132 dr. C. E. BEEVOR and MR. V, HORSLEY ON THE EXCITATION OF THE 
between the representation of the upper and the lower limbs respectively ; this we 
similarly find to be the case in the Orang (see fig, 4, p. 150), 
The Intraparietal Sulcus 
In the present brain runs a characteristic course, viz., from near the lower end of 
the ascending parietal gyrus (where it bifurcates) upwards and backwards. At the 
bifurcation one limb runs forwards and upwards towards the genu of the Fissure of 
Rolando, and the other downwards towards the Fissure of Sylvius. As it courses 
across the parietal lobe it follows in a fairly parallel manner the direction of the Fissure 
of Rolando. 
The post-central sulcus (Ramus Superior of the intraparietal sulcus, according to 
Cunningham, ‘ Joiirn, of Anat. and Physiol.,’ 1889) is well marked, and begins near 
the longitudinal fissure at the upper margin of the hemisphere, and follows close round, 
strictly parallel to the Fissure of Roiando, reaching down the ascending parietal 
convolution almost as far as the genu of the Fissure of Rolando, vide figs. 2 and 3. 
Convolutions. 
The ascending frontal and parietal convolutions have the same position and relations 
as in Man and are bound by the same fissures and sulci. 
The three frontal convolutions present more difficulty, as their boundaries are not 
so evident. The division between the superior and middle convolutions is sufficiently 
indicated by the superior frontal sulcus; but the line of separation between the 
middle and inferior convolutions is not so evident; we think, however, that the 
sulcus “ iv” i.e., the inferior frontal sulcus, marks the boundary between the two. 
Around the posterior end of this sulcus, as will be subsequently seen, is situated the 
focus of the representation of the movement of turning the eyes to the opposite side; 
and since this particular focus of representation in the Bonnet surrounds the posterior 
end of the sulcus tv, we think we are justified in regarding the sulcus marked I.F.S., in 
figs. 1, 2, and 3, as homologous to the tv or inferior frontal sulcus of the Bonnet, and, 
consequently, as the dividing line between the middle and infenor fronted convolutions. 
For topographical purposes, as was shown many years ago, the sulci are of very 
much more importance than the convolutions which they fashion, consequently, we 
need not prolong the description of the latter. We must, however, recall attention 
to the -determination by the sulci of representation or its absence, a point which we 
first observed in the Bonnet. To repeat, representation of movement in the cortex is 
found only on the summits of the gyri of the convoluted surface, and the approach of 
a sulcus dimpling the cortex at all makes it at once inexcitable. This, true of the 
Bonnet, is still more marked in the Orang, where, in addition, as wall be seen, we have 
slands of smooth cortex, even between foci, which are inexcitable. 
In the yet more highly divided human cortex this principle naturally must be held 
to prevail with still more force, as seems, in fact, to be the case, see p. 152, et seq. 
