OCCIPITAL LOBE. 151 



The superior parietal convolution or superior parietal lobule (fig. 101) 

 is that part of the parietal lobe which lies above the pars honzontalis of the intra- 

 parietal sulcus, and behind the upper part of the last described convolution, from 

 which it is imperfectly separated by the upper part of the postcentral sulcus. Its 

 posterior limit is the boundary of the parietal lobe, viz., the parieto-occipifcal fissure, 

 round the extremity of which an annectent convolution connects this lobule with 

 the occipital lobe (upper parieto-occipital annectent gyrus). The superior parietal is 

 continuous on the mesial surface of the hemisphere with the quadrate lobule. It is 

 often deeply cut into by an ascending branch of the intraparietal sulcus. 



The quadrate lobule (precuneus, fig. 102) is the part of the parietal lobe which 

 is seen on the mesial surface of the hemisphere. It is bounded in front by the 

 posterior limb of the paracentral fissure, and behind by the parieto-occipital fissure, 

 and is incompletely separated from the posterior portion of the limbic lobe by the 

 postlimbic fissure. Its surface is indented by one or two furrows which cut the 

 margin of the hemisphere, and extend a variable distance over the surface of the 

 lobule (precuneate sulci). 



The inferior parietal lobule is embraced within the curve of the intraparietal 

 sulcus. It is bounded behind by the lower limb of the anterior occipital fissure. It 

 is divided into three gyri, viz. : the supramarginal, the angular, and the postparietal 

 (fig. 101). The supramarginal convolution lies behind and below the anterior 

 part of the intraparietal sulcus, beneath the lower end of which it is continuous with 

 the ascending parietal convolution. It arches round the upturned extremity of the 

 fissure of Sylvius, to become continuous with the superior temporal convolution 

 below, and with the angular behind. The angular gyrus, connected in front with 

 the supramarginal, bends over the end of the parallel (first temporal) sulcus, and is 

 continued below into the first and second temporal gyri. Behind, it is separated 

 from the postparietal gyrus by a shallow vertical sulcus, which may be absent. The 

 postparietal gyrus curves round the upturned end of the second temporal sulcus. 

 (In the brain figured this appears like a continuation of the first temporal sulcus.) 

 It is continued below into the second and third temporal gyri. 



OCCIPITAIi LOBE. This lobe is very well marked off in Quadrumana by 

 the deep cleft (Affenspalte) before mentioned (p. 144) ; but in man the line of 

 demarcation between it and the adjacent parietal and temporal lobes is less distinct. 

 It is relatively much larger in Quadrumana, especially in the lower monkeys, than 

 in man. 



The lobe is of pyramidal shape, lying behind the parietal and temporal lobes. 

 In the adult its length, as compared with that of the whole hemisphere, is about as 

 21 to 100 (occipital index, Cunningham). It has two free surfaces, an external or 

 lateral, and a mesial. It occupies the superior fossa of the occipital bone. On the 

 external surface it is bounded in front by the parieto-occipital fissure, by the anterior 

 occipital sulcus, and by a line connecting the lower end of this sulcus with the 

 posterior extremity of the calcarine fissure. The line in question mainly corresponds 

 with a horizontally-directed sulcus, which is termed by Eberstaller the lateral occipital : 

 this sulcus may therefore be regarded as the lower boundary of the occipital lobe on 

 the lateral surface. On the mesial surface of the hemisphere the lobe is marked off 

 by the parieto-occipital fissure from the quadrate lobule, and by the posterior calcarine 

 from the temporal lobe (fig. 102). 



In assigning the above limits to the occipital lobe I have followed Eberstaller. and have 

 departed from the usual description which includes in the lobe all the parts behind the plane 

 of the parieto-occipital fissure, and therefore the posterior parts of the parietal and temporal 

 lobes as here described. It seems to me, however, that Eberstaller's definition, although it 

 confines the occipital lobe in man to relatively small limits, has the advantage of assigning 

 well-marked anatomical boundaries to it and to the adjacent lobes, thus rendering a descrip- 

 tion of this part of the brain more easy. 



