PARIETO-OCCIPITAL FISSURE. 143 



conjoined fissure almost as far as the upper margin of the hemisphere ; this conjunction is 

 sometimes found in the human subject. 



Fissure of Rolando. The fissure of Rolando, 1 or central sulcus (Huschke) 

 (fig. 101, Ro. ; and figs. 105, 107 (monkey) ), extends across the lateral convex surface 

 of the hemisphere interrupting the general longitudinal course of the gyri and sulci. 

 The parallel convolutions which bound it are named respectively the ascending 

 frontal and ascending parietal convolution or the central gyri. It begins above, near 

 the vertex or highest point of the hemisphere, somewhat behind the middle of 

 the great longitudinal fissure, and passes downwards and forwards to end near 

 the middle of the fissure of Sylvius, the posterior limb of which it sometimes (but 

 rarely) joins. The junction is effected by the medium of a small sulcus, precenlral 

 transverse sulcus (figs. 101, 106, p. c. tr.\ the inferior transverse sulcus of Eberstaller, 

 which is usually separated from the lower end of the Rolandic fissure by an annectent 

 gyrus joining the two central convolutions. The Rolandic fissure usually reaches 

 the superior border of the hemisphere, but sometimes stops short of that line ; it 

 often exhibits a hook -like backward inclination at this extremity. As just mentioned, 

 it passes obliquely downwards and forwards, but not in a perfectly straight course, 

 for it has certain well-marked curves. At about the junction of its upper and middle 

 thirds it is generally curved with the concavity forwards ; just below its middle 

 third it has a second bend with the concavity backwards, while in the lowest part its 

 direction is nearly vertical, with a tendency of the lowermost end to curve backwards. 

 The upper curve sometimes forms a very distinct bend (superior genu, fig. 101, #), 

 representing a deep bay which is here found in the brain of the anthropoid apes (fig. 

 105). The middle part of the fissure is then nearly horizontal. It is very rarely 

 interrupted in its course, although on separating its lips it may often be seen that 

 there is a tendency to the appearance of an annectent gyrus about the level of the 

 superior geiiu, and it is here that the interruption is liable to occur. The fissure 

 of Rolando appears early (end of fifth month), being laid down in two parts, an 

 upper shorter, and a lower longer portion (Cunningham). In this double mode of 

 origin it resembles the precentral and postcentral sulci. 



The acute angle which the slope of the fissure of Rolando forms with the superior border 

 of the hemisphere is known as the Rolandic angle. According to Cunningham it averages 

 71*7. The two Rolandic fissures form therefore when looked at from above, a wide V, open 

 forwards and forming an angle of 143. The length of the fissure is about |ths of the whole 

 length of the hemisphere. It is relatively longer and more curved in the anthropoid apes than 

 in man. 



This fissure has once been found duplicated, the two fissures replacing it being separated 

 by a gyrus (g. Rolandicus) along their whole length (Griacomini). The condition was present 

 in both hemispheres of the same brain. 



Parieto-occipital fissure. The parieto-occiintal fissure is best marked on the 

 mesial surface of the hemisphere, where it appears as a deep cleft (fig. 102, p.-o.), 

 extending downwards and a little forwards from the margin of this surface to near 

 the posterior extremity of the corpus callosum, where it usually joins the calcarine 

 fissure, the two together forming a Y which encloses a wedge-shaped portion of the 

 occipital lobe (cuneus). On the convex surface the fissure is continued transversely 

 outwards for a variable distance, generally only a few millimeters (external part of 

 the parieto-occipital fissure (fig. 101, p.-o.} ). This fissure is here taken as the division 

 between the parietal and occipital lobes. The size of its external portion depends 

 (inversely) on the size of the annectent gyrus which curves round its outer extremity 

 and connects the parietal with the occipital lobe. In Quadrumana (figs. 105, 107) the 

 external portion of this fissure is concealed within a deep transverse cleft (Affenspalte) 



1 Although thus named by Leuret (Anatomie compares du systeme nerveux, 1839), from attention 

 having been directed to it by Rolando a few years previously, this fissure was noticed and figured by Vicq- 

 d'Azyr (Traite' d'anatomie et de physiologic, 1796). 



