THE BRAIN. 



33 



The central lobe or island of Reil, also called the insula, consists of five 

 to seven convolutions which radiate upward; it. can be seen by separating the two 

 sides of the anterior portion of the fissure of Sylvius. 



THE FISSURES AND CONVOLUTIONS OF THE BRAIN. 



The surface of the brain is wrinkled or thrown into folds, producing elevations 

 and depressions. The elevations are called convolutions or gyri, and the depres- 

 sions, fissures or sulci. 



The fissures are called main or subsidiary fissures, according to their impor- 

 tance. The five main fissures are the longitudinal fissrire, which separates the 

 hemispheres; the transverse fissure, which separates the cerebrum and cerebellum 

 and communicates with the third ventricle ; the fissure of Sylvius ; the fissure of 

 Rolando, or central fissure, and the parieto-occipital fissure. 



THE LATERAL SURFACE OF THE HEMISPHERES. 



The frontal lobe has a superior, an inferior, and a precentral sulcus. The first 

 two divide the anterior portion into the s^lperior, middle, and inferior frontal convo- 



Sulcus praecentralis 



Sulcus centralis (Rolandi) 

 Sulcus postcentralis 



^Sulcus interparietalis 



Fissura parieto- 

 occipitalis 



Sulcus occipitalis 

 iransversus 



Sulcus frontalis 

 superior 



Sulcus occipitalis 

 lateralis 



XSF W^IM^^lP^^rM -* o.-socc . 



Sulcus frontalis 

 inferior 



Fissura lateralis (Sylvii) 



Sulcus temporalis superior 



Sulcus temporalis rnedius 



FIG. 41. Fissures, sulci, and gyri (convolutions) of the lateral surface of the cerebral hemisphere. 



lutions. That portion of the inferior or third left frontal convolution which surrounds 

 the ascending limb of the fissure of Sylvius is called Brocrf s convolution, and is 

 the centre for speech. Posterior to these and running upward and backward, form- 

 ing the anterior wall of the central fissure, is the precentral or ascending frontal 

 convolution. 



The convolution forming the anterior extremity of the parietal lobe and the 

 posterior wall of the central sulcus or fissure of Rolando is called the postcentral or 

 ascending parietal convohdion. Immediately behind it is the postcentral or inter- 

 parietal sulcus. The upper portion of this sulcus divides, one branch going up- 

 ward and one backward. Immediately above the posterior branch is the superior 

 parietal gyrus or lobule, and below it and surrounding the posterior extremity of the 

 fissure of Sylvius is the sripra marginal gyru s. Posterior to the supramarginal gyrus 

 and surrounding the posterior extremity of the superior temporal, or temporosphe- 

 noidal sulcus is the convolution known as the angular gyrus. 



The occipital lobe on its convex surface is divided into superior and inferior 

 occipital convolutions by the lateral occipital sulcus. 



The temporal or temporosphenoidal lobe is also divided into superior, middle, 

 and inferior, or first, second, and third temporal convolutions by the superior, or par- 

 allel, and middle fissures. On the under surface is a fourth temporal convolution, 

 separated from the third by the inferior temporal fissure. These fissures may not be 

 distinct. 



