162 
DR C. B. BEEVOR AND PROFESSOR V. HORSLEY ON A 
Thumb .—The limitation of the representation of the thumb in the cortex (see 
fig. 6) is a matter of great interest, considering that it is the most highly differen¬ 
tiated member of the body. The representation is limited to the ascending frontal 
and parietal convolutions. No movement could be elicited by stimulating the centres 
1, 1', 5, and 11 ; to these we must add 2 and 2', as being centres in which the thumb 
was only exceptionally represented, i.e., in one-third of the total number of cases for 
2, and one-fifth for 2'. It is interesting to observe that the thumb obeys the same 
general rules respecting the relations of the movements of flexion and extension as do 
the digits ; thus extension precedes flexion in the ascending frontal and follows 
flexion in the ascending parietal. The movement of opposition, which is, of course, 
the most highly differentiated one, was only obtained on stimulating the lowest part of 
the thumb area, viz., the centres 7 and 5 '; this is in perfect harmony with the general 
plan of representation, as we have found it to exist in the outer convex surface of the 
cortex. (See Axioms 1 and 2.) 
We may here refer to the fact that in 5', in the ascending frontal gyrus, we Obtained 
in two cases abduction of the thumb, whereas adduction was present in two cases in 
centre 7 in the ascending parietal ; this is a further illustration of the fact that 
extension is represented in the ascending frontal, and flexion in the ascending parietal 
convolutions. 
Before leaving the consideration of the representation of the thumb, we would 
briefly chaw attention to the fact that the representation of the thumb extends lower 
down anteriorly, i.e., just behind the prcecentral sulcus, than that of any other part 
of the upper limb. Thus it is represented at centre 5', but not in centre 5, &c. 
(See Table 3.) 
We employ throughout our paper this expression—Primary Movement—in a very 
definite sense, namely, to express that movement which is represented above all 
others at one particular spot in the cortex. This use of the expression is in harmony 
with Dr. Hitghlings Jackson’s view, viz., that cerebral localisation is in the main a 
matter of degree of representation of several movements, and not the close limitation 
of any one. We were brought to the necessity of closely examining this point, not 
from the considerations of theories, but by observing with what remarkable constancy 
the various joints of the limb took up movement in series according to the part of 
the cortex stimulated, and how invariably one joint would commence the action when 
we adopted a method of excitation which we may call instantaneous or minimal 
stimulation. 
Priority in the Order of Movements. (See fig. 7.) 
We may now return with advantage to a detailed consideration of Axiom 1, viz., 
that the larger muscles are represented in the upper part of the motor area for the 
upper limb, while the smaller ones have their centres in the lower part of that area. 
