ON THE FUNCTIONS OF THE CEREBRAL CORTEX. 
11 
trunk ; (5) of the lower (pelvic) part of the trunk ; (6) of the leg at the hip ; (7) of 
the lower leg at the knee ; (8) of the foot and toes.* The part which is concerned 
with the rotation of the head is very small, and close to the margin ; it belongs to the 
large head-area of the external surface. The part concerned with movements of the 
upper limb, which is, as we have alrea,dy stated, a portion of the arm-area, extends 
back upon the gyrus raarginalis to about the level of the anterior end of the sulcus x. 
Then comes a small portion of the convolution, excitation of which is followed by 
movements of the trunk muscles, causing rotation and arching of the spine. This 
may be termed the trunk-area. It overlaps the arm-area in front and the leg-area 
behind, and about corresponds in length with the sulcus x, being seldom more than 
half-an-inch in length. In some cases it appears to extend a short way over the 
margin towards x. It is certainly not a little remarkable that the numerous and 
powerful muscles of the spine should be governed from so small a portion of the 
cerebral cortex, but it is to be remembered that the movements of which the spine 
is capable are comparatively few and simple. All the rest of the convolution, from 
about the level of the middle of x to the posterior limit of the gyrus, forms, together 
with the corresponding strip upon the external surface, the leg-area. 
In this marginal portion of the leg-area, as in the external portion, we can 
distinguish between the movements caused by excitation of successive parts from 
before backwards. When the electrodes are applied to the anterior part the most 
marked movement usually obtained is at the hip joint, but here it is a movement of 
extension instead of the flexion produced by excitation of the corresponding part of 
the external portion of the area. It is usually accompa.nied by extension or lateral 
movement of the tail. The movements in question are obviously caused in chief part 
by the glutaei muscles, and this anterior end of the marginal leg-area may be 
distinguished as the glutceus-centre, but the hamstrings are also usually thrown into 
contraction at the same time. Next comes a part, opposite the upper end of the 
ascending frontal gyrus, excitation of which is followed by well-marked action of the 
hamstrings, either alone or accompanied by contraction of the calf-muscles and 
peronsei. This is, therefore, an extension of the hamstring -centre of the external 
surface. It overlaps the glutseus-centre in front, and the succeeding centre behind. 
Lastly, the movements most characteristic of excitation of the posterior part of the 
area are similar to those caused by excitation of the corresponding part of the external 
surface, viz., movements of the foot and toes ; flexion of the foot and extension of 
the toes being those which we have most frequently obtained. They are often 
accompanied by flexion at the knee, due to hamstring action. This part of the leg- 
area may, therefore, be distinguished as the foot-centre.f 
* The movements here mentioned are the primary movements, but, as will be seen from the previous 
description, they are almost invariably complicated by secondary movements, which are usually the 
primary movements produced by excitation at the adjacent parts. 
t The part of the marginal convolution which is concerned with movements of the leg and foot is 
that portion which is often known as the paracentral lobule. 
c 2 
