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Proceedings of Boyal Society of Edinburgh. [jan. 31 , 
it can. There is always more or less diffusion of the tract imme- 
diately and for some distance below the point of lesion, if that be 
cortical, and the explanation I think is to be found in the inter- 
position of this plexus. The plexus is a means of reduction and 
association, a means by which the action of the many fibres coming 
from a particular cortical area may be combined and correlated in 
the few. 
But the direct fibres entering the inner capsule are not all motor 
in their function. There are many other bands which enter it and 
whose function varies. Thus there is a large contingent of fibres 
which passes into its posterior limb from the parieto-occipital region, 
and whose function, there cannot be much doubt, is sensory. It 
has been shown, over and over again, that when it is destroyed 
hemianassthesia results. Then there is a large band of fibres 
which comes from the prefrontal region, and which enters the 
anterior limb on its way back to the anterior nucleus of the 
thalamus, to which it becomes attached. The geniculate bodies and 
the pulvinar finally are connected by direct bands with the occipital 
region. 
Plexus most abundant in Man . — One of the main differences which 
exist between the brain of Man and that of the lower mammalian 
types consists in the disproportional size of the white and grey 
matters. In Man the white matter is relatively more abundant than 
in the brain of any other mammal I have examined, and the lower 
we go in the scale the greater the disproportion appears to be. 
Now, the cause of this seems mainly to reside in the fact that 
this intertwining plexus which ramifies through the whole centrum 
ovale is vastly more abundant in Man than in the lower animals, 
and hence, probably, the superiority of the human brain as an 
instrument of association may be accounted for. 
Connections of Thalamus Opticus . — I have said that a large 
number of callosal fibres pass into the thalamus opticus. They lose 
themselves in it, apparently by becoming connected with a dense 
plexus. In conclusion, let me ask the question whether there are 
any fibres which leave the thalamus, and, if so, where they go to ? 
Do fibres descend from the thalamus into the cerebral peduncle, 
ultimately to enter the spinal cord? I am becoming daily more 
and more convinced that, if such do exist, they must be small in 
