THE CEREBRUM 405 



lobe is entirely under cover of the parietal bone ; whilst, on 

 the other hand, the parietal bone covers all the parietal lobe 

 and parts of each of the other lobes a considerable part of 

 the frontal lobe, and smaller portions of the occipital and 

 temporal lobes. Further, each of the lobes forms part not 

 only of the supero-lateral but also, at least, of one other surface 

 of the hemisphere, where it lies in relation with parts other 

 than the bones after which it is named (Fig. 177). 



Lobus Frontalis. The frontal lobe appears on the supero- 

 lateral, the inferior, and the medial surfaces of the hemisphere. 



The Supero-lateral Surface of the Frontal Lobe is bounded, 

 above, by the supero-medial border ; below, by the super- 

 ciliary border and the anterior part of the posterior ramus of 

 the lateral fissure; and it extends, antero-posteriorly, from 

 the frontal pole to the central sulcus. It is divided by three 

 sulci into four chief gyri. The sulci are the precentral, and 

 the superior and inferior frontal. The gyri are the anterior 

 central and the superior, middle and inferior frontal. 



Sulcus Prcec entrails. The precentral sulcus consists of 

 two parts, superior and inferior : they run obliquely upwards 

 and backwards, parallel with the central sulcus of the hemi- 

 sphere, and although they are sometimes continuous in the 

 adult they are developed independently of one another. The 

 superior precentral sulcus is generally connected with the 

 superior frontal sulcus. The inferior precentral sulcus has 

 usually two limbs, a vertical, which lies parallel to the lower 

 part of the central sulcus, and a horizontal or oblique limb 

 which extends forwards into the middle frontal gyrus (Figs. 

 138, 152). 



Sulcus Frontalis Superior (Figs. 138, 152). The superior 

 frontal sulcus extends forwards from the superior precentral 

 sulcus. 



Sulcus Frontalis Inferior (Figs. 138, 152). The inferior 

 frontal sulcus commences posteriorly in the angle between 

 the vertical and the horizontal or oblique part of the inferior 

 precentral sulcus, and, not uncommonly, it is confluent with 

 one or other of the two parts. As it passes forwards it 

 descends towards the superciliary border and ends a short 

 distance from it in a terminal bifurcation. 



Gyrus Centralis Anterior. The anterior central gyrus 

 extends, obliquely, across the supero-lateral surface of the 

 hemisphere, from the supero-medial border above, to the 

 in 26 b 



