II36 



HUMAN ANATOMY. 



secondary importance, while those of primary value are comparatively obscure in the 

 fully formed human brain. The interlobar fissures, six in number, are : ( i ) the 

 fissure of Sylvius, {2) the cetitral fissure, (^3) \^\^ parieto-occipital fissure, (4) the 

 collateral fissure, (5) the calloso-marginal fissure and (6) the limiting sulcus ofi 

 Reil. 



The lobes marked off by these fissures with varying degrees of certainty are : ( i ) 

 t\\Qfirontal, (2) thft parietal, (3) t\\Q temporal, {4.) \.\vq occipital, (5) the lifu die, and 

 (6) the insula. An additional division, (^7) the olfiactory lobe, although of impor- 

 tance as representing the peripheral part of the rhinencephalon of osmatic animals (as 

 those possessing the sense of smell in a high degree are called), is not related to the 

 foregoing sulci and comprises the rudimentary olfactory bulb and tract and associated 

 parts (page 1151). It will be of advantage to describe the interlobar fissures as pre- 

 paratory to a detailed consideration of the lobes. 



The fissure of Sylvius (tissura cerebri lateralis) is the most conspicuous fissure 

 of the hemisphere. It begins on the inferior surface of the brain in a depression, the 

 vallecula Sylvii, which opens out on the anterior perforated space. The first part of 

 the fissure, its stem, passes horizontally outward to the lateral surface of the hemi- 

 sphere, forming a deep cleft which separates the orbital area from the underlying tem- 



FiG. 981. 



Rolandic figure 



Inferior precentral sulcus 



Inferior frontal sulcus 



Ascending: limb 



Posterior limb 



Orbital surface 

 Horizontal limb 



Portion of lateral surface of right hemisphere, showing ascending, horizontal and posterior limbs of Sylvian 

 fissure radiating from Sylvian point. B, T, O, pars basalis, triangularis and orbitalis of inferior frontal gyrus : 

 .S'7', superior temporal gyrus. 



poral pole. On reaching the surface at the Sylvia?! point, the fissure divides (Fig. 

 981) into {a) a short anterior horizontal branch, (b) a somewhat longer anterior 

 ascending branch, and (c) a long posterior branch. 



The atiterior horizontal branch (ramus anterior horizontalis), about 2 cm. in 

 length, extends forward into the inferior frontal gyrus parallel to and just above the 

 infero-lateral border, and forms the lower limit of the pars triangularis (page 1141). 



The anterior ascending branch (ramus anterior ascendens) passes upward and 

 slightly forward into the hind-part of the inferior frontal convolution for a distance of 

 about 3 cm. The frequently observed variations in the relation and arrangement of 

 the antCx-ior branches of the Sylvian fissure — the ascending and horizontal limbs in 

 many cases arising from a common arm, sometimes being fused into a single sulcus, 

 or again being absent — are due to atypical growth of the opercula, particularly of 

 the frontal. 



The posterior branch (ramus posterior), the main continuation of the fissure and 

 about 8 cm. in length, is directed horizontally backward, with a slight inclination 

 upward. It forms a very evident boundary between the anterior parts of the parietal 

 and temporal lobes which it separates by a deep cleft that usually ends behind in an 

 ascending limb surrounded by the angular gyrus (Fig. 980). Not infrequently the 

 fissure ends by dividing into two short arms, one of which penetrates the parietal 

 lobe while the other arches downward into the temporal lobe. 



