192 



ANATOilY OF THE CEXTEAL NERVOUS SYSTEM. 



are distinctly to be seen. All the rest of the brain is still smooth (compare 

 Fig. 23). 



From this developmental period on, iissures (sulci) are formed on the 

 surface of the hemisphere by local elevations (gyri) of the cortex. These 

 sulci and gyri increase more and more in the later months of fetal life, until, 

 at the time of birth, almost all the fissures and convolutions which the adult 

 brain will possess are clearly defined. 



The following drawings, purely diagrammatic, may serve as guides in 

 the study of the surface of the hemispheres. Only the more important 

 and constant fissures and convolutions are therein indicated. A simple 



Fig. 129. — The left hemisphere ^vith the flssura Sylvii drawn apart in order 

 to show the insula {In). Sc, Sulcus centralis, ffca, gcp, Gyrus centralis anterior 

 and posterior. Fop, Fissura parieto-oceipitalis. (After Henle.) 



diagram is more easily remembered than representations of the surface of the 

 brain, which reproduce the smaller gyri and shallower sulci, all of which 

 are inconstant, alongside of those which are more pronounced and constant. 

 First locate the fissure of Sylvius. It separates the greatest part of the 

 temporal lobe from the rest of the brain. A long posterior limb and one or 

 two short anterior branches, which are directed upward, are distinguished 

 on it. The mass of the brain which lies at the point where these join one 

 another covers the island of Eeil and is called the operculum. If those parts 

 of the brain which surround the fissure of Sylvius are separated from one 



