64 
DR. C. E. BEEVOR AND MR. V. HORSLEY ON THE EXCITABLE 
The Confi(juration of ihe Basal Ganglia. 
It is now necessary to consider in detail the shapes and relative positions of the 
three chief divisions of basal grey matter, to the fibres of the internal capsule. 
We think we may best accomplish this by describing in succession from above down 
the sections typical of each group of experiments (see p. 65). The position of the 
genu, which, as will be seen in fig. 1, gradually moves backwards and inwards, as we 
pass down the capsule to near the base, i.e., until the posterior limb, as seen in these 
sections, changes from an oblique antero-posterior direction to an almost transverse 
position. This is strikingly exhibited in Table II., calculated from the observed 
dimensions of the capsule in each experiment. 
Table II. 
Group. 
Average total length 
of capsule. 
Length of Anterior 
Limb. 
Length of Posterior 
Limb. 
m.m. 
m.m. 
m.m. 
I. 
18-2 
7 
11-2 
II. 
17 
6 
IL 
III. 
18 
7 
11 
IV. 
20 
9 
11 
V. 
21 
10 
11 
VI. 
20 
9 
11 
VII. 
17 
2 
15 
vin. 
12 
0 
12 
The genu is of course the point of junction of the two limbs, and it will be seen at 
once that while the length of the posterior limb remains remarkably constant until 
its axis finally changes direction, the anterior limb on the other hand increases in 
size from above down, lying in the notch between the caudate and lenticular nuclei 
(see figs. 1, 2, and 5), until the section approaching the grey matter of the base of the 
brain, it rapidly disappears (as in Group VIII.), see fig. 1. 
It will be convenient next to consider two angles; firstly that which the anterior 
limb makes with the posterior limb in forming the genu, and secondly the angle of 
inclination of the long axis of the posterior limb to the middle line. By subtraction 
of these two from 180° we naturally obtain at once the inclination of the anterior 
limb to the mesial plane. 
Since the angle of the genu is correlated with that of the inclination of the 
anterior limb, we set both out in the following Table III. (The angles we measured 
with a goniometer on photographs of the specimens made just after each experiment.) 
Each of the angles typical of the several groups was found by taking the average of 
all the angles of the capsules contained in each group. 
