96 



Cerebral Convolutions. 



calloso-marg. 



paneto-occip 



Pli de pass, 

 sup. 



Pli dt. 

 pass, inf 



?. operc 



430. Left Hemisphere, from the outside. After Eberstaller. 



The brain, Encephalon, is divided into four principal parts, the c e r e b r u in, 

 cerebellum, Pans Varolii and Medulla oljlonyata. 



The cerebrum is divided into two hemispheres, right and left. Each hemi- 

 sphere has a dorsal or outer, convex surface, in contact with the vault 

 of the cranium, an inner, flat surface, which forms one side of the longi- 

 tudinal fissure, and an irregular under surface, in which is a dee}) cleft, 

 the fissure of Sylvius. The anterior extremity is called frontal pole, the 

 posterior, occipital pole. 



The entire surface of the hemispheres presents a number of convoluted emi- 

 nences, the convolutions or G-yri, separated from each other by fissures or 

 Sulci. At certain places, several, otherwise separate sulci may blend tog-ether, and this 

 is the chief cause for the great variety in the convolutions found in different 

 brains; if this is the case, a concealed convolution will always be found beneath 

 the union of the fissures; the places where these deep gyri are most frequent, 

 are marked by points in the diagram. 



The deepest of all the fissures is the Fissure of Sylvius, Fissura 

 Sylviij it commences as Fossa Sylvii, at the under surface of the brain, turns 

 outwards and backwards (Ramus posterior fissnrae Sylvii) and ends by divi- 

 ding into a Itamus ascendens and a short Hamus desccndens. At the place where 

 it curves around, the Rainus anterior ascenders branches off, often also a 

 liamus anterior horizontal! ft. The fissure of Sylvius divides the frontal lobe and 

 partly also the parietal lobe from the temporal lobe; the central lobe or 

 island of Reil, Insula Iteilii, lies deeply within its commencement, being rarely 

 visible except when the lips of the fissure are separated; the parts of the hemi- 

 sphere covering it, are called Operculum. 



The central fissure or fissure of Rolando, Snlcns centralis s. 

 Rolandi, is situated about the middle of the outer surface of the hemisphere; it 

 commences near the longitudinal fissure and runs downwards and forwards to 

 terminate a little above the horizontal limb of the fissure of Sylvius ; the parallel 

 convolutions which bound it, are named the ascending frontal, or Gyrus 

 centralis anterior and the ascending parietal, or Gyrus centralis posterior. That 

 portion of the brain, situated in front of the fissure of Rolando, and above and 

 in front of the fissure of Sylvius is called the frontal lobo ? Lolus frontalis. 



