THE BRAIN. 497 



third . month of intra-uterine life. It commences at the base of 

 the brain, and runs .outward, backward, and upward, giving off a 

 short anterior branch or limb. The continuation of the fissure 

 backward from where this branch is given off is the posterior 

 branch or horizontal limb, which ends in the parietal lobe. The 

 fissure of Sylvius is the boundary between the frontal and parietal 

 lobes on the one hand and the temporosphenoidal on the other. 

 At the middle and anterior part of this fissure, deeply situated, is 

 the island of Reil, or insula, or central lobe. 



(2) The fissure of Rolando starts near the median line, and runs 

 downward and forward nearly to the fissure of Sylvius. It is 

 the boundary between the frontal and parietal lobes. 



FIG. 288. Inner surface of right hemisphere : A, ascending frontal ; B, ascend- 

 ing parietal convolution ; c, terminal portion of the sulcus centralis, or fissure of 

 Eolaudo; (7(7, corpus callosum, longitudinally divided; Cf, collateral or occipito- 

 temporal fissure (Ecker) ; cm, sulcus callosomarginalis ; D, gyrus descendens ; Fi, 

 median aspect of the first frontal convolution ; (?/, gyrus fornicatus ; H, gyrus hip- 

 pocampi ; h, sulcus hippocampi, or dentate fissure; 0, sulcus occipitalis transversus; 

 oc, calcarine fissure ; oc', superior, oc", inferior ramus of the same ; Oz, cuneus ; po, 

 parieto-occipital fissure; Pi", precuneus ; Tt, gyrus occipitotemporalis lateralis 

 (lobulus fusiformis) ; Ts, gyrus occipitotemporalis medialis (lobulus lingualis) ; U, 

 uncinate gyrus. 



(3) The parieto-occipital fissure is about half-way between the 

 fissure of Rolando and the posterior extremity of the brain, and 

 is the boundary between the parietal and occipital lobes. 



Lobes of the Cerebrum. The lobes of the cerebrum are (1) the 

 frontal, (2) the parietal, (3) the occipital, (4) the temporosphen- 

 oidal, and (5) the central, or island of Reil. 



(1) The frontal lobe is above the fissure of Sylvius and in front 

 of the fissure of Rolando. It is divided into 4 convolutions by 

 3 sulci, or fissures as they are sometimes called. The precentral 

 fissure or sulcus is in front of the fissure of Rolando, and the 

 convolution between the two is the ascending frontal. From the 

 upper extremity of this sulcus the superior frontal sulcus runs 

 downward and forward between the superior and middle frontal 



32 



