II38 



HUMAN ANATOMY. 



owing to a marked angular backward projection of the substance of the precentral 

 convolution, situated about the junction of the upper and middle thirds of the fissure. 

 In consequence, the fissure presents in this part of its course a distinct curve, with 

 the concavity directed forward, the upper and lower limits of this bend consti- 

 tuting the superior and the inferior genu respectively (Fig. 980). The cortical 

 tissue filling this recess is of importance, since it represents the part of the precentral 

 gyrus devoted to the motor centre for the arm. Below the inferior genu the fissure 

 descends almost vertically, its lower end often bending slightly backward. The 

 angle which the general direction of the central fissure makes with the mesial plane 

 in the adult brain is on an average 71.7° (Cunningham), the Rolandic angle, as it 

 is called, of the two sides subtending therefore about 143° (Fig. 984). 



Fig. 984. 



Vf. 



Superior aspect of cerebral hemispheres; LF, longitudinal lisMiie; r., ;, Rolandic fissure; sg, /!?■, its superior 

 and inferior genu ; J. /c, superior precentral ; j.y, /.y", superior and inferior frontal ;/>/«, paramedian ;/>i,/-, /J >',/'', 

 inferior, superior, horizontal and occipital limbs of interparietal; /-<?, parieto-occipital ; t.o., l.o., transverse and 

 lateral occipital; Sasc, ascending limb of Sylvian ; f^asc, t-asc, ascending limbs of superior and middle temporal. 



Since the central fissure is usually developed from two separate parts, a longer 

 lower and a short upper (Cunningham, Retzius) which later become continuous, a 

 deep annectant gyrus is generally found crossing the bottom of the sulcus at the 

 junction of its upper and middle thirds. In exceptional cases the original separation 

 is continued by the deep annectant gyrus maintaining its superficial relations, the 

 adult fissure then being interrupted by the bridge which ordinarily is limited to the 

 bottom of the cleft. As a variation of very great rarity, completed doubling of the 

 central fissure has been observed. 



The parieto-occipital fissure (fisstira parieto-occipitalis) is seen chiefly on the 

 mesial surface of the hemisphere (Fig. 987), where it appears as a deep cleft which 

 extends from a point on the supero-mesial border of the hemisphere, about 4 cm. in 

 front of the occipital pole, downward and forward. This inner part of the fissure, 



