The four sets of gyri included between these three sets of arched sulci and the fissure of Sylvius unite caudad and craniad to form single gyri. The caudal one is the gyrus compositus posterior (6). The cranial one (less regular than the caudal one) is the sigmoid gyrus (7). The latter curves about the cruciate sulcus (f) ([described] below).
In the cranial part of the lateral surface of the hemisphere are certain sulci and gyri which do not belong to the system above described. The cruciate sulcus (f) is a short transverse sulcus passing from the lateral surface of the hemisphere onto its medial surface, where it extends caudad about a centimeter. Curving around the lateral end of this, separated from it by part of the sigmoid gyrus, is the short coronal sulcus (g). Connected usually with the end of the lateral sulcus (e) and running nearly parallel with the cruciate sulcus is the short sulcus ansatus (h).
At its ventral end the fissure of Sylvius (a) joins a longitudinal groove passing craniad and caudad from the point of junction. That part which passes craniad is the sulcus rhinalis (i); it forms the dorsolateral boundary of the olfactory tract (8). Passing dorsocraniad from the sulcus rhinalis is a sulcus which separates a very small cranial lobe from the rest of the hemisphere; this is the supraorbital or præsylvian sulcus (j). The portion of the hemisphere craniad of this sulcus is the orbital gyrus (9).
The sulcus rhinalis (i) is continuous caudad with the sulcus rhinalis posterior (or sulcus postrhinalis) (k). This extends caudad onto that portion of the hemisphere which faces the cerebellum. It forms the lateral boundary of a large elongated oval lobe lying at the side of the ventral floor of the midbrain and ’tween-brain. This lobe has been called tractus postrhinalis or lobus pyriformis ([Fig. 145], 10; [Fig. 138], f).
In addition to the sulci and gyri above mentioned, any given specimen will usually show a number of small inconstant sulci and gyri in various regions; these inconstant structures will not be here described.
Fig. 146.—Diagram of the Sulci and Gyri on the Medial Surface of the Hemisphere.
a, sulcus splenialis; b, sulcus marginalis; c, sulcus cruciatus; d, sulcus falcialis; e, sulcus supracallosalis; f, sulcus rhinalis posterior; g, hippocampal sulcus; h, corpus callosum. 1, gyrus marginalis; 2, gyrus fornicatus; 3, gyrus compositus posterior.
On the medial surface of the hemisphere ([Fig. 146]), the following arrangement of sulci and gyri seems to be typical. Some distance from the dorsal margin a long sulcus runs parallel with the margin; this is the sulcus splenialis (a). The marginal gyrus (1) is dorsad of the splenial sulcus, passing onto the lateral surface of the hemisphere, where it is bounded by the lateral sulcus. The splenial sulcus extends onto the caudal surface of the hemisphere. A shallow sulcus marginalis (b) occurs frequently between the sulcus splenialis and the dorsal margin of the hemisphere. Cranioventrad or the cranial end of the splenial sulcus is that portion of the sulcus cruciatus (c) that lies on the medial surface of the hemisphere. Ventrad of this is a short shallow furrow which has been called the sulcus falcialis (d).
Immediately dorsad of the corpus callosum (h), separating its dorsal surface from a part of the hemisphere, is the supracallosal or callosal sulcus (e). Between the supracallosal (e) and splenial (a) sulci is the gyrus fornicatus (2).