72 . Flower On the Cerebral Commissures [Feb. 9, 



the differentiating characters to be enumerated, some members of the 

 higher section present an approximation to the lower; but, as far as is 

 known at present, there is still a wide interval between them without any 

 connecting link. 



The differences are manifold, but all have a certain relation to, and even 

 a partial dependence on, each other. They may be enumerated under the 

 following heads : 



1. The peculiar arrangement of the folding of the inner wall of the 

 cerebral hemisphere. A deep fissure, with corresponding projection within, 

 is continued forwards from the hippocampal fissure, almost the whole 

 length of the inner wall. 



2. The altered relation (consequent upon this disposition of the inner 

 wall) and the very small development of the upper transverse commissural 

 fibres (corpus callosum). 



3. The immense increase in amount, and probably in function, of the 

 inferior set of transverse commissural fibres (anterior commissure). 



Each of these propositions must now be considered a little more closely. 

 Arguing from our knowledge of the development of the brain in placental 

 mammals (for of that of the marsupials we have at present no information), 

 it may be supposed that the first-named is also first in order of time in 

 the gradual evolution of the cerebral structures. Before any trace of the 

 budding out of the fibres which shoot across the chasm separating the 

 hollow sac-like hemispheres, before the differentiation of a portion of the 

 septal area into the anterior commissure, that remarkable folding of the 

 inner wall, indicated by the deep (hippocampal) furrow on the surface 

 and the corresponding rounded projection iu the interior, has already be- 

 come distinctly manifest. 



Now the first rudiment of the upper transverse commissure is found 

 undoubtedly at the spot, afterwards situated near its middle, 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 hemi- 

 spheres (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 has to be brought into communication. In the marsu- 

 pials, on the other hand, the prolonged internal convolution or hippo- 

 campus spreading up to and beyond this point, 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 can alone be 

 brought into connexion with its fellow. 



Can this transverse commissure, of which the relation is so disturbed by 

 the disposition of the inner wall of the hemisphere, be regarded as homo- 

 logous to the entire " corpus callosum " of the placental mammals ? or is 

 it, as has been suggested, to be looked upon as only representing the 



