58 
DR. C. R, BEEVOR AND MR. V. HORSLEY ON THE EXCITABLE 
Whatever be their function it is evident that the fibres entering the anterior limb 
are arranged much as is indicated in fig. 3, and that, regarding the hemisphere as 
fixed with its greatest length in a horizontal plane, we may group these fibres, accord¬ 
ing to tlieir vertical relations to such a plane, into :— 
a. Superior and Descending. 
h. Horizontal. 
c. Inferior and Ascending. See also figs. 3, 5, G. 
But further, on account of the manner in which the grey cortical mantle of the 
frontal lobe presents, roughly speaking, the form of a cap or segment of a globe, we 
must, in addition, sub-divide these fibres according to their relative position about the 
horizontal plane of the hemisphere. Hence we must speak of a middle bundle, viz., 
those from the tip and anterior part of the lobe ; of an internal group, viz., those 
coming from the mesial surface, i.e., of the anterior third of the marginal and neigh¬ 
bouring gyri; and of an external group, i.e.. those coming from the outer part of 
lower frontal gyri (of course excitable). 
Many of these bundles will be referred to under the next heading, and the 
destination of the remainder, i.e., of the truly prmfrontal, we have already discussed. 
To sum up then, it is evident that we have excitable fibres coming into the 
anterior limb from the so-called motor cortex lying on the external aspect of the 
hemisphere in the opercular region, and tlmse we regard as motor. The mass, 
however, of the anterior limb is composed of the bundles of fibres coming from the 
most anterior and mesial surfaces of the frontal lobe, these being non-excitable. 
Looked at in their sagittal arrangement, we find that the excitability diminishes 
from above down, or, in other words, the pyramidal portion decreases and the 
prmfrontal and non-excitable increases. 
11. Excitahle or Pijramidcd, Fronto-Parietal .—This is the collection of fibres 
commonly known as the pyramidal tract, and which on being stimulated produce 
movement in a distinctive manner. Strictly speaking, the term pyramidal tract 
should apply only to those fibres contained in the pyramids, i.e., those for the muscles 
of the limbs and trunk. In fact, many authors divide the “ motor ” capsular fibres 
into two groups, one devoted to the cerebral nerves, and the other termed the 
pyramidal fibres. This distinction we consider fair, but tlie term pyramidal tract 
has been so extended as to include these two groups of so-called motor fibres. But, 
while we agree in general to this distinction, it must not be forgotten that these two 
sets of fibres are not in utterly distinct compartments, but that the area of each 
overlaps. 
We would prefer, tlierefore, to employ the term excitable to express the whole 
number of fibres possessing “ efterent ” or “ motor” function. 
These fibres wm presume come from most of the excitable portions of the cortex. 
