1887.] Prof. D. J. Hamilton on the Cortex of the Brain. 531 
number. Certain bands of fibres which are not callosal, no doubt 
enter the thalamus, but these seem to connect it with parts of the 
cortex which experiment has shown are associated with definite and 
well-located functions. Thus there are the three so-called 'peduncles 
of the thalamus, uniting it respectively with the prefrontal region, 
with the nucleus amygdalaris in the temporo-sphenoidal lobe, 
and with the hippocampus major. But these differ entirely in their 
nature from the fibres which are supposed to leave the thalamus 
and to pass downwards with the other descending cerebral tracts, and 
it seems to me that the latter, if they do exist, must be in small 
quantity. 
How, then, is the thalamus connected peripherally, and what is it3 
use ? As yet any statement on this subject must necessarily be 
largely conjectural, but, all things considered, there is reason for at 
least supposing that this ganglion is largely concerned with the 
education of the brain through the optic nerves and corpus callosum . 
Of all the nerves in the body the optics are those by which the 
brain is mainly educated. They are in constant use, imperceptibly 
opening up the cortical grey matter to impressions made upon the 
periphery by light vibrations. What is the connecting link between 
the peripheral retina and the central cortex 1 The visual centre 
is said to be located in the occipital lobe, but the optic must 
subserve a far wider function in educating other parts of the cortex 
as well as this small area % How is it that the motor centres, for 
instance, are educated to a particular complicated act, purely through 
the sense of sight ? What is the mechanism by which a sudden 
visual impression, accompanied by a sense of danger, will serve to 
throw the body instantaneously into a complicated attitude of 
defence 1 ? This introduces far too wide a subject of discussion 
to take up at present ; but it seems to me likely that the callosal 
fibres entering the thalamus constitute the substratum by which 
these acts are accomplished ; that they are, in fact, the means 
by which the opposite side of the brain is educated through 
vision. 
Where the corpus callosum has been destroyed in infancy, im- 
becility seems to have been the invariable result. There are certain 
records of congenital deficiencies of this body which have been 
unaccompanied by any symptoms of note, more especially one 
