648 
MR. W. H. FLOWER ON THE CEREBRAL COMMISSURES 
septal area into the anterior commissure, that remarkable folding of the inner wall, indi- 
cated by the deep furrow on the surface and the corresponding rounded projection in 
the interior, has already become distinctly manifest, and the future form of the ventri- 
cular cavity, with its elevations and depressions, has been sketched out. Now the first 
rudiment of the upper transverse commissure is found undoubtedly at the spot after- 
wards situated near its middle — that part to which in the lowest placental mammals it 
is almost entirely confined. This spot is situated a little way above and in front of the 
anterior end of the ventricular aperture, at the upper edge of the region of adherence of 
the two hemispheres (the future septal area). In the placental mammals this part is in 
direct relation to the great mass of the internal medullary substance of the hemispheres, 
which have to be brought into communication. In the Marsupial, on the other hand, 
the prolonged internal convolution or hippocampus extending up to and beyond this 
part, forms the inner wall of the hemisphere from which the fibres pass across, and it 
is necessarily through the medium of this convolution, and following the circuitous 
course of its relief in the ventricle, that the upper part of the hemisphere alone can be 
brought into connexion. 
Can this transverse commissure, of which the relation is so disturbed by the dispo- 
sition of the inner wall of the hemisphere, be regarded as homologous with the entire 
corpus callosum of the placental mammals 1 or is it, as has been suggested by Professor 
Owen, to be looked upon as only representing the psalterial fibres or transverse com- 
missure of the hippocampi'? Undoubtedly a large proportion of its fibres do come 
under the latter category. But even if they should nominally be all so included, it is 
important to bear in mind that we have still a disposition in the marsupial brain very 
different from that which would remain in the brain of any placental mammal after the 
upper and main part of the corpus callosum had been cut away. In the latter case the 
commissure of a very small part of the inner wall of the hemisphere alone is left, that 
part folded into the hippocampus. In the former there is a commissure, feeble it may 
be, but radiating over the whole of the inner wall, from its most anterior to its posterior 
limits. Granted that only the psalterial fibres are represented in the upper commissure 
of the marsupial brain, why should the name of “ corpus callosum ” be refused to it 1 
These fibres are part of the great system of transverse fibres bringing the two hemi- 
spheres into connexion with each other ; they are inseparably mingled at the points of 
contact with the fibres of the main body of the corpus callosum, and are only separated 
from it in consequence of the peculiar form of the special portions of the hemisphere 
they unite. Indeed, as mentioned before, they are not more distinct than is the part 
called “ rostrum ” in front. And although they blend at each extremity with the fibres 
of the diverging posterior crura of the fornix, they certainly cannot be in any sense 
confounded with that body, the essential character of which is that it is a longitudinal 
commissure consisting of two halves closely applied in the middle, but each composed 
of fibres belonging to a single hemisphere only. 
But is the main part of the corpus callosum of the placental mammal not also repre- 
