414 



THE NERVOUS SYSTEM. 



early as the third month ; and in the adult it forms a basis for the 

 whole topographical division of the hemispheres. It commences as a 

 transverse indentation on the under surface of the brain, running 

 thence outward, backward, and upward, to form the anterior boundary 



FIG. 110. 



PLAN OF THE HUMAN BRAIN, IN PROFILE; showing its Fissures and Convolutions. S. Fissure of 

 Sylvius ; S'. Anterior Branch ; S". Posterior Branch ; B. Fissure of Rolando ; P. Parietal Fissure. 



of the temporal lobe. In some of the inferior animals all the convolu- 

 tions on the convexity of the hemispheres follow the course of this 

 fissure, bending round its upper extremity in a loop-like form ; and in 

 the human brain a similar general arrangement is distinctly visible. 



On the outer side of the cerebral hemisphere the fissure of Sylvius 

 presents, in man, two distinct branches, namely, a shorter, anterior 

 branch (S r ), and a longer, posterior branch (S r/ ). At its middle and 

 anterior portions, the fissure is very deep, concealing beneath its folds 

 a group of short radiating convolutions on the lower and lateral surface 

 of the brain, called the " Island of Reil," or the Insula. 



Externally the insula is covered by the convolutions included between 

 the anterior and posterior branches of the fissure of Sylvius, which pro- 

 ject downward from above and overlap, at this point, the deep-seated 

 parts. This portion of the cortical mass is known as the " Opercu- 

 lum," or cover. 



The second important fissure, on the convexity of the hemisphere, 

 is the Fissure of Eolando (R). This fissure runs from near the median 

 line outward and forward, reaching nearly to the fissure of Sylvius, 

 and forming the boundary between the frontal and parietal portions of 

 the hemisphere. It is bordered by two convolutions, running parallel 

 with itself, namely, the "anterior and posterior central convolutions." 



The third principal fissure is the Parietal Fissure (P). It starts 



