EXTERNAL SURFACE. 



525 



the under surface of the brain, close to the anterior perforated spot, 

 and passes transversely outwards to the lateral surface of the hemi- 

 sphere, where it divides into a short anterior ascending limb, and a 

 longer posterior horizontal limb. The iissure in the base is a deep 

 cleft, of which the posterior lip projects over the anterior. This part 

 differs from other fissures in being due to a fold of the whole brain 

 in its development, the others being due merely to duplications of the 

 cortical layer. AVitliin the fissure of Sylvius is the isolated group of 

 convolutions of the island of Keil or " central lobe." 



The anterior or ascending limb (/. S>/. a), about an inch in length, 

 runs upwards and forwards into the frontal lobe, the lowest convolu- 

 tion of which curves round it. 



The posterior or horizontal limb (/. Si/. jO passes backwards, ascend- 



Fig. S70. 



ituictal J. 



Tronlal.X. 



Temporo — Spheuuidal . L . 



Fig. 370. — Convolutions of Outer Surface of Right Hemisphere. From a simply- 

 convoluted European Brain. About A 



The convolutions are, for the most part, indicated by Roman, the fissures by italic 

 letters. The dotted lines indicate the divisions into lobes, the names of which are given 

 in full at the margin of the hemisphere. /. Ho., fissiu-e of Rolando ; par.-oc.f., parieto- 

 occipital fissure;/. St/, a., anterior limb, and/. Sy. p., posterior limb, of the fissure of 

 Sylvius ; s.fr. c, m. fr. c, i. fr. c, superior, middle, and inferior frontal convolutions ; 

 asc. fr. c. , ascending ditto ; asc. par. c. , ascending parietal convolution ; Sup. par. 1. , 

 superior parietal lobules ; s. m. c. , supra-marginal convolution ; ang. c. , angular convolu- 

 tion; int. 'par.f., intra-parietal fissure ; s. oc. c, m. oc. c, i. oc. c, superior, middle, 

 and inferior occipital convolutions; s. t.-s. c, m. t.-s. c, i. t.-s. c, superior, middle 

 and inferior temporo-sphenoidal convolutions; fll. /., parallel fissure; o^ a", o?, a*, 

 first, second, third and fourth annectant convolutions ; c. L., -vrithin the fissui-e of Sylvius, 

 central lobe, or Island of Reil. 



ing sHghtly, through the middle third of the hemisphere. Its extremity 

 is usually l3ent vertically upwards. It separates the parietal lobe 

 above, from the temporo-sphenoidal lobe below it. 



The group of convolutions, which occupy the angle between the 

 two divisions of the fissure of Sylvius, has been collectively termed 

 the "operculum.^' 



The fissure of Rolando {fig. 370, fEo.) or Central Sulcus, extends 



