FIBRES OF THE INTERNAL CAPSULE OF THE BONNET MONKEY. 
77 
Lateral Arrangement of the Excitable Fibres in the Internal Ca'psule. 
We now introduce a new branch of this subject, namely, the relations in lateral 
juxtaposition of the efferent fibres. 
So far we have described the antero-posterior arrangement of these fibres, but we 
have postponed the consideration of the results obtained by exciting the capsule 
in individual points of its breadth. 
As the capsule varied in width from 1 to 3 mm., and as the part of the sections 
involving the inner zones of the lenticular nucleus was also constituted of white fibres 
in and around those zones, we have sometimes found five rows of excitable points (each 
1 mm. square). The result obtained from these points we shall now analyse. Altogether 
we were able to obtain two or more rows of 1 mm. squares in 26 experiments. 
Anatomically speaking, in horizontal transverse section we may regard the fibres of 
the internal capsule as falling into three groups, viz. :— {a) those in the mid line ; 
(6) those next the lenticular nucleus, and (c) those on the inner side next the optic 
thalamus, &c. 
Of the 26 cases above-mentioned, in seven the excitability appeared to be equally 
distributed throughout the breadth of the capsule. However, besides these seven, we 
found that in 15 the most excitable part was that next the lenticular nucleus, 
this gradually decreasing towards the optic thalamus, but in the remaining four the 
capsule was most excitable alongside of the thalamus. Considering the anatomical 
arrangement of the cortex mantle to the position of the basal ganglia and internal 
capsule, it is not at all improbable that these results may be referable to the fibres 
entering the capsule from above in an oblique direction, and hence in a horizontal 
section, they will offer for excitation not only a transversely cut end alongside the 
lenticular nucleus, but also longitudinal surfaces, which are progressively less and less 
exposed to excitation as we move across the capsule from the lenticular nucleus to the 
optic thalamus. 
If correct, this view would postulate the theory that the fibres forming the part of the 
capsule nearest the middle line are inexcitable. This, at any rate, is evidently strongly 
suggested by the experiments. Moreover, in a frontal microscopic section of the 
region, the fibres which emerge from the gyrus fornicatus on reaching the capsule, are 
seen to occupy the most internal position therein, and excitation of the gyrus 
fornicatus has always failed to elicit movement. 
To sum up this question, it is self-evident that, the lateral arrangement of the fibres 
is infinitely less important than that of the antero-posterior direction, and principally 
because the function of each point in any given transverse row of squares is the same 
in character though diminishing in intensity, as we pass from the outer to the 
inner side of the capsule. 
