646 THE NERVOUS SYSTEM. 



limb, which passes forward, and a short ascending limb, which passes upward into 

 the inferior frontal convolution. It is then continued backward as the horizontal 

 limb, which terminates by an upward inflexion in the parietal lobe. It occupies 

 the middle third of the lateral surface of the hemisphere. 



The Fissure of Rolando is situated about the middle of the outer surface of the 

 hemisphere, and, coursing obliquely downward arid forward, divides the surface 

 of the hemisphere into approximately equal parts. It commences at or near the 

 longitudinal fissure, a little behind its mid-point, and runs sinuously downward 

 and forward, to terminate a little above the horizontal limb of the fissure of 

 Sylvius, and about half an inch behind the ascending limb of the same fissure. 

 It forms two chief curves : the upper or superior genu is concave forward and 

 upward, while the lower or inferior genu has its concavity directed backward. 



The parieto-occipital fissure is only seen to a slight extent on the outer surface 

 of the hemisphere, being situated for the most part on its mesial aspect. The 

 portion on the outer surface is called the external parieto-occipital fissure, to dis- 

 tinguish it from the part continued on to the internal surface, which is termed the 

 internal parieto-occipital fissure. The external parieto-occipital fissure commences 

 about midway between the posterior extremity or occipital pole of the brain and 

 the fissure of Rolando, and runs downward and outward for about an inch. 



These three fissures divide the external surface of the hemisphere into four 

 lobes: the frontal, the parietal, the occipital, and the temporal. To these must be 

 added (1) the central lobe, or island of Reil, which is situated deeply in the Sylvian 

 fissure, and (2) the olfactory lobe, which is found at the base of the brain and was 

 formerly described under the name of the olfactory nerve. 



The Lobes on the External Surface. The lobes on the external surface have 

 received their names from the bones of the skull with which they are most nearly 

 in relation, but it must be borne in rnind that they do not correspond in shape or 

 limit with the bone after which they are named. The division is, moreover, to a 

 certain extent artificial, as will be seen from the following description. If a line 

 is drawn in continuation of the external parieto-occipital fissure downward and 

 outward to the lower border of the hemisphere, it will impinge on a slight notch, 

 the pre-occipital notch, and if a second line is prolonged backward from the hori- 

 zontal part of the fissure of Sylvius to join the first, the division of the outer surface 

 of the hemisphere into four lobes will be accomplished (Fig. 344). The portion in 

 front of the fissure of Rolando is the frontal lobe ; that behind the fissure of 

 Rolando and above the fissure of Sylvius is the parietal lobe ; the portion behind 

 the parieto-occipital fissure and its continuation is the occipital lobe ; and the part 

 below the fissure of Sylvius and in front of the occipital lobe is the temporal lobe. 



The Fissures and Lobes of the Mesial and Tentorial Surfaces. The mesial sur- 

 face of the cerebrum can only be fully viewed by dividing the corpus callosum and 

 the structures beneath it longitudinally in the middle line ; in order to expose the 

 tentorial surface, the pons Varolii, cerebellum, and medulla must be removed, by 

 division of the crus cerebri on either side. When this has been done, a section 

 such as is represented in Fig. 345 will be shown. The parts in the centre, below 

 the corpus callosum, belong to the interior of the brain, and will be disregarded 

 for the present, while the lobes and fissures of the remaining portion of the hemi- 

 sphere are considered. The fissures are five in number, in addition to a small part 

 of the fissure of Sylvius, the commencement of which is seen, separating the 

 frontal and temporal lobes. These fissures are named the calloso-marginal, the 

 internal parieto-occipital, the calcarine, the collateral, and the dentate or hippo- 

 campal. 



The calloso-marginal fissure commences 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 in front of the genu of that body, it con- 

 tinues from before backward, between the upper margin of the hemisphere and 

 the convolution of the corpus callosum, to about midway between the anterior 

 and posterior extremities of the brain, where it ascends to reach the upper margin 



