I be- 

 lieve that the use of that term for an encephalic region cannot be 

 justified. The word is, however, a convenient descriptive term 

 for the constriction in question and if retained in our nomenclature 

 at all it should be used only in that sense. 



Rostrad of the rhombencephalon the evidence of the primary 

 metamerism has almost entirely disappeared from the adult human 

 brain. In lower brains, even in their embryonic conditions, it is 

 very difficult to decipher the vestiges of metamerism in these regions. 



