The _Central Lobe_ of the hemisphere, more usually called the _insula_ or _island of Reil_, does not come to the surface of the hemisphere, but lies deeply within the Sylvian fissure, the opercula forming the margin of which, conceal it. It consists of four or five short gyri, which radiate from the _locus perforatus anticus_, situated at the inner end of the fissure. This lobe is almost entirely surrounded by a deep sulcus called the limiting sulcus of Reil, which insulates it from the adjacent gyri. It lies opposite the upper part of the ali-sphenoid, where it articulates with the parietal and squamous-temporal.
[Illustration: FIG. 10.--Orbital surface of the left frontal lobe and the island of Reil; the tip of the temporo-sphenoidal lobe has been removed to display the latter.
17. Convolution of the margin of the longitudinal fissure. O. Olfactory fissure, over which the olfactory peduncle and lobe are situated. TR. Orbital sulcus. 1" 1"'. Convolutions on the orbital suface. 1,1,1,1. Under surface of infero-frontal convolution. 4. Under surface of ascending frontal; and 5, of ascending parietal convolutions. C. Central lobe or insula.]
In front of the central lobe, on the base of the brain, are the _orbital gyri_, which are separated from one another by the _orbital sulcus_. This is usually H-shaped, and the gyri are therefore anterior, posterior, external and internal. Bisecting the internal orbital gyrus is an antero-posteripr sulcus (_s. rectus_), beneath which lies the olfactory lobe, bulbous in front, for the olfactory nerves to arise from.