864 THE NERVOUS SYSTEM 



marked parieto-occipital fissure. It comprises the constantly defined, wedge- 

 shaped lobule known as the cuneus, and the posterior and mesial extremity of the 

 lingual gyrus. Since the greater 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. 671, 676). The cuneus 

 is separated from the lingual gyrus by the posterior portion of the calcarine 

 fissure, which always terminates in a bifurcation, one limb 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 fissure begins on the tentorial surface in the posterior portion of the hippocampal 

 gyrus of the limbic lobe, below the splenium of the corpus callosum, and extends backward 

 across the internal occipital border of the hemisphere. It then bends downward and proceeds 

 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 medial wall of the posterior cornu of the lateral ventricle, known a^ the 

 calcar avis or hippocampus minor. It is developed separately from the posterior portion, which 

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

 birth. 



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

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

 more than 10 mm. {external parieto-occipital fissure), thence it extends vertically downward 

 across the mesial surface {internal parieto-occipital fissure), and terminates by joining the cal- 

 carine 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. Neither 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 transitory gyrus 

 (deep annectant gyrus), which is the gyrus cunei, superficial in the foetus. In the higher apes 

 and in micro-cephalic idiots this gyrus may be on the surface or partially submerged. Two 

 other transitory gyri (annectant gjTi) 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 cuneus and the lingual gyrus. Of these, the anterior 

 cuneo-lingual gyrus crosses the floor 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 tentorial 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 connecting the gyrus cinguli with the hippocampal gyrus, just below 

 the splenium of the corpus callosum (see fig. 671). The gyri blending the occip- 

 ital and temporal lobes across this line are the lingual gyrus, already mentioned, 

 and the fusiform gyrus (occipito-temporal convolution). In fact, the tentorial 

 surface of the lol^e may be considered as nothing more than the posterior ex- 

 tremity 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 gj^i are separated by the collateral fissure 

 the posterior end of which extends into the occipital lobe. The fusiform gyrus is 

 bounded laterally by the inferior temporal sulcus, which sometimes is continuous 

 by a lateral twig, across the posterior end of this gyrus, with the collateral fissure. 



The Rhinencephalon 



The rhinencephalon or olfactory brain includes those portions of the cerebral 

 hemisphere which are chiefly concerned as the central components of the olfactory 

 apparatus. Owing to the preponderant development of the other divisions of the 

 hemisphere, the parts comprising this division appear relatively but feebly de- 

 veloped in the human })rain. In most of the mammals the sense of smell is 

 relatively much more highly developed, and in many of the larger mammals ■ 

 the parts comprising the rhinencephalon arc of greater absolute size than in man, 

 though their cerebral hemispheres may be considera))ly smaller. In the human 

 foetus the parts of the rhinence[)halon are relatively much more prominent than 

 after the completed differentiations into the adult condition. In the broader 



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