868 THE NERVOUS SYSTEM 



and 671). It is bounded externally by the anterior end of the calcarine fissure, 

 and incloses the posterior turn of the cingulum. 



►' The hippocampus is the name applied to the curved appearances produced in 

 the floor of the lateral ventricle by the peculiar foldings of this part of the cerebral 

 cortex. The hippoca7?ipal gyrus (gyrus of the hippocampus) is the main gyrus of 

 the tentorial surface of the limbic lobe. Externally it is separated from the fusi- 

 form gyrus by the collateral fissure, and it is bounded internally by the hippo- 

 campal or, more inclusive, the chorioid fissure. Posteriorly it is partially divided 

 by the calcarine fissure into the lingual gyrus (of the temporal lobe) and the 

 isthmus of the gyrus fornicatus. Its anterior extremity is hooked backward and 

 is known as the uncus (gyrus uncinatus). This is almost entirely separated from 

 the temporal lobe by a groove, the temporal notch. If the hippocampal fissure 

 be opened up, the dentate gyrus or fascia and the fimbria will be seen. These lie 

 side by side, separated by the shallow fimbrio-dentate sulcus (fig. 690.) 



The free edge of the dentate gyrus presents a peculiarly notched appearance, produced by 

 numerous parallel grooves cutting it transversely. Its posterior end, sometimes called the 

 fasciola cinerea, continues backward over the splenium of the corpus callosum, and upon the 

 upper surface of the corpus callosum appears as a thin strip of grey substance which contains 

 embedded in it the medial and lateral longitudinal strice. This thin strip is sometimes called the 

 supracallosal gyrus {gyrus epicallosus, induseum griseum), and is thought to represent a ves- 

 tigial part of the hippocampal gyrus. Closely beneath the splenium of the corpus callosum, on 

 the supero-mesial side of the hippocampal gyrus and mesial to the dentate gyrus, there sometimes 

 occur suggestions of round or oval elevations of the grey substance which have been called 

 the "callosal convolutions" or gyri Andreoe Retzii. Rarely are they strongly developed, but when 

 so they often produce a spiral appearance. 



The fimbria is but the fimbriated, free border of the posterior end or origin 

 of the fornix, so folded as to project into the hippocampal fissure, parallel with the 

 dentate gyrus (fig. 690). It is a conspicuous band composed almost entirely of 

 white substance, continuous laterally with the thick stratum covering the ven- 

 tricular surface of the hippocampus. It begins anteriorly in the hook or recurved 

 extremity of the uncus. Traced backward, it is seen so curve upward, and within 

 the ventricle it becomes part of the general accumulation of the white substance 

 (alveus) of the surface of the hippocampus, which accumulation is the beginning 

 of the fornix. The free border of the fimbria (seen in section) is known as the 

 t(B7iia fimhricB. The fimbria is separated from the cerebral peduncles by the 

 chorioid fissure, the thin, non-nervous floor of which alone intervenes between the 

 exterior of the brain and the cavity of the lateral ventricle within. 



The hippocampal fissure attains its greatest depth between the dentate gyrus 

 and the hippocampal gyrus, and the resulting eminence produced in the floor of 

 the lateral ventricle is known as the hippocampus major, as distinguished from 

 the lesser eminence produced posteriorly by the end of the calcarine fissure and 

 known as the hippocampus minor [calcar avis]. The collateral fissure may like- 

 wise produce a bulging in the wall of the ventricle, the collateral eminence. In 

 transverse sections of the hippocampus major, the layers of grey and white sub- 

 stance present a coiled appearance known as the cornu ammonis. Externally 

 the medial surface of the hippocampal gyrus adjoining the dentate gyrus has 

 reflected over it a delicate reticular layer of white substance known as the sub- 

 stantia reticularis alba (Arnoldi). 



The fornix is the great association pathway of the limbic lobe, and appears to 

 be wholly concerned in the apparatus of the rhinencephalon. It is a bilateral 

 structure arched beneath the corpus callosum, with which it is connected ante- 

 riorly by the septum pellucidum. Posteriorly it passes in contact with the 

 splenium. It consists of two prominent strips of white substance, one for each 

 h(;mis[)h('re, the ends of which are separate from each other, while their inter- 

 mediate parts are fuse(l across the mid-line. These run above the chorioid tela of 

 the third ventricle;, and their lateral edges {tmnice fornicis) rest, on each side, along 

 the line of the taenia chorioidea. The posterior, separate ends are known as the 

 posterior pillars or crura of the fornix; the fused, intermediate portion is the body, 

 and the separate, anterior ends ;ire the anterior pillars or columns of the fornix. 



The posterior pillars | crura] of the fornix. — Wlien s(!en from the medial 

 aspect of the hemisphere, the fused ])()rti()n of the fornix, in the separation of the 

 hemispheres, is split along the mid-line (fig. 671). The half under examination 



