THE TELENCEPHALON. 1147 



ends, after a short bend outward, by cutting into the hmbic lobe just below the 

 splenium of the corpus callosum (Fig. 987). This incision divides the posterior 

 extremity of the hippocampal gyrus into a narrow upper tract, the isthmus, which 

 links the gyrus with the callosal convolution, and a broader lower arm, which 

 establishes continuity between the hippocampal and lingual gyri. A short distance 

 in front of its middle, the calcarine fissure is joined by the lower end of the parieto- 

 occipital sulcus, the two furrows forming a •> shaped sulcus, between whose 

 diverging limbs lies the triangular cuneus. Although usually appearing as one 

 continuous fissure, the parieto-occipital and calcarine sulci are incompletely separated 

 by a deep annectant gyrus, which connects the cuneus with the limbic lobe. The 

 calcarine fissure itself is subdivided by a second sunken gyrus into an anterior and a 

 posterior part. The latter, the posterior calcarine fissure, is shorter and shallower, 

 than the front part and is not a total fissure. The other portion, the a7iterior calca- 

 rine fissure, is not only the deeper but completely invaginates the brain-wall, thereby 

 giving rise to the elevation known as the calcar avis, seen on the inner boundary of 

 the posterior horn of the lateral ventricle. 



The cuneus forms the chief part of the mesial aspect of the occipital lobe. It 

 is triangular in outline and lies between the parieto-occipital sulcus in front and the 

 posterior limb of the calcarine fissure below, whilst above and behind it reaches the 

 superior border of the hemisphere (Fig. 987). Its surface is frequently impressed 

 by one or more shallow vertical furrows. 



The lingual gyrus, also called the infracalcarine , is the irregular elongated 

 tract bounded mesially and above by the calcarine fissure, and laterally and below by 

 the collateral (Fig. 989). Its rounded hind-end lies in the occipital lobe, whilst its 

 tapering and greatly narrowed front-end is continuous with the hippocampal convo- 

 lution. The gyrus fits into the angle between the falx cerebri and the tentorium 

 and therefore bears the internal occipital border of the hemisphere and appears on 

 both the mesial and the tentorial surfaces. It is usually modelled by irregular shallow 

 furrows which break up the larger tentorial aspect into uncertain secondary gyri. 



The inferior or tentorial surface of the occipital lobe is continuous with the more 

 extensive similar surface of the temporal lobe resting upon the tentorium. In addi- 

 tion to the tentorial part of the lingual gyrus, this aspect of the lobe is occupied by 

 the posterior part of the occipito-temporal gyrus. The latter includes an irreg- 

 ular fusiform tract, bounded by the collateral fissure internally and by the inferior 

 temporal sulcus laterally (Fig. 989). As expressed by its name, the occipito- 

 temporal convolution belongs partly to the occipital and partly to the temporal 

 lobe and extends from the occipital to the temporal pole. Its surface is broken 

 by a number of irregularly disposed furrows which add to the uncertainty of its 

 outer boundary. 



The Temporal Lobe. — The temporal lobe includes the irregularly pyramidal 

 division of the cerebral hemisphere, whose apex is lodged within the middle fossa of 

 the skull and whose succeeding part forms the conspicuous dependent mass seen on 

 the infero-lateral surface of the hemicerebrum. In front it is separated from the 

 frontal lobe by the stem of the Sylvian fissure; above it is marked off from the pari- 

 etal lobe by the posterior limb of the Sylvian fissure and the arbitrary line prolonged 

 backward in the direction of this sulcus; externally and below it is defined by the 

 infero-lateral border of the hemisphere; and mesially it is separated from the limbic 

 lobe by the collateral fissure. Its posterior border, however, on both the lateral and 

 the inferior (tentorial) surface is arbitrary and indicated by the lines already men- 

 tioned (pages 1 143 and 1146) which afford the conventional demarcation between 

 the occipital and temporal lobes. 



The temporal lobe presents three surfaces, the convex lateral, the inferior 

 (largely tentorial), and the buried superior or opercular. Of these the lateral and 

 inferior are separated by a border so broad and rounded that the surfaces pass insen- 

 sibly into each other. Its tip corresponds with the temporal pole of the hemisphere 

 and underlies the posterior part of the orbital surface of the frontal lobe, which it 

 partially masks. 



The lateral stirface of the temporal lobe is modelled by two fissures, the superior 

 and the middle temporal, and three convolutions, the superior, the middle and the 



