184 A MANUAL OF ANATOMY. 



angle, opening forward, of about 71 degrees, with the 

 longitudinal fissure. 



The course of the fissure is not perfectly straight but 

 winding, and may be divided into three segments or thirds ; 

 these segments make with each other angles or bends 

 (genua) ; the angle formed between the upper and middle 

 thirds points backward, that formed by the middle and 

 lower thirds points forward (superior and inferior genu). 

 Fig. 26. 



The fissure of Rolando separates the frontal (anterior) 

 from the parietal (posterior) lobe. 



The Parieto-occipital Fissure. Figs. 26, 27. 



As this shows only slightly upon the outer surface of 

 the cerebrum its description will be omitted until the inner 

 surface of a hemisphere is described. Suffice to say that 

 its external portion is very short (one-half of an inch) and 

 is found midway between the fissure of Rolando and the 

 apex of the hemisphere behind, and partially divides the 

 parietal from the occipital lobe. 



The Frontal Lobe. Figs. 23, 24, 26. 



The frontal lobe is situated at the anterior part of the 

 cerebrum. It was thought to correspond to the limits of 

 the frontal bone hence its name but such is not the 

 case, as the lobe reaches backward beneath the anterior 

 third of the parietal bone. 



The frontal lobe presents three surfaces, the external, 

 internal, and inferior ; these are also called the frontal, 

 mesial, and orbital. The external surface of the lobe is 

 cut off from the rest of the cerebrum by the fissure of 

 Rolando behind and the fissure of Sylvius below. The 

 inferior surface rests upon the orbital plate of the frontal 

 bone. The internal surface is separated from the corres- 



