532 Proceedings of Royal Society of Edinburgh. [jan. 31 , 
described by Eicbler [Arch, f Psycliiat., vol. viii. p. 355). It seems, 
however, that in these cases we have to do not with a deficiency 
in the actual callosal fibres, but with a malformation by which 
they have failed to decussate in the middle line, just as so frequently 
happens in the anterior pyramids. 
On the supposition that the thalamus subserves the purpose of 
concentrating the fibres which educate the higher centres through 
the optic, it can readily be understood how, if it were destroyed in 
adult life, no very evident symptoms might follow. It has already 
to a great extent subserved its purpose. The higher centres have 
been educated, and are capable of discharging their functions apart 
from the channels through which that education has been imparted. 
It has played its part, so to speak, in infancy and youtb, and may 
now in a manner be considered as functionally inert. The im- 
pressions made upon the cerebral cortex through it are quite pos- 
sibly recalled by a perfectly different set of channels ; for I do not 
see why in the internal economy of the brain there may not be 
paths for educating the higher centres through vision, hearing, 
touch, and so on, and a whole set of other paths by which the 
results of this education may be brought into action. If such be 
the case — and I advance this with all due caution — the callosal 
system of fibres might be regarded as the great educating 
system ; while the direct bundles to which I have adverted would 
constitute the means of adapting this education to a utilitarian 
purpose. 
In the case of those born blind, the education of the cortex would 
of course be carried on through other channels, namely, through 
those of the remaining special senses. The nuclei of the nerves 
connected with these are situated in great part in the pons, medulla 
oblongata, and spinal cord, and these again, as already mentioned, 
appear to be extensively united to crossed callosal fibres which have 
descended in the inner capsule. By the agency of these callosal 
fibres they are placed in communication with the cerebral cortex on 
the opposite side of the body. The same mechanism for educating 
the cortex in fact prevails here as in the case of the optic ; that is 
to say, there is, firstly, the peripheral nerve to receive the impres- 
sion ; secondly, an intermediate nucleus to which the nerve is 
bound ; and, thirdly, a system of fibres (callosal) by means of 
