642 CEREBRAL COMMISSURES OF THE MAMMALIA, 



comparative papers of Sir William Turner,* may be quoted as 

 examples] is that all mammals possess a corpus callosum. This 

 opinion is still actively upheld by Heri'ick in his numerous papers, 

 although in a different (but equally mistaken) sense to all other 

 writers. So that to-day there is still considerable uncertainty 

 upon this question, in spite of the short but clear and convincing 

 paper of Symington, f 



So far no attempt has been made to explain either the arrange- 

 ment of the fibres in this commissure or the difference in shape 

 which it presents in different animals, and which so deceived 

 Flower. This paper aims at offering an explanation of these 

 appearances. 



If a sagittal section be made through the mesial wall of the 

 cerebral hemisphere of Perameles, i.e., a short distance from the 

 mesial plane of the brain, the section stained by Pal's method 

 presents an appearance which is represented in a purely schematic 

 manner in fig. 1. In the middle of the figure is seen the section 

 of the commissure {ps.), which has given rise to so much discussion, 

 and radiating from it the fibres (/. ) of the alveus are schematically 

 represented proceeding to all parts of the hippocampus, whose 

 contour is represented by the hippocampal fissure (Jif. ) From the 

 concavity of the hook formed by the commissure are to be seen 

 issuing the fibres of the anterior pillars of the fornix {af.) and the 

 precommissural fibres of Huxley {a.) separated by the anterior 

 commissure (ac). 



In fig. 2 the corresponding structures in Orjiithorhynchus are 

 represented in a similarly schematic manner. The psalterium or 

 superior commissure here presents only a slight hook at its 

 posterior extremity corresponding to the rudimentary posterior 

 descending cornu of the hippocampus. Consequently the com- 

 missure is not folded in bilaminar form as it is in Perameles. In 

 fig. 3 the condition found in the placental mammal is diagrammati- 

 cally represented. 



* '.'The Convohitions of the Brain," .Journ. of Anat. Vol. xxv. 1890; 

 " The Cerebrum of Ornithorhyuchiis" Journ. of Anat. Vol. xxvi. 

 t Loc. cit. 



