THE MORPHOLOGY OF THE SEPTUM, HIPPOCAMPUS, 



AND PALLIAL COMMISSURES IN REPLILES 



AND MAMMALS^ 



J. B. JOHNSTON 



Institute of Anatomy, University of Minnesota 



NINETY-THREE FIGURES 



In the mammalian brain the hippocampus extends from the 

 base of the olfactory peduncle over the corpus callosum and 

 bends down into the temporal lobe. Over the corpus callosum 

 there is a well developed hippocampus in monotremes and mar- 

 supials, while in higher mammals it is reduced to a slender ves- 

 tige consisting of the stria longitudinalis and indusium. The 

 telencephalic commissures in monotremes and marsupials form 

 two transverse bundles in the rostral wall of the third ventricle. 

 We owe our knowledge of the history of the pallial commissures 

 in mammals chiefly to the work of Elliot Smith. This author 

 states that these commissures are both contained in the lamina 

 terminalis. The upper (dorsal) commissure represents the com- 

 missure hippocampi or psalterium; the lower (ventral) contains 

 the commissura anterior and the fibers which serve the functions 

 of the corpus callosum. In .mammals as the general pallium 

 grows in extent there is a corresponding increase in the number 

 of corpus callosum fibers. These fibers are transferred from the 

 lower to the upper bundle in the lamina terminalis, in which 

 corpus callosum and psalterium then lie side by side. As the 

 pallium grows the corpus callosum becomes larger, rises up and 

 bends on itself until it finally forms the great arched structure 

 which we know in higher mammals and man. 



During all this process two changes have taken place in the 

 lamina terminalis. First, it was invaded by cells from the neigh- 

 boring medial portion of the olfactory lobe so that the paired 



1 Neurological studies, University of Minnesota, no. 18. 



371 



