1887.] Prof. D. J. Hamilton on the Cortex of the Brain. 521 
mass of fibres (see PI. XIY. fig. 1), which leaving this body and con- 
tinuous with it turns upwards, outwards, and downwards in the 
centrum ovale. The arcuate mass varies somewhat in shape at 
different parts of the brain. Thus, anteriorly it represents an almost 
complete semicircle, while posteriorly it becomes more pointed. The 
fibres entering into the composition of the arched mass subse- 
quently pass into the inner and outer capsules. The greater hulk 
of them, however, enters the inner capsule, and in its anterior limb 
the capsule is almost entirely composed of them ; while a consider- 
able portion also seems to run into the outer capsule, constituting 
the inner of the two kyers of which it consists. Their further 
course and attachments to underlying parts will be subsequently 
considered. 
In a former paper [Journal of Anatomy and Physiology , vol. xix., 
1885, p. 385) 1 have named this mass of fibres the “crossed 
callosal tract”; and as all my work since then has tended to fully 
bear out the view I at that time entertained of its significance, I 
propose still to adhere to this nomenclature. 
In order to get at once to the gist of the arguments I intend 
using to explain the nature of this crossed callosal tract, I shall 
start with the postulate that it is mainly composed of callosal fibres 
which have arisen from the cortex, which have crossed in the corpus 
callosum, and which, instead of turning upwards to become attached 
to points in the opposite cortex corresponding with those from 
which they sprang, are now turning downwards into the two capsules 
to become subsequently united to the basal or other ganglia presently 
to be enumerated. 
If it be true that the crossed callosal tract represents the fibres 
derived from the opposite cortex, which have passed over in the 
corpus callosum, and which are now turning down to the two 
capsules, the following data ought to admit of verification : — 
1. The crossed callosal tract ought to be capable of being dis- 
sected out ; 
2. It ought to be co-extensive with the corpus callosum ; and 
3. It should be possible to trace the fibres microscopically as 
they turn downwards. 
I shall consider each of these in order. 
