404 'Journal of Comparative Neurology and Psychology. 



and acustencephalon illustrates my own view of the proper bal- 

 ance to be struck between the claims of the functional systems 

 and of convenience in the recognition of topographic landmarks 

 for didactic and descriptive purposes, in a general scheme of the 

 subdivision of the brain. 



The pedunculus cerebri is a convenient, but purely artificial, 

 topographic region, including the tegmentum, substantia nigra, 

 basis pedunculi. III nerve, etc. — in short all of the mesencephalon 

 of the BNA after the subtraction of the colliculus superior and col- 

 liculus inferior (or the single colliculus of lower vertebrates where 

 there is but one). It will doubtless be found possible to subdivide 

 it or distribute it to neighboring regions, but at present it may be 

 better to use this term as a makeshift than to propose a new sub- 

 division, on the basis of our very imperfect knowledge of the com- 

 parative anatomy of this part of the brain. 



The nomenclature of the diencephalon, after the separation of 

 the optic centers to form the ophthalmencephalon, requires very 

 little modification of existing usage. The term rnedithalamus 

 may be applied to such parts of the thalamus and metathalamus 

 (BNA) as remain after subtraction of the optic centers (pulvinar, 

 geniculatum laterale, etc.) The term hypothalamus- may be 

 retained for all of the region so designated in the BNA save the optic 

 chiasma. The epithalamus remains as defined by the BNA. The 

 medithalamus is a derivative of the central gray of the first, sec- 

 ond and third neuromeres. The hypothalamus includes the hypo- 

 physis and important correlation centers, chiefly of visceral sen- 

 sation (olfactory and gustatory). The ^/)z if Aa/*^^^/ includes the 

 epiphysis and important olfactory centers in the habenula. 



The exact boundaries of these thalamic subdivisions for verte- 

 brates in general cannot be defined in the present state of our knowl- 

 edge. Johnston has discussed the limits of these regions in lower 

 vertebrates in his text book,^ to which the reader is referred for 

 a summary review of our present knowledge regarding this diffi- 

 cult subject. Much more extensive comparative study will be 

 necessary before attempting anything but a provisional analysis 

 of the thalamus region and classification of its nuclei and fiber 

 tracts. 



The telencephalon is well named. It is terminal, not only in 

 position, but also in point of time, having been added relatively late 



^ Johnston, J. B. The nervous system of vertebrates, Philadelphia. 1906, chapter 17; 



