684 THE SERVOUS SYSTEM 



tills peculiarity is shared, however (hut uut so coustantlj'), hy the i)ra3central sulcus and hy tlie 

 vertical part of the intrai)arietal fissure. It is one of the uiost iiiii>ortaiit landmarks on the cere- 

 hral surface, as the principal motor centres of the cortex are situated around it. 



The Lobes of the Cerebral Hemispheres avith the Fissures and 



Convolutions 



Each cerebral liemisphere is divided into five lobes, viz. the frontal, jiarietal, 

 occipital, teniporo-sphenoidal, and the central lobe or island of Reil. Four of these 

 lobes are visible on the peripheral part of the hemispheres; the fifth, or island of 

 Reil, is deeply i)laced and is concealed by the operculum. 



Two other lobes, the liml)ic and the olfactory lobe, are also described. 



The FRONTAL LOBE occupies the fore part of the hemisphere and presents 

 three surfaces: a convex supero-external or frontal, an inferior or orbital, and 

 an internal or mesial. The convolutions and fissvu-es on the mesial and tentorial 

 surfaces of the hemisj^here will be described sei^arately. The frontal lobe is bounded 

 beliind bv the fissure of Rolando, and below, for al)outits posterior half, by the stem 

 and posterior limb of the fissure of Sylvius. It is limited internally, l\v the calloso- 

 marginal fissure. 



The frontal surface is separated from the orbital liy a well-marked angle 

 which may be called the orbital margin. This margin is usually destitute of fis- 

 sures, and forms a curve concentric witli the orbital arch of the frontal bone. 

 Three fissures on the frontal surface recjuire descrij^tion: one of these, running in a 

 transverse direction, is termed the prajcentral, the other two, which run parallel to 

 the great longitudinal fissure, are named superior and inferior frontal. 



The praecentral sulcus is placed in front of and parallel to the fissure of 

 Rolando, and is usually divided by a sagittally-placed convolution into tAvo parts, 

 a superior and an inferior praecentral; the former is usually continuous with the 

 superior frontal fissure. 



The superior frontal fissure is sometimes discontinuous Avith the praecentral. 

 At its orl)ital end it (,>ften jjasses into a coronally placed, cuiTcd fissure, termed the 

 sulcus fronto-marginalis. The latter is situated just aboA'e the orbital margin of 

 the frontal lol)e. 



The inferior frontal fissure may lie continuous with the pnrcentral, luit a 

 narroAv gyrus usually separates the tAvo fissures. 



The convolutions on the frontal surface are four in number: one directed trans- 

 A'crsely to the long axis of the cerebrum, the ascending frontal; and three lying 

 in an antero-posterior direction, named superior, middle, and inferior frontal. 



The ascending frontal convolution bounds the fissure of Rolando in front, 

 and is placed l)etween that fissure and the prsecentral sulcus. It is connected at 

 the lower end of the fissure of Rolando Avith the ascending parietal convolution, and 

 losses into the })aracentral lobule on the inner face of the hemisphere. 



The superior, middle, and inferior frontal convolutions occupy the remainder of 

 the frontal surface, and are marked off by the superior and inferior frontal sulci. 

 They pass forwards, and become continuous Avith the convolutions on the orbital 

 surface. They are not simple, but are traversed lioth in a coronal and sagittal 

 direction by tertiary fissures. 



The superior frontal convolution is continuous l)ehind Avith the ascending 

 frontal, and ])asscs in front into the gyrus rectus and into the internal and anterior 

 orbital couA'olutions. It is continuous with tlie marginal convolution on the inner 

 face of the hemisphere. 



The middle frontal convolution is continuous behind Avith the ascending 

 frontal Avhen tlie superior and inferior praecentral fissures are discontinuous. It 

 ])asses in front into the anterior orbital gyrus. It is almost invariably continuous 

 Avith the superior and inferior frontal convolutions at the orbital margin of the 

 frontal lobe, and is often connected to these convolutions l)y secondary gyri passing 

 across the su]ierior and inferior frontal fissures. 



The inferior frontal convolution is continuous behind with the ascending 

 frontal, and joins the external orbital gyrus aroimd the orbital margin. The 



