MOTOR CORTEX AND INTERNAL CAPSULE IN AN ORANG-OUTANG. 131 
It will now be important to note the position of the genu of this fissure in 
the Orang and Man, as compared wdth the Bonnet Monkey. In the last, it is 
situated at the junction of the middle and lower thirds of the fissure, in the Orang 
and in Man it is found at about the mid-point between the upper margin of the 
hemisphere and the Fissure of Sylvius, or higher. We would suggest that this 
is due to the greater development in the higher animals of the convolutions about 
the lower end of the Fissure of Rolando, including the third frontal gyrus. We 
think this is borne out by the fact that it is the lower part of the Fissure of Rolando, 
i.e., that part below the genu, which is so much lengthened. 
Of further curvation, the Fissure of Rolando often exhibits at its upper end 
a slight bend forwards which, in man, is surmounted by a small curve in the opposite 
direction. 
The Prcecentrcd Sulcus 
Consists of a vertical arm, which springs from close above the Fissure of Sylvius, 
and ascends till it bifurcates just below the horizontal level of the genu of the 
Fissure of Rolando, into a posterior ascending branch and an anterior horizontal. 
Where this latter joins the vertical limb, it forms a right angle anteriorly, which, as 
will be subsequently seen, is just as in the Bonnet Monkey, the focus for the 
representation of the movement of turning the head and eyes to the opposite side. 
In this angle, formed by the limbs of the praecentral sulcus, there is another sulcus, 
directed downwards and forwards, and which we believe to be the homologue of the 
sulcus which we have temporarily called “iv” in the Bonnet Monkey.* In the 
anthropoids, its anterior end seems to extend down to the orbital surface of the lobe, 
and to wind round in front of the anterior extremity of the Fissure of Sylvius. 
(We believe that this sulcus corresponds to the inferior frontal sulcus of Man.) 
We have pointed out in a previous communication (‘ Phil. Trans.,’ B, 1887) that 
the sulcus in the Bonnet Monkey, which Professor Schafer provisionally termed 
X, is the superior frontal sulcus of human nomenclature. We also showed that 
it consisted in its simplest condition of a vertical and a longitudinal limb. Now it 
has been proposed by Jensen to call the vertical limb the superior prsecentral sulcus; 
wdthout giving our support to this step, we wish to emphasize the correctness of the 
description, and to point out that in the Orang this limb is well marked. As a rule, 
we believe that it will be found that this limb, or its secondary continuation, reaches 
down to the horizontal level of the genu of the Fissure of Rolando. For localising 
purposes, however, as we have shown in the Bonnet, the important feature of the 
superior frontal sulcus is not so much the vertical as the horizontal limb ; for, on 
continuing the direction of the latter limb backwards across tlie ascending frontal and 
parietal convolutions it constitutes, according to our observation, the boundary 
* ‘ Phil. Trans.,’ B, 1888. 
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