56 
DR. C. E. BEEVOR AND MR. V. HORSLEY ON THE EXCITABLE 
a photograph of a transverse vertical section of a Monkey’s [Macacus sinicus) right 
hemisphere. This superior level (black clotted line) is seen to be a plane directed 
slightly obliquely downwards and outwards, and resting on the upper surfaces of the 
caudate and lenticular nuclei. 
The lower level is scarcely a. plane, owing to the configuration of the basal ganglia, 
as about to be described, but its general direction is roughly represented by a plane 
inclined upwards and backwards in a line with the upper surface of the optic tract 
and drawn from the optic chiasma backwards and upwards to the posterior extremity, 
or the pulvinar, of the optic thalamus. 
Of course these levels are purely arbitrary divisions of the fibres of the pyramidal 
tract, separating a part of them from the centrum ovale above and the crus cerebri 
below. 
The internal capsule may be described as consisting of bundles of fibres, arranged 
like the rays of a fan, of wdiich the handle if broadened would represent the crus 
cerebri, while if the fan be considered to be held laterally in a plane parallel to the 
sagittal section of the hemisphere, the outside rays would correspond respectively 
with what we shall hereafter term the antero-inferior and postero-superior borders of 
the internal capsule, this is represented in fig. 3, which is a photograph of a sagittal 
section of a Monkey’s brain {Macacus sinicus). 
Viewed in horizontal section the capsule is limited by lines drawn at right 
angles to the anterior and posterior extremities of the lenticular nucleus. 
Although this view of the fibres composing the internal capsule is useful to bear in 
mind, it nevertheless does not express the whole facts of the case. In the first place, 
we have to remember that extending down towards the base of the brain, i.e., towards 
the basal ganglia and internal capsule, is a large number of fibres converging from all 
parts of the cortex mantle. 
Ar ranged in order from before back, these fibres may be enumerated as follows, 
irrespective of the impulses which they are generally considered to convey, classified 
according to the region of the cortex with which they are apparently connected. 
I. Prsefrontal. 
II. Excitable, Pyramidal, or Fronto-Parietal. 
III. Temporal. 
IV. Occiplto-Temporah 
V. Occipital. 
Th ese fibres are shown in their relation to the basal ganglia, anterior commissure, 
&c., in the accompanying fig.s. 4, 5, G. These are drawn from a dissection of a 
hardened Monkey’s {M. sinicus) brain in which the large majority of the hemisphere 
has been cut away, leaving uninjured the nuclei of the corpus striatum. In fig. 4 the 
ventricular aspect of the mass thus obtained, and also in fig. 6, is seen a dotted line 
in front, showing liow much of the grey matter forming the anterior perforated spot. 
