778 THE NERVOUS SYSTEM. 



limited by the fissure of Sylvius ; behind, on its external surface, it is connected 

 with the parietal and occipital lobes, and is limited artificially by a line continuing 

 the direction of the external parieto-occipital fissure across the outer surface of the 

 hemisphere. It is divided into three convolutions by two sulci. The superior of 

 these runs parallel to the horizontal limb of the fissure of Sylvius. It is named 

 the superior or first temporal or parallel sulcus, and it is well marked and con- 

 stant. The second, the middle or second temporal, is not so well marked or con- 

 stant ; it takes the same course at a lower level. 



The superior or first temporal convolution is situated between the horizontal 

 limb of the fissure of Sylvius and the superior temporal sulcus. It is continuous 

 behind with the supramarginal convolution. 



The middle or second temporal convolution is situated between the superior 

 and middle sulci of the same name, and is continuous behind with the angular 

 and middle occipital convolutions. 



The inferior or third temporal convolution is situated below the middle tem- 

 poral sulcus, and is separated from the external occipito-temporal (fourth tem- 

 poral) convolution, on the under (mesial) surface of the hemisphere, by a sulcus 

 which is called the inferior or third temporal sulcus. It is connected with the 

 inferior occipital convolution. 



The central lobe, or island of Reil (Figs. 467, 468), is situated in the fissure 

 of Sylvius; at the base of the brain it is separated, in front, from the posterior 

 orbital convolution by a nearly transverse sulcus, the anterior sulcus of Reil : 



externally, from the inferior frontal con- 

 volution and the lower ends of the ascend- 

 ing frontal and parietal convolutions by 

 another deep sulcus, the external side us 

 of Reil ; and posteriorly, from the tem- 

 poral lobe by a third sulcus, the pos- 

 terior sulcus of Reil. It is a triangu- 

 lar-shaped (apex downward) prominent 

 cluster of about six convolutions, the 

 gyri operti, so called from being covered 

 in by the gyri bounding the fissure. By 

 the removal of these convolutions the ex- 

 traventricular part of the corpus striatum 



FIG. 468. External surface of left hemisphere. Wrt , 1 'M KQ ^r>olirl 



The island of Reil is shown by raising the opercula W(n BU - 



and bending outward the upper edge of the tern- These Various Sulci of Reil, taken 



poral lobe, a and b are the portions of the insula . ' . 



separated by the sulcus centralis insulx. (Gegenbaur.) together, Constitute the SulcUS limitans 



insulce. The sulcus centralis insulce di- 

 vides the lobe into a pre-central and a post-central lobule, of which the former 

 corresponds to, or may be regarded as part of, the frontal lobe ; and the latter to 

 the parietal and temporal lobes. Those portions of the corresponding lobes from 

 which the above-mentioned sulci separate the island overlap it in the normal con- 

 dition, and are known as the opercula. 



THE MESIAL LOBES AND FISSURES OF THE HEMISPHERE. The arrangement 

 of the convolutions in this region is less complex: they are generally well defined, 

 and, some of them being of great length, there is not the same subdivision into 

 smaller lobes as on the external surface (Figs. 469, 470). The fissures on the 

 internal surface are five in number, and are named the calloso-marginal, the 

 parieto-occipital, the calcarine, the collateral, and the dentate. 



The calloso-marginal fissure is seen in front, commencing below the anterior 

 extremity of the corpus callosum : it at first runs forward and upward, parallel 

 with the rostrum of the corpus callosum, and, winding round the genu of that 

 body, it continues from before backward, between the upper margin of the hemi- 

 sphere and the convolution of the corpus callosum, to about midway between the 

 anterior and posterior extremities of the brain, where it turns upward to reach the 

 upper margin of the inner surface of the hemisphere (paracentral fissure of 



