THE NERVOUS SYSTEM 1383 



horizontally outwards to the Sylvian point on the external surface 

 of the hemisphere, where it divides into three diverging branches. 

 It is a deep cleft, which is overhung posteriorly by the front part 

 of the temporal lobe, and it separates the orbital surface of the 

 frontal lobe from the temporal lobe. The posterior or Sylvian 

 border of the small wing of the sphenoid bone faces the fissure, 

 which lodges the middle cerebral artery. The limbs into which 

 the fissure divides at the Sylvian point are, as stated, three in 

 number, namely, anterior horizontal, anterior ascending, and 

 posterior horizontal. The anterior horizontal limb passes forwards 

 into the frontal lobe, its length being about | inch. The anterior 

 ascending limb passes upwards and slightly forwards into the frontal 

 lobe for about i inch, but its length is variable. The posterior 

 horizontal limb is the longest and most conspicuous. It passes 

 backwards on the external surface of the hemisphere for at lejist 

 2 inches, having portions of the frontal and parietal lobes above it, 

 and the temporal lobe below it. Finally it turns upwards into the 

 parietal lobe for a very short distance. 



The fissure of Rolando, also known as the cen£ra/sM/c«s, com- 

 mences at the supero-mesial border of the hemisphere, alif fle behind 

 its mid-point, and terminates above the centre of the posterior 

 horizontcd limb of the fissure of Sylvius. It does not usually open 

 into this limb, but may do so. Superiorly the fissure in most 

 cases intersects the supero-mesial border to reach the internal 

 surface of the hemisphere, upon which it passes backwards for a 

 very short distance. The direction of the fissure of Rolando is 

 irregularly downwards and forwards over the external surface of 

 the hemisphere, and it separates the frontal from the parietal 

 lobe. It describes t^'o bends. The upper genu has its concavity 

 directed forwards, and is situated about the junction of the upper 

 and middle thirds of the fissure. The lower genu has its concavity 

 directed backwards, and is situated on a more anterior plane than 

 the upper genu. Below the lower genu the direction of the fissure 

 is almost vertical, with a slight inclination backwards. 



The parieto-occipital fissure is situated about 2 inches behind 

 the upper end of the fissure of Rolando, and it separates the parietal 

 from the occipital lobe. It is composed of two limbs, external and 

 internal, which are continuous with each other at the supero-mesijQ 

 border of the hemisphere, where they form a right angle. The 

 external limb is situated on the external surface of the hemisphere, 

 [- upon which it passes transversely outwards for about | inch, when 

 it is arrested by the convolution which connects the parietal and 

 occipital lobes. The internal limb appears as a deep, almost vertical, 

 deft on the internal surface of the hemisphere, which opens into the 

 calcarine fissure a short distance behind the splenium of the corpus 

 callosum. 



The calloso-marginal fissure is situated on the internal surface of 

 the hemisphere. It commences below the rostrum of the corpus 

 callosum, contiguous to the locus perforatus anticus, and, bending 

 round the genu, it passes backwards above the corpus callosum. 



