234 



THE NERVOUS SYSTEM 



the base, nearly reaching the posterior ramus of the lateral fissure. It makes 

 an angle of about 70 degrees with the dorsal border. It is customary to recog- 

 nize two knee-like bends in this sulcus; one located at the junction of the dorsal 

 and middle thirds with concavity forward, and the other at the junction of the 

 middle and basal thirds with concavity backward. If the margins of the sulcus 

 are pressed apart a deep annectant gyrus may often be seen extending across 

 it, by which the continuity of the sulcus is to some extent interrupted. This is 

 explained by the fact that the sulcus usually develops in two pieces, which be- 

 come united as the depth of the sulcus increases. 



Lobes, The frontal lobe lies dorsal to the lateral cerebral fissure and rostral 

 to the central sulcus (Fig. 167). The remainder of the dorsolateral surface is 

 subdivided rather arbitrarily into the parietal, occipital, and temporal lobes. 



Frontal pole 



Lateral ( Ant. hor. ram. 

 cerebral I Ant. ascend, ram. ' 

 fissure { Post. ram. " 



Temporal pole' 



-Parietal lobe 

 ,'- Temporal lobe 



-- Parieto-occipital fissure 

 r - Occipital lobe 



Preoccipital notch 

 ''Occipital pole 



Fig. 167. Diagram of the lobes on the lateral aspect of the human cerebral hemisphere. 



The rostral border of the occipital lobe is usually placed at a line joining the end 

 of the parieto-occipital fissure with the preoccipital notch. The latter is a 

 slight indentation on the lateral border of the hemisphere about 4 cm. rostral 

 to the occipital pole; while the parieto-occipital fissure is a deep cleft on the 

 median surface (Fig. 170), which cuts through the dorsal border about midway 

 between the occipital pole and the central sulcus, but a little nearer the former. 

 The parietal lobe is situated between the central sulcus and the imaginary line 

 joining the parieto-occipital fissure with the preoccipital notch. It lies dorsal 

 to the lateral fissure and an imaginary line connecting that fissure with the 

 middle of the preceding line. The remainder of the dorsolateral surface belongs 

 to the temporal lobe. 



The Frontal Lobe. The rostral part of the hemisphere is formed by the 



