46 



LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



island of Reil. The island is, therefore, that part of the cortex 

 which immediately adjoins the ganglia of the cerebrum. At 

 first it lies on the surface, but little by little it is covered in i>y 

 the overlapping hemispheres. 



You will easily find the fissure of Sylvius in the adult 

 brain. It is the largest of the sulci, and on separating its walls 

 you will discover the island, and see that it is traversed by a 

 number of perpendicular and oblique sulci. In the sixth month 



Gca So 



FIG. 27. 



The left hemisphere with the fissure of Sylvius drawn apart in order to show the con- 

 volutions in the island (if Reil (In). Sc, sulcus rvntralis ; (ica. (jcp. gyms cen trails, 

 anterior and posterior: Fop, fissura parieto-occipitalis. (After Henle.) 



of pregnancy the two divisions of the fissure of Sylvius the 

 anterior and posterior are plainly to be seen. All the rest of 

 the brain is still smooth. (Compare Fig. 7.) 



After this period, furrows (sulci or fissures) are developed 

 on the surface of the hemispheres by means of local elevations 

 which increase more and more during the later months of fcetal 

 life until birth, at which time almost all the fissures and convo- 

 lutions which the adult brain will possess are clearly marked out. 



The following purely diagrammatic drawings may serve as 



