THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 335 



then backward to become the pillars of the fornix. From the 

 rudimentary hippocampus above the callosum similar fibers run 

 in the striae Lancisii and eventually break through the callosum 

 or run around its anterior border and through the septum to join 

 the pillars of the fornix. The latter then run into the thalamus 

 and end in the corpus mammillare. Fibers of the olfactory tracts 

 and fibers from the tuberculum olfactorium and precommissural 

 body enter the hippocampus by way of the septum and fimbria 

 or as fibers perforating the callosum. Fibers running by the 

 same routes come to the hippocampus from the pyriform lobe 

 and nucleus amygdalae. The hippocampus also receives or sends 

 out fibers through the corpus striatum. It is possible that these 

 latter include the equivalent of the tractus lobo-epistriaticus of 

 fishes and that they bring up gustatory impulses from the hypothal- 

 amus. It is especially interestmg m this connection to note that 

 gustatory sensation is thought by Flechsig to be localized in the 

 hippocampus or the area immediately adjoining it. 



A remarkable Constance and similarity of structure is seen in 

 the olfactory central apparatus throughout the classes of vertebrates. 

 Speaking broadly, the primary (bulb) and secondary centers (lobe) 

 and cortex with their respective tracts are already formed in 

 selachians upon a plan which is retained in all higher vertebrates. 

 A relative increase in the tertiary (cortical) centers and an increas- 

 ing complexity of structure in these centers is the chief difference 

 between higher and lower vertebrates. In fishes, amphibia and 

 reptiles, as far as known, the whole forebrain with the exception 

 of the center for the nervus terminalis in those forms which possess 

 it, is devoted to the olfactory functions. The cortical center also 

 serves the gustatory system. Apparently suddenly in monotremes 

 appears a large lateral and dorsal cortex which is devoted to 

 somatic functions. This is the neopallium whose structure and 

 functions will be considered in the next chapter. Its position and 

 the relations of its commissure show that it began its history in a 

 dorso-lateral position at the anterior end of the forebrain between 

 the mesial and lateral olfactory nuclei, and that it spread back 

 from this point, pushing the two olfactory areas down upon the 

 ventral surface and crowding the olfactory cortex to the extreme 



