2T,2 JoUKNAJ. OF COMHARATIVK NeUROI.OGY. 



SUMMARY OF OLFACTORY LOBES. 



1. The olfactory lobes are exceedingly variable in size 

 and position, but exhibit no decided differences in structure. 



2. The primary condition is similar to the permanent 

 condition in Sauropsida, i.e., the lobes are sessile or attached 

 by short crura to the base of the cerebrum; but their con- 

 nection with the cerebrum is more accidential than essential. 



3. Whether they become stalked or not depends on 

 whether the growth of the head removes the peripheral 

 organs of smell from the brain at an early period, and 

 whether, in case it is so removed, the olfactory nerve or crus 

 is elongated. The probable determinant for the latter is the 

 relative rate of development of the various regions of the 

 head. The olfactories are always sessile in an early stage. 



4. The crura contain two distinct tracts, forming a radix 

 mesalis and lateralis. 



^. The lateral ventricle is frequently extended into the 

 the crura, but the form it assumes varies. In the extreme 

 case the crus is a hollow cylinder, with the fibres chiefly 

 collected in the ventral portion. In other cases the ventricle 

 simply extends a short distance along the dorsal surface of the 

 crus. 



6. In some cases there is a rudimentary olfactory ven- 

 tricle in the substance of the lobe. 



7. The radix mesalis enters a special mesaxial lobe of 

 the cerebrum and its fibres decussate in the commissura 

 interloborum, forming with the next a hippocampal com- 

 missure and fornix. 



8. The radix lateralis follows a gentle groove (sinus 

 rhinalis) on the ventral surface of the hemispheres to a point 

 opposite the decussation of the radix mesalis when it turns 

 abruptly mesad and crosses to the opposite side; but probably 

 also gives off fibres to the hippocampus. According to 

 Owen the olfactory lobes are sessile in Perca, Scomber, 

 Esox, Pleuronectes, Blennius, Anguilla, Gasterosteus, Eper- 



