1887.] Prof. D. J. Hamilton on the Cortex of the Brain. 529 
oblongata , while there is a probability of certain of them even ex- 
tending down to the spinal cord. 
The Direct Fibres . — This paper, however, is concerned with the 
connections between the cerebral cortex and the centres lower down, 
and as yet I have referred to only one set, namely, those which are 
callosal and which cross from the opposite side. There are others, 
of course, which run down directly, and of these the motor fibres 
are among the most important. These direct motor fibres lie to 
the outside of the crossed callosal tract (fig. 2, d.f.), and, like it, 
bend somewhat outwards in circumventing the ventricle. Those 
derived from the marginal gyrus seem, at least in the sheep, to lie 
in very close apposition with the fibres of the crossed callosal tract. 
In Man I calculate that about one-third of the fibres entering the 
anterior two-thirds of the posterior limb of the inner capsule are 
direct, while the remainder are crossed callosal. 
Prom experiments made upon the cortex, it is evident that these 
direct fibres are derived from a wide area, one, indeed, so wide that 
it comes to be a question how it is that they are so few in number 
when they decussate in the medulla and become connected directly, 
or through the intermediation of the spinal cord, with the peri- 
pheral nerves. The notion at present held by most physiologists is 
that from the motor cells of the cortex fibres issue which are con- 
tinuously prolonged downwards to the spinal cord. But if we con- 
sider the matter for a moment, it is evident that they must have 
been much reduced in hulk by the time they have reached the 
medulla, and that the pyramidal fibres of the medulla or cord 
cannot represent the whole of the motor fibres derived from the 
motor area. How, then, is this sudden falling off to be accounted 
for My present conviction is that the direct continuity of the 
process of a ganglion motor cell in the cortex with the pyramidal 
tracts of the spinal cord is a myth. I am strongly inclined to 
believe that, just as in the case of many of the callosal fibres, the 
motor fibres break up into a plexus, from which again fresh fibres, 
those which enter the pyramidal tracts, take their origin. When 
the pyramidal tracts in the cord are affected by secondary degenera- 
tion, they are mapped out with the utmost precision, and the 
degeneration never overlaps them. Can the same be said of the 
degeneration further up in the centrum ovale ? I do not think that 
