488 



PROF. O. ELLIOT SMITH OX THE MOKPHOLOOT OF THE 



this part of the paratermiual body presents a relationship to the hippocampus which 

 is analoo-ous (topographically) to the fimbria in the Mammalian brain. 



That the commissiira aherrans arises from the hippocampus is an undoubted fact ; for, 

 as one of the coronal sections (fig. 10) in the introductory part of this memoir shows, its 

 relationship to the caudal portion of the hippocampus is exactly analogous to that of the 

 dorsal commissure to the same formation iurther forward. Therefore Rabl-Riickhard 

 had some justification for calling this structure the commissura fornicis, especially as it 

 springs from that portion of the hippocampus which persists in the Eutherian brain. 

 But this name is objectionable, in that the designation '\fornicis " is equally applicable 

 to the commissural fibres which spring h-om the cephalic ])ortion of the hippocampus 

 and from the commissimi dorsaUs. Movc-over, this commissure has a peculiarly 



aberrant course in comparison with the usual course of the fornix — and hence it 



deserves a special name. The term " commissura aberrcms \liippocampt\ '' seemed to 

 me to fulfil these requirements. 



Fig. 22. 



This is diagrammatic, the relations of the various parts oot being drawn to scale. The true proportions 

 are shown in figs. 1 and 2, and the seiies of conin^il sections of Splienndon. 



The explanation of the peculiar caudal extension of the paratermiual l)ody is a matter 

 of extreme difficulty, and one which needs a much fullei- knowledge of its beliaviour in 

 certain other Vertel)rate types than I at present possess. The question is complicated by 

 the fact that this caudal extension seems in some way to be associated with the presence 

 of the commissura aherrans. So far as I am a^^■are. it occurs in all Reptilian^brains in 

 which the latter commissure is found, and is lacking in the Chelonia and other Reptiles 

 in which the aberrant commissxtre' is not developed. One might be led to believe that 

 the coiiiiit.is.sura aherrans is merely a part of the commissura dorsalis (for both are 

 hippocampal) which had in some way become separated from the parent commissui-e and 

 in its wandering caudalward had drawn back on each side a prolongation of its matrix 

 of paratermiual body. But the features of the region in the adult render such an 



