312 DR. G. ELLIOT SMITH ON 



outline, and the greater part of its free surface is covered by a very definite layer of 

 medullated nerve-fibres — the alveus. 



The anterior border of the formation is rounded, and from the antero-mesial part of 

 the cui've a small triangular mass of fibres — -the fimbria — projects. This spur serves to 

 indicate the line of demarcation between the ventricular and extrave utricular surfaces 

 of the hippocampus. The whole of the ventricular and the greater part of the extra- 

 ventricular surfaces (in this section) are covered with a A'ery well-defined layer of 

 medullated nerve-fibres — the alveus. Extending in the lateral direction from the fimbria, 

 the alveus extends outward and then sweeps in a large curve caudally and extends back- 

 ward as far as the posterior aspect of the ventricle, of which it forms the mesial wall. 

 From the extent and degree of curvature of this large alveus-coated sui"face in the figure, 

 we can appreciate more readily the great size of the Ijulging of the hippocampus into the 

 ventricle (fig. 19). The alveus gathers fibres from the whole of the deep or ventricular 

 surface of the hippocampus and conveys them by an oblique course to the iimbria, which 

 is composed of a large mass of such fibres gathered into a compact bundle. But the 

 alveus is not confined to the ventricular aspect of the hippocampus. Eor we find an 

 extraventricular alveus extending backward from the fimbria upon the mesial surface of 

 the hemisphere for a considerable distance until it meets a grey band of peculiar 

 constitution — the fascia dentata. 



The fascia dentata is morphologically the extremely specialized margin of the costa, 

 which has undergone a peculiarly modified hypertrophy in its superficial layers, resulting 

 in that curiously folded layer which is known in human anatomy as the fascia dentata. 

 [It is not advisable to apply the term gyrus to this structure, as many writers, following- 

 Huxley, are in the habit of doing, because it is not a gyrus in the ordinary sense of the 

 term, but merely the peculiarly modified sujjerficial region of a cortical area.] 



An exceedingly small fragment of the fascia dentata makes its appearance upon the 

 surface in the section we are considering, since almost the whole of its real surface is 

 hidden from view l)y being opposed to the morphological sui'face of the hippocampus and 

 the adjoining cortical area. 



In the section we find the superficial fragment of the fascia dentata at the posterior 

 extremity of the extraventricular alveus ; from this point the fascia dentata extends for 

 a considerable distance in the lateral direction and in apposition with the area of cortex 

 which adjoins the hippocampus proper. The fascia dentata then bends forward, its surface 

 now coming into contact Avith the morphological surface of the hippocampus ; in the rest 

 of its course the fascia dentata pursues a course parallel to the alveus : that is to say, it 

 pi'oceeds forward and then curves mesially and then slightly backward. The potential 

 cleft which extends into the cortex in the interval between the fascia dentata and the 

 geoeral cortex and separates t];e morphological surface of the fascia dentata from the 

 surfaces of the hippocampus and the adjoining cortex is the fissura hippocampi. The 

 deeper parts of this "fissure" are merely potential, because the surface of the fascia 

 dentata becomes adherent to the opposed surfaces, especially that of the hippocamjjus. 

 The hippocampal fissiu'e is unique in its nature and mode of formation, and cannot be 

 classed with any other fissures of the brain. 



