MINUTE ANALYSIS OF CORTICAL CENTRES. 
155 
volution, and that that part which lies between it and the top of the prsecentral sulcus 
is certainly the second or middle frontal convolution. Finally, all that portion of the 
cortex which lies below the anterior horizontal limb of the prsecentral sulcus must at 
present be regarded as of uncertain denomination, since, for several reasons which need 
not be detailed here, the upper third of it would appear to belong to the middle 
frontal convolution, while there can be little doubt that the lower two-thirds are 
homologous with the third or inferior frontal convolution of Man. We will now 
return to the consideration of the evidence on which we ground the opinion that x is 
really the representative of the posterior extremity of the superior frontal sulcus in 
Man. This evidence we will consider under the following heading’s :— 
1. Morphological significance as an important sulcus. 
2. Functional significance as an important sulcus. 
3. Anatomical form and direction. 
4. Value as a determinant of localisation of function. 
1. Morphological Significance.- —We have already stated that Professor Schafer 
had previously observed the constancy of this small sulcus, which opinion is fully con¬ 
firmed by our experience. He has further shown on a transverse section that it was 
marked by a distinct folding in of grey matter—the sure indication of an important 
sulcus. 
2. Functional Significance. —We have found that if electrodes are applied to the 
sulcus itself no movement follows, whereas, if they be just shifted to either border, the 
characteristic effects about to be described are obtained. Here we have a functional 
proof that this sulcus, although small, definitely divides two portions of the motor 
cortex. These portions, we shall see directly, have a different function. 
3. Anatomical Form and Direction. —With one exception, we have found that the 
most constant direction of this sulcus is antero-posterior, though frequently it is 
slightly oblique. The exception referred to was found in a pig-tailed Monkey (Macacus 
nemestriniLs), in which specimen the sulcus was almost vertical, and consequently 
parallel, to the fissure of Polando. Now we would submit that we have here faithful 
imitations of the varieties of form presented by the superior frontal sulcus in Man. 
For, as is well known, that sulcus is most frequently a more or less straight antero- 
posteriorly directed line, wfifile sometimes, though more rarely, it resembles a miniature 
prsecentral sulcus in possessing a vertical stem parallel to the fissure of Polando, 
from the middle of which there runs forwards an antero-posterior limb. If, therefore, 
we regard this sulcus, x, as the posterior extremity of the superior frontal sulcus, its 
variations in form will be most easily understood. Finally, we would observe that the 
superior frontal sulcus in Man commences posteriorly, well behind the line of the 
prsecentral sulcus, directed vertically upwards ; in fact, it begins posteriorly in the 
middle of the ascending frontal convolution. It is just this first part of the superior 
frontal sulcus which, lying over the anterior half of the breadth of the ascending 
frontal convolution, forms the sulcus, x, of the Monkey’s brain. 
x 2 
