GYRUS FORNICATUS.
To the peripheral region of the rhinencephalon, the lobus olfactorius, is attached the central district. Here the gyrus fornicatus first claims closer attention. It is an annular tract on the medial surface of the hemisphere, encircled by the cerebral mantle and composed of two chief convolutions, the gyms cinguli and the gyms hippocampi, connected with each other by means of the isthmus.
The gyrus cinguli forms the arching convolution, paralleling the convex upper surface of the corpus callosum, between the sulcus cinguli and the sulcus corporis callosi. It presents numerous variations in consequence of the inconstant relations of the sulcus cinguli. The latter, in fact, does not represent a simple fissure, but consists of several parts, known as the pars anterior, pars intermedia and pars posterior. As a result, numerous annectant or bridging convolutions arise, which unite the gyms cinguli with the neighboring convolutions of the pallium. When the composite parts join to form a simple sulcus, the course already described (page 22) as typical is observed. In its entire path, several incisions, some deep, branch toward the frontal lobe, while those passing into the gyrus cinguli are few and mostly short. The surface of the gyrus cinguli exhibits likewise some shallow furrows. Owing to these peculiarities, as well as to its smooth surface, the gyrus is more or less clearly defined from the adjacent convolutions. Accordingly, the gyms cinguli takes the following course. It begins narrow beneath the knee of the corpus callosum, as the direct continuation of Broca's field. In its further course, around the genu and over the truncus corporis callosi, the convolution is broader. Farther behind, at the bend around the splenium, it again distinctly narrows and, where it is deeply incised by the fissura parieto-occipitalis, passes over into the isthmus gyri fornicati.
When the sulcus cinguli does not form a simple furrow, the convolution assumes an entirely different character. Doubling and splitting of the convolution may exist, as well as separation into two, three or four parts. Concerning the annectant convolutions, it may be noted that of these one is fairly constant in the fore part of the gyrus cinguli and establishes a connection between the latter and the gyms frontalis superior. A second annectant convolution is found in the middle portion, the connection between the gyms and the lobulus paracentralis, while a third appears in the posterior division, providing continuity with the praecuneus. The last connection is often double in consequence • of the sulcus subparietalis existing as a separate furrow and not as the hind part of the chief fissure. In such cases the gyrus cinguli appears to radiate into the praecuneus.
The chief variations of the gyms cinguli are found mostly in its front part. Here the convolution may be doubled by an inner or outer parallel fissure. If an outer secondary fissure be present, the gyrus cinguli proper appears markedly narrowed at the knee of the callosum ; in such case, the convolution lying between the secondary fissure and the sulcus cinguli proper must be reckoned as part of the gyms cinguli.
The delimitation of the gyrus becomes more difficult when it consists of several pieces. Then each portion behind appears as a wedge beneath the part in front, and the entire convolution is markedly narrowed, particularly towards the genu corporis callosi. The convolution appears notched in its upper part. For this reason Rolando compared it to a cock's comb and called it the " ridged convolution " ; hence also the designation of the sulcus cinguli as the " festooned fissure" (Pozzi).
In consequence of the deep incision into the gyrus fornicatus by the common limb of the parieto-occipital and calcarine fissures behind the splenium, the isthmus is pro duced ; this marks the transition of the gyrus cinguli into the gyrus hippocampi.
The gyrus hippocampi proceeds forward, becomes broader and, at the level of the substantia perforata anterior, bends around the front end of the fissura hippocampi to form the uncus. On its outer side, the gyrus hippocampi is bounded by the common stem of the parieto-occipital and calcarine fissures, the anterior part of the collateral fissure and the fissura rhinica.
As the gyrus cinguli, so also the gyrus hippocampi exhibits connections with the convolutions lying to its outer side. In this relation the great variability of the fissura collateralis comes into consideration. When the fissura rhinica is connected with the fissura collateralis, two annectant convolutions are found, an anterior and a posterior. The former, the gyrus anterior, joins the front part of the gyrus hippocampi with the temporal pole and is one of the most constant bridges. The other, the gyrus connects the gyrus hippocampi with the gyrus lingualis. The last-named bridge is mostly superficial, but may present mani fold variations. It may be divided by a longitudinal furrow into two parts, of which one or the other is deeply placed and the remaining one is superficial. Quite rarely, the entire bridge may be deeply situated, in which case the collateral fissure ends in the calcarine. In the event of the fissura rhinica being separated from .the fissura collater alis, a third bridge, the gyrus is present.
The surface of the gyrus hippocampi, from where the gyrus approaches the hind end of the callosum forward, particularly toward the bottom of the fissupt hippocampi, exhibits a lighter color. This tract is known as the substantia reticularis alba (Arnold). Moreover, mention must be made of the peculiar character of the surface of that part of the gyrus which lies between the fissura rhinica and the fissura hippocampi. Here the surface is covered with numerous small nodules or wartlike elevations, designated as verrucae gyri hippocampi.