THE <'i-:iti-:iut.\L HEMISPHERES 843 



constant in shape. Tl extend- a variable distance transversely across (lie superior 

 poll ion of the lobe, and. as noted above, it is frequently continuous with the inter- 

 parietal sulcus through its occipital ranius, and when so. it appears as the posterior 

 terminal bifurcation of this sulcus (fig. (i'_M). When detached, ii often occurs merely 

 as a definite furrow with few ranii. and sometimes the ramus by which it otherwise 

 would join the interparietal sulcus is entirely absent. ('_') The lateral occipital 

 sulcus is always short . and has its deepest portion below t he transverse sulcus. It 

 usually has a somewhat oblique course towards the supero-niesial border. Some- 

 times it occurs in .several detached pieces, then known collectively as the lateral 

 iiccijiiliil xnlri. 



Therefore, the gyri of the convex surface of the lobe are also variable. They 

 are not sullicient ly constant to merit individual names. The lateral occipital sulcus 

 or siil.-i roughly divide them into an inferior and lateral area, known as the -lateral 

 occipital gyri, and into a superior area, the superior occipital gyri. The lateral 

 area is continuous into the gyri of the temporal lobe, while the superior area is con- 

 tinuous into the gyri of the parietal lobe. 



The mcxiiil xiii-jare of the occipital lobe is separated from that of the parietal lobe 

 (precuneus) and from the gyrus cinguli of the limbic lobe by the well-marked 

 parieto-occipital fissure. It comprises the constantly denned, wedge-shaped lobule 

 known as the cuneus, and the posterior and mesial extremity of the lingual gyrus. 

 Since the '^renter portion of the length of the lingual gyrus is involved in the basal 

 surface of the temporal lobe, this gyrus as a whole has been considered as belonging 

 to the temporal lobe (see figs. GlOand ('J4). The cuneus is separated from the lingual 

 gyrus by t he posterior portion of the calcarine fissure, which always terminates in a 

 bifurcation, one lirnb of which invades the cuneus near the superomesial border. In 

 addition the cuneus may contain other twigs from both the fissures bounding it, and 

 also, when wide, may contain one or more short, detached sulci cunei. 



The calcarine fissure and the parieto-occipital fissure are almost invariably joined in 

 the human brain, forming a Y-shaped figure, the prongs of which give the cuneus its shape. 

 The calcarine figure begins on the tentorial surface in the posterior portion of the bippocampal 

 gvrus of the limbic lobe, below the splenium of the corpus callosum, and extends backwards 

 aero-^ the internal occipital border of the hemisphere. It then bends downwards andproe. 

 to its terminal bifurcation in the polar portion of the occipital lobe. The stem or hippocampal 

 portion of the fissure is deeper than the posterior or occipital portion. It produces a well-marked 

 eminence in the inner wall of the posterior cornu of the lateral ventricle, known as the calcar 

 orw or ////I/).). -n in/nix minor. It is developed separately from the posterior portion, which also 

 first appears as two grooves. All three parts are usually continuous with each other before birth. 



The parieto-occipital Insure usually appears from the first as a continuous groove. It 

 begins in the supero-niesial border of the hemisphere, rarely extending into the convex surface 

 more than 10 mm. (external parieto-occipited /ixxure), thence it extends vertically downwards 

 across the mesial surface (inli rn/il /HI rii lo-occiptial fissure), and terminates by joining the calcarine 

 fissure at the region of the downward bend of the latter, or at about the junction of its anterior 

 and middle thirds. In certain of the lower apes and in the brain of the chimpanzee there is no 

 junction between the. two fissures, they being kept apart by a narrow neck of cortex, the gyrus 

 cunei. Xeit her are they joined in the human foetus. If in the adult human brain the region of 

 their junction be opened widely, there will be found a submerged gyrus transitivus (deep annectant 

 gyrus), which is the gyrus cunei. superficial in the foetus. In the higher apes and in micro- 

 ei-pinlie idiots this gyrus may be on the surface or partially submerged. T\\o oilier gyri transit ivi 

 (aiinectaut gyri) are to be found by pressing open the calcarine fissure, and they mark the points 

 at which its three original grooves became continuous during its development into a boundary 

 between the runeus and the lingual gyrus. Of thc.se, the anterior cuneo-lingual gyrus < n 

 the lloor of the calcarine fissure on the posterior side of its junction with the parieto-occipital 

 fissure, and therefore near the gyrus cunei. The posterior cuneo-lingual gyrus occurs near the 

 region of the terminal bifurcation of the fissure. 



The tfiitorial surface of the occipital lobe is blended intimately with that of the 

 temporal lobe, from which it is separated only by an arbitrary line drawn to join the 

 line of demarcation for the convex surface, at the region of the pre-occipital notch, 

 and thence to the isthmus of the gyrus fornicatus the narrow neck of cortex con- 

 necting the gyrus cinguli with the hippocampal gyrus. just below the splenium of the 

 corpus callosuni (see fig. 619). The gyri blending the occipital and temporal lobes 

 across this line are the Ihi/junl </i/ri<s, already mentioned, and the fusiform gyrus 

 (occipito-temporal convolution). In fact, the tentorial surface of the lobe may be 

 considered as nothing more than the posterior extremity of the fusiform gyrus. and the 

 inferior portion of the same extremity of the lingual gyrus. The former is often 

 somewhat broken up and is then continuous into the lateral occipital gyri. The 

 two gyri are separated by 1 he ntllntcnil fissure, the end of which extends into the 



