6 Journal of Comparative Neurology. 



Univ., Vol. VI,) that there is no direct connection between the tractus 

 prsecommissuralis cruris, or internal olfactory tract, and the olfactory 

 substance. On the other hand, the fibres of the tract seem to be 

 largely derived from the cortex cruris and its forward extension into 

 the pes. Of this relation we have been long convinced in the opos- 

 sum, as well as lower vertebrates, though it would be premature to 

 decide that there is no secondary connection with the pero. A few 

 general considerations might be advan ced to indicate the bearing of 

 the conclusion suggested. 



1. The olfactory, as a sensory nerve, should be derived from 

 the dorsal part of the primitive medullary tube or embryonic vesicle. 

 According to the above suggestions, such an origin is the only one. 

 The three superficial radices ultimately reach the gyrus fornicatus. 

 Now, if the callosum be considered the partial homologue of the 

 dorsal commissure of the cord, of which there can be little doubt, the 

 cephalic part of the hippocampus, i. e. gyrus fornicatus, is the homo- 

 logue of the most dorsi-mesal part of the vesicle. The commissura 

 fornicis is also a portion of the dorsal commissural system and the 

 connected parts of the hippocampus are morphologically part of dor- 

 si-meson. The enormous development of the parietal part of the first 

 embryonic vesicle which serves to bring much of the ventral surface 

 dorsad, especially cephalad, causes a revolution correlated with the 

 flextures which obscure the priaiitive simplicity, but in the rodents 

 and marsupials, it is nevertheless sufficiently obvious. 



2. The olfactory, as a sensory nerve, should have a trophic 

 ganglion. This might lie upon the nerve itself, occupying a distinct 

 fossa in the skull, like the Gasserian ; it might be carried peripherad 

 and become associated with the end organ, as in the auditory ; or it 

 might fuse with the brain itself to form an apparently organic unity. 

 To us the latter seems the actual state of the case. If the tracts are 

 really superficial, lying, for the most part, ectad to the neuroglia layer 

 and, in spite of subdividing into several bundles, reach the same part 

 of the brain; and if the so-called deep olfactory tract or olfactory 

 bundle of the prascommissura springs, like other fascicles of that 

 commissure, from cortex cells, there seems to be no reason for doubt- 

 ing (what is a priori so probable) that the adhesion of the pero to the 

 pes is a comparatively subordinate character. In this case, while 

 there may be more or less fusion and interblending of the two, there 



