Fkbkuaey 4, 1898.] 



SCIENCE. 



151 



cephalad of the mesencephal comprises dor- 

 sal and ventral zones demarcated by an 

 alleged sulcus connecting the mesocele with 

 the reeessus opticus. 



1. Doubtless all members of this society 

 have discarded the anthropotomic estimate 

 of the olfactory bulbs and their crura as 

 constituting merely a ' first pair of cerebral 

 nerves.' But not all, perhaps, fully realize 

 that, notwithstanding their complete ab- 

 sence in certain adult Cetacea, in most 

 Mammals the olfactory bulbs are quite mas- 

 sive ; that in Batrachians, Reptiles and most 

 Selachians they constitute a large propor- 

 tion of the brain ; and that in lamprej'S and 

 hags they equal in size ' the cerebral hem- 

 ispheres.' 



Had the study of the vertebrate brain 

 begun with Myxine or Bdellostoma the 

 olfactory bulbs would have been unhesi- 

 tatingly assigned a rank at least equal to 

 that of either of the three following sub- 

 divisions. 



Whatever the ontogeny in a given case, 

 it is probable that phylogetically the smell- 

 ing portion of the brain preceded the re- 

 flective. 



" The revolution, so to speak, of the 

 ' hemisphere ' about the olfactory axis ac- 

 cords with other considerations which have 

 led Spitzka and the writer independently 

 to consider the prevailing idea that the ol- 

 factory lobes are mere appendages of the 

 cerebrum as nearly the reverse of the 

 truth."* 



2. The Diencephalic Flexure. With Rep- 

 tiles, Birds and Mammals, the forms with 

 which most anatomists are more familiar, 

 the first (cephalic or ' anterior ') of the series 

 of cavities seems to be the ' ventriculus ter- 

 tius '; indeed, in some Birds and Mammals 

 the recess at the root of the optic nerve 

 actually lies farthest cephalad. This con- 

 dition seems to be associated with the gen- 



* The Dipuoan Brain, American Naturalist, June, 

 1887, p. 546. 



eral crowding of the cerebrum dorsad and 

 caudad over the other parts of the brain. 

 It is discussed briefly in the Amerimn Asso- 

 ciation Proceedings, 1887, pp. 250-251 ; Amer- 

 ican Naturalist, October, 1, 1887, 914-917 ; 

 Reference Handbook of the Medical Sci- 

 ences, VIII., 112 , and Journal of Comparative 

 Neurology, VI., 128. 



The following propositions seem to me 

 warranted by the conditions in Batrachians 

 and ' fishes :' 



However numerous or sharp the dorso- 

 ventral flexures of a given brain, for com- 

 parison with other brains or with an ideal 

 schema the axis is to be regarded as 

 straight. 



Whatever its actual position, the aula or 

 mesal space between the two portas ( ' for- 

 amina of Monro ' ) constitutes the cephalic 

 member of a longitudinal series of cavities. 



From the standpoint of comparative 

 neurology the terma (^'lamina terminalis ') 

 is a constituent of the floor of the encepha- 

 lic cavities ; its dorso-ventral position in 

 Reptiles, Birds and Mammals no more con- 

 verts it into a morphologic end-wall of 

 those cavities than its dorso-caudal inclina- 

 tion in certain forms entitles it to be inter- 

 preted as a portion of the roof. 



3. In order to be entitled to rank as a 

 definitive segment must a given region ex- 

 hibit the dorsal and ventral zones of His ? 



Conceding, for the present, the constancy 

 and significance of these zones in the myel 

 (spinal cord) and in the brain as far as the 

 cephalic orifice of the mesocele (' aque- 

 duct'), are they represented in the region 

 beyond ? 



In the absence of complete developmental 

 and histologic evidence on that point, my 

 provisional answer in the negative is based 

 upon two very different considerations : 



First, the general distinctions between the 

 parts derived from the first encephalic vesicle 

 and the rest of the cerebro- spinal axis. 

 Secondly, the unsatisfactory presentation of 



