644 CEREBRAL COMMISSURES OF THE MAMMALIA, 



lowlier Eutheria, e.g., Mole (Ganser*) it is formed of psalterial 

 fibres, and is not therefore strictly a splenium corporis caUosi, 

 but corresponds rather with the " splenium " of the marsupial 

 commissure. Ganser, from such considerations, and recently Paul 

 Martin t from a study of the development of the corpus callosum 

 in the cat, have come to the conclusion that the separation of the 

 psalterium from the corpus callosum is purely artificial and serves 

 no useful purpose, since both commissures connect parts of the cortex 

 cereljri. These writers therefore include both structures under 

 the term corpus callosum, and divide it into dorsal and ventral 

 layers. But as I have already pointed out, the superior com- 

 missure of Proto- and Metatheria is formed solely of fibres derived 

 from cells lying in the hippocampal region. This in Eutheria is 

 the only distinguishing feature of the psalterium, that it is formed 

 of fibi-es whose " Ursprungzellen " lie in the hippocampus, as 

 distinct from callosal fibres, which consist solely of axis-cylinders 

 and collaterals of cells lying in the mantle proper. The whole 

 superior commissure of the non-placental mammal is strictly 

 homologous then with the ventral layer of the great cerebral 

 commissure of the placental mammal, while the dorsal layer of 

 the latter is a pallial commissure superadded to the ventral, and 

 superseding the dorsal, layer of the hippocampal commissure 

 present in the Marsupials and Monotremes. If then it be 

 permissible to consider the ventral layer of the Eutherian com- 

 missure as truly corpus callosum, we must, of course, admit that 

 the Proto- and Metatheria have a corresponding structure, consti- 

 tuting, however, a corpus callosum, none of whose fibres arise in 

 the mantle proper and which is, therefore, not strictly comparable 

 to the great white band seen on divaricating the hemispheres of a 

 higher mammal. This relationship of the superior limb of the 

 hippocampal commissure in the Metatheria was not appreciated 

 by Flower, who, from the resemblance of the whole commissure 

 in sagittal section to the combined corpus callosum and psalterium 



* " Vergleichend-anatom. Studien ii. d. Gehirn des Maulwurfs," 

 Morph. Jahrb. Bd. vii. 1882. 

 t "Zur Entwick. des Gehirnbalkens beider Katze," Anat. Anzeiger, 1894. 



