_A_, frontal lobe; _B_, temporal lobe; _C_, occipital lobe. _a_, lateral fissure or fissure of Sylvius; _b_, sulcus anterior; _c_, sulcus posterior (the broken line connecting the ends of _a_ and _b_ serves to show how the first arch is completed to form the ectosylvian sulcus, as in the dog); _d_, sulcus suprasylvius; _e_, sulcus lateralis; _f_, sulcus cruciatus; _g_, sulcus coronalis; _h_, sulcus ansatus; _i_, sulcus rhinalis; _j_, sulcus præsylvius; _k_, sulcus rhinalis posterior. 1, anterior Sylvian gyrus; 2, posterior Sylvian gyrus; 3, gyrus ectosylvius; 4, gyrus suprasylvius; 5, gyrus marginalis; 6, gyrus compositus posterior; 7, sigmoid gyrus; 8, olfactory tract; 9, orbital gyrus; 10, lobus pyriformis; 11, olfactory bulb.] The gyri included between the fissure of Sylvius on the one hand and the anterior and posterior sulci on the other are known as the =anterior= (1) and =posterior= (2) =Sylvian gyri=. The second arch is formed by the =suprasylvian sulcus= (=sulcus suprasylvius=) (_d_). The gyrus between the anterior and posterior sulci ventrad and the suprasylvian sulcus dorsad is the =ectosylvian gyrus= (3) (because dorsad of the ectosylvian sulcus). The third arched sulcus, next to the medial margin of the hemisphere, is variable: it forms the =lateral sulcus= (_e_). The gyrus lying between the suprasylvian and lateral sulci is the =suprasylvian gyrus= (4) (because dorsad of the suprasylvian sulcus). The lateral sulcus is sometimes broken into two (as in Fig. 137, _h_), the two ends overlapping so as to leave a small gyrus between the two ends. The gyrus dorsomediad of the lateral sulcus, extending onto the medial surface of the hemisphere as far as the splenial sulcus, is the =marginal gyrus= (5). 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_).