Herrick, Cerebrum of the Opossum. 7 



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.] 



Callosum and hippocatnpal eoinniissure. 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 dx)rsal 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- 



