SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 411 



The relations of the commissures to the fimbria, fornix superior 

 and the stria Lancisii are surprisingly primitive. In marsupials 

 and lower mammals, as in selachians, the two coimnissures are 

 interwoven with longitudinal fibers of the fimbria system (olfacto- 

 cortical) on their way to the hippocampus. When the corpus 

 callosum increased in size, separated from the hippocampal com- 

 missure and arched dorsally to form the splenium, it carried up 

 on its dorsal surface such fibers as ran over it. These are the 

 stria medialis Lancisii. The fibers which were interwoven with 

 the callosal fibers have retained that relation and are the fornix 

 superior and perforating fibers of Kolliker; the fibers (olfacto- 

 cortical and fornix colunins) which were primitively interwoven 

 or intermingled with the hippocampal commissure have retained 

 that relation and form the mammalian fimbria. 



This interpretation is illustrated in figures 92 and 93 showing 

 the relations of the fornix system to the commissures in the bat, 

 which is intermediate between the marsupials and higher mam- 

 mals, and in the rat, which presents a commissure system essen- 

 tially like that of man. Figures 35 and 38, showing the dispo- 

 sition of fibers in the opossum should be compared with these. 



REMARKS ON THIS REGION IN THE AMPHIBIANS 



In the writer's study of the selachian brain ('11) comparisons 

 were made with the brain of the frog with reference to the posi- 

 tion of the primordium hippocampi and its boundaries. At that 

 time the writer accepted the sulcus limitans of Elliot Smith ('03) 

 as the limit of the hippocampal formation and believed it to be 

 the homologue of the sulcus which marks the boundary of the 

 pallium in selachians. On this basis figure 75 of that paper 

 was constructed. As has been stated elsewhere in this paper, 

 the selachian sulcus is not the equivalent of the sulcus limitans 

 of Smith in the reptile, while that in the frog is the same. 



Anyone who will read the summary of errors and confusion 

 in the literature of the amphibian septal region given by Elliot 

 Smith ('03, p. 495) will understand that it would be useless for 

 me to review this literature, especially as the result would be 

 to note several additions to the list, made by later authors. 



