THE NERVOUS SYSTEM 1395 



locus perforatus anticus, below the rostrum of the corpus callosum, 

 and it terminates below the splenium of that body. Between these 

 two points it pursues a sweeping course, passing forwards beneath 

 the rostrum, upwards in front of the genu, backwards above the 

 body of the corpus callosum, and finally curving roimd the splenium, 

 to be continued into the hippocampail gyrus through the isthmus. 

 It is bounded superiorly by the calloso-marginal fissure, which 

 separates it from the marginal gyrus and paracentral lobule, and, 

 posterior to the latter, it is partially separated from the praecuneus 

 by the postlimbic sulcus. The caUosal gyrus is separated from the 

 corpus callosmn by the eallosal sulcus. 



The hippocampal gyrus constitutes the lower part of the limbic 

 lobe, the upper part being formed by the eallosal gyrus. Posteriorly, 

 below the splenium of the corpus callosum, it is continuous above 

 with the eallosal g}Tiis throiigh means of the isthmus, and behind 

 and below with the lingual gyrus. As it passes forwards it has the 

 hippocampal or dentate fissure above it, and the anterior part of 

 the collateral fissure below it. Anteriorly, near the apex of the 

 temporal pole and close behind the locus perforatus anticus, it forms 

 an enlargement, known as the caput gyri hippocampi, which is 

 separated from the temporal pole by a slight fissure, called the 

 incisura temporalis. From the caput a hook-hke process, known 

 as the uncus, passes backwards fcr a short distance above the 

 Jinterior part of the hippocampal or dentate fissure. The caput 

 represents the largely-developed lobus pyriformls of many mam- 

 mals, and it constitutes the olfactory centre of the cerebral cortex. 

 Along with the uncus it forms part of the rhinencephalon or rhino- 

 paUium, the rest of the hippocampal gyrus belonging to the neo- 

 pallium. 



The part of the gyrus which hes below the hippocampal fissure is 

 known as the subiadum. 



The incisura temporalis, which, as stated, separates the caput gyri 

 hippocampi from the temporal pole, represents the rhinal fissure, 

 which bounds the weU-developed rhinencephalon in some anim als 



The cingiilum is connected with the limbic lobe, which is com- 

 posed of the eallosal and hippocampal gyri. According to Cajal, 

 its fibres arise as the axons of cells of the caUosal gyrus, upon the 

 under sm-face of which gyrus the cinguliun hes. On entering the 

 cingulima some fibres pass forwards, and others backwards, whilst a 

 few are described as branching into forward and backward branches. 

 The anterior branches pass as far as the caput of the corpus striatum, 

 where they are described as blending with the fibres which enter 

 the internal capsule. Some may even pass to the cortex of the 

 prefrontal region. The posterior branches turn round the splenimn 

 and then he upon the subiculum, or upper part of the hippocampal 

 g>Tus, as far forwards as the caput and uncus. The posterior 

 fibres are described as ending in the cortex of (i) the subiculmn 

 hippocampi and (2) occipital lobe. The cingulum belongs to the 

 class of long association fibres. 



