THE CEREBRUM 473 



sphere, and it encloses the posterior horn of the lateral 

 ventricle, v On the surface it is very imperfectly mapped off 

 from the parietal and temporal lobes, which lie anterior to it. 

 It presents three surfaces and an apex or occipital pole. On 

 the medial aspect of the hemisphere it is separated from the 

 parietal lobe (i.e. the praecuneus) by the parieto-occipital 

 fissure. On the tentorial part of the inferior surface it is not 

 separated either from the temporal lob.e or from the gyrus 

 hippocampi, which lie anterior to it. It is necessary, there- 

 fore, on this aspect, to employ an arbitrary line of demarcation, 

 which extends from the prse-occipital notch, on the infero- 

 lateral border of the hemisphere, to the isthmus of the gyrus 

 fornicatus, which lies below the posterior end of the corpus 

 callosum. On the lateral surface the parieto-occipital fissure, 

 and an arbitrary line from that fissure to the prae-occipital 

 notch, may be regarded as separating the occipital from the 

 parietal and temporal lobes. 



Medial Aspect of the Occipital Lobe. On the medial 

 surface are (i) the calcarine fissure; (2) the cuneus ; and 

 (3) part of the gyrus lingualis. 



The calcarine fissure commences below the isthmus of 

 the gyrus fornicatus and takes a curved course towards the 

 occipital pole where it bifurcates into short branches. At a 

 point somewhat nearer its anterior than its posterior extremity 

 it is joined by the parieto-occipital fissure and the two fissures 

 together form a >--shaped figure. 



When calcarine and parieto-occipital fissures are fully opened up, so 

 as to expose the bottom in each case, two well-marked deep or submerged 

 gyri will be displayed (Fig. 189). One of these, the gyrus cunei, marks 

 off the parieto-occipital fissure from the calcarine fissure ; the other in- 

 terrupts the calcarine fissure immediately posterior to its junction with 

 the parieto-occipital. It is called the anterior cuneo-lingual deep gyrus, 

 and it divides the calcarine fissure into an anterior and a posterior part. 

 The anterior part of the calcarinefissure corresponds very nearly to the stem 

 of the >- -shaped fissural arrangement. It is very deep and, being a 

 complete fissure, it gives rise to an elevation, called the calcar avis, on the 

 medial wall of the posterior horn of the lateral ventricle. The posterior part 

 of the calcarine fissure is much shallower. 



The cuneus (Fig. 187) is the wedge-shaped or triangular 

 district on the medial aspect of the occipital lobe between 

 the parieto-occipital and calcarine fissures. 



The gyrus lingualis (Figs. 187, 188) is a well-marked gyrus 

 situated between the calcarine fissure above and the pos- 



