GYRUS DENTATUS.
When, in order to determine the depth of the hippocampal fissure, the gyrus hippocampi is pulled downward, one sees a gray notched or corrugated band, the fascia dentata (Tarini) or the gyrus demeans of Huxley. Farther inward and over the gyrus dentatus, is seen a white band, which passes from the uncus gyri hippocampi backward ; this is the fimbria hippocampi. In its further course the fimbria is con tinuous with the fornix.
The gyrus dentatus is separated from the gyrus hippocampi by the fissura hippo campi, and from the fimbria by the sulcus Following the dentate gyrus farther backward, it is seen to run at first parallel with the fimbria to the splenium corporis callosi. Here the gyrus leaves the fimbria, loses its incisions and knobs, becomes smooth and, as the fasciola cinerea, passes around the callosum to spread out over the latter as a thin lamella of gray substance, the induseum griseum. In the middle, the induseum exhibits the striae longiludinales mediales or striae Lancisii, while on each side, in the sulcus corporis callosi, lies the stria longitudinalis lateralis or iaenia tecta (Fig. 33). Induseum and striae longitudinales run forward around the knee of the callosum and in their farther course pass into the gyms subcallosus, with which, in turn, Broca's band (page 3o) joins.
According to most authors, the fasciola cinerea constitutes the direct continuation of the gyrus dentatus. As shown by Retzius, however, the gyrus dentatus is not pro longed directly into the fasciola cinerea (Fig. 34). On examining the area beneath the splenium where the gyrus dentatus leaves the fimbria, a thin strand is seen next the fascia dentata, which likewise sinks into the depth of the sulcus fimbrio-dentatus between the fascia dentata and the fimbria. This small cylindrical strand was called by Retzius the gyrus fasciolaris. It is separated from the gyrus dentatus by a small furrow, the sulcus dentato-fasciolaris, and forms, by union with the pointed end of the gyrus dentatus, the fasciola cinerea of the authors. The fasciola extends as a gray semi cylinder strand around the splenium and, on the surface of the callosum, continues as a broad plate, the gyrus efticallosus (Retzius) or induseum griseum.
Retzius agrees with Zuckerkandl, that the striae longitudinales mediales and laterales correspond to local elevations of the induseum. In front, they pass into the gyrus subcallosus and also in part, at least so far as the taenia tecta is concerned, into the substance lying lateral to this gyrus. Retzius further notes, that a portion of the
gray lamella covering the callosum branches off at the posterior limit of the splenium to continue on the lower surface of the latter and there form an induseum inferius. Since this structure often resembles a convolution, it has been designated by Retzius as the gyrus subsfilenialis.
Following the gyrus dentatus forward, the gyrus hippocampi being drawn down ward, one perceives that the dentate gyms here likewise gradually separates from the fimbria and then, after a bend at right angles—the angulus gyri—passes as a smooth slightly convex band onto the uncus. This band of Giacomini, as it is termed, passes over the under surface of the uncus, from the outer side onward and somewhat back ward, and thence continues onto its upper surface. The band courses from the inner side forward and outward, as far as a thin sheet of medullary substance, the velum lerminale (Abbey), adhering to the uncus. This entire course is plainly exposed after removal of the gyms hippocampi.
Retzius recognizes two divisions of -the gyms dentatus, a longitudinal and a transverse. The former, proceeding from the angulus gyri dentati, extends backward within the fissura hippocampi. The transverse division, on the contrary, proceeding from the angulus, represents the front end of the convolution. The transverse part the limbus Giacomini—is further subdivided into a pars occulta, which lies buried in the hippocampal fissure, and a pars afierta, which is visible on the upper surface of the uncus. In front, the pars occulta is limited by a furrow that corresponds morphologi cally to the end of the fissura hippocampi. Behind, the limitation is usually less definite, at times the limbus Giacomini appearing to pass over into this part.
On the portion of the under surface of the uncus lying in front of Giacomini's band, one distinguishes two, often only one, or occasionally three, sulci and convolutions, which pass from the anterior limiting fissure. These are designated as the sulci and gyri digilali externi. Small tip-like extensions of the Giacomini band radiate forward for a short distance within the sulci digitati ; conse quently, this part of the limbus appears more or less festooned. The anterior termi nation of Giacomini's band is as yet undetermined.