FRONTAL AND PARIETAL LOBES 685 



ascending and anterior limits of the fissure of Sylvius cut into the substance of this 

 convohition, and divide it posteriorly into three parts, viz. pars or])italis, pars 

 triangularis, anil ])ars l)asilaris. The pars orbitalis is situated l)elo\v the anterior 

 linilj of the Hyivian fissure. The pars triangularis is included between the 

 ascending and anterior limits of the fissure. The pars basilaris is placed Ijehind 

 the ascending limb of the Sylvian fissure between the latter and the inferior prte- 

 central sulcus. 



This cuiivolutiun is of great physiological interest, as the faculty of articulate speech is local- 

 ised in its posterior i)art (Broca's convolution). Disease of tliis liuiitcd portion of brain substance 

 on the left side i)r(Kluces apliasia, except in lelt-handed ])ersons, in whom the speech-centre is 

 placed in the corresponding position on the right side. The siieech-centre is limited to the pars 

 basilaris. According toFerrier, the speech-centre includes (in addition to tlie aliovej the inferior 

 extremity of the ascending frontal convolution and a small part of the a.scending jjarietal imme- 

 diately behind the lower end of the fissure of Rolando. 



The orbital surface of the frontal lolje appears flattened in brains hardened in 

 the ordinary manner; but when the brain is hardened in situ (fig. 410) it i)rc- 

 sents a ver\'^ characteristic concavity, being moulded on the orbital plate of the 

 frontal bone. The outer half of the orbital surface looks directly downwards, but 

 the inner half, following the curve of the orbital plate, looks much more outwards 

 than downwards, and forms a tolerably sharp edge where it meets the internal 

 surface. Two fissures are situated on this surface, viz. the triradiate and the 

 olfactory. 



The triradiate fissure, as its name implies, is formed by three branches, which 

 radiate from a common point. One branch is directed forwards, anotlier l)ackwards 

 and inwards, and the third outwards. Another branch frequently springs from 

 the external division near its centre, and in such cases the triradiate fissure assumes 

 an H -shaped outline. 



The olfactory or straight sulcus is placed a short distance external to the great 

 longitudinal fissure, and lodges the olfactory tract and bulb. 



The convolutions on the orljital surface comprise the three orbital convolutions 

 and the straight convolution. 



The orbital convolutions are Avedge-shaped masses placed between the limbs of 

 the triradiate fissure. They are called, from their position, internal, anterior, and 

 posterior. They are continuous with the su])erior, middle, and iid'eiior frontal 

 convolutions ros] x'ctively. 



The straight convolution, or gyrus rectus, is situated betAveen the sulcus of 

 the same name and the margin of the great longitudinal fissure. It is continuous 

 in front Avith the superior frontal convolution, and on the inner margin of the 

 hemisi»here with the marginal gyrus. It is sometimes described as a part of the 

 internal orljital convolution. 



The convolutions on the internal surface of the frontal lobe are the marginal and the 

 paracentral. The student is recommended to defer the study of these till a later period (page 

 690). 



The PARIETAL LOBE occupics the portion of the convex surface betAveen the 

 frontal and occijdtal lobes and above the temporo-sphenoidal lobe. It is bounded in 

 front by the fissure of Rolando. Internally it is continued into the mesial surface 

 of the hemisphere, Avhere it is marked oft' from the adjacent lobes by the upturned 

 end of the calloso-marginal fissure in front, by the intornal i>arieto-o(ci]>ital fissure 

 behind, and more ol)Scurely below by the variable sulcus subparietalis. On the 

 convex surface of the hemisphere the posterior limits of the parietal lobe are in a 

 great measure artificial. The external parieto-occipital fissure marks it oft' for a short 

 distance, and the l)oundarv is then crossed by convolutions termed annectant gyri, 

 Avhich run in a sagittal direction and so bring the parietal and occi})ital lobes into 

 direct continuity. There is, hoAvever, a notch, the pree-occipitnl notch (fig. 410), 

 placed at the loAver margin of the hemisphere between the occipital and temporal 

 lobes, and if a line be draAvn from the extremity of the external jtarieto-occipital 

 fissure to this notch, the upper ])art of the line, together Avith the external parieto- 

 occipital fissure, separates the parietal from the occipital lobe. The parietal lolje 



