OF DENISON UNIVERSITY. 
8l 
is not a morphological but simply a mechanical connection between 
them. 
3. The ganglion of the olfactory, if the latter be a true sensory 
nerve, should have chiefly trophic functions. There are several hints 
that the olfactory nerve is trophic. The fact that the fibres of the fifth 
or seventh nerve may upon occasion supply the place of the olfactory, 
if authenticated, might be significant in this connection. 
4. The structure of the pero, with its large cells, might also be 
interpreted in this way. The fact that the fibres lose their sheaths in 
their passage through the pero, but acquire them at either exit seems 
favorable to this view. 
5. The facts of comparative anatomy seem to us to admit of this 
interpretation. Of course a more careful and extensive comparison 
of data especially from embryology is necessary before the view thus 
tentatively suggested could be seriously advocated. 
[It may be added that a somewhat extended study of the olfactory 
radices in lower vertebrates seems to confirm the above suggestions. 
In several papers in the Journal of Comparative Neurology, for 1891, 
the writer has shown the essential distinctness of pero and pes and the 
tracts related. It is especially evident in fishes where a distinct radix 
lateralis and mesalis relate the one to the pero the other to the pes, the 
former ending in the hippocampus, the latter entering the praecommis- 
sura.] 
Callosimi and hippocampal commissure. It is not necessary to re- 
count the various opinions and discussions of the callosum in the mar- 
supials. Until Osborn, most authors had agreed that the callosum is 
absent and functionally replaced by the anterior commissure. Pro- 
fessor Osborn has done much to place this whole subject in its proper 
light and we agree with him in respect to the essential homologies of 
the dorsal commissural system. In one group of fishes the callosum 
is present, as we have endeavored to prove in several recent papers, 
and is thoroughly distinct from the anterior commissure. The incom- 
plete development of the cerebrum and especially the suppression of 
the free cortex causes the callosum to appear greatly displaced and it 
accordingly lies far cephalad in contiguity with the lamina terminalis 
in front and fornix body behind. We have too much evidence that 
structures pertaining to the brain when once acquired are not easily 
lost to be surprised if the callosum in some form exists in all verte- 
