74 THE CEREBRUM. 



by the lateral fossa and lateral fissure of the cerebrum, overlapped 

 by the temporal lobe. More accurately, the posterior boundary 

 is the anterior perforated substance and the anterior part of the 

 circular sulcus. The orbital lobe is concave transversely and 

 is divided by the triradiate or H-shaped sulcus orbitalis, made 

 up of the medial orbital, the transverse orbital and the lateral orbital 

 sulci; and by the olfactory sulcus, which is close to the longitud- 

 inal fissure and parallel with it. Five gyri are thus formed : 



Straight (g. rectus) 



Medial orbital (g. orbitalis medialis) 



Gyri 



Lateral orbital (g. orbitalis lateralis), not constant 

 Anterior orbital (g. orbitalis anterior) 

 Posterior orbital (g. orbitalis posterior). 



The gyrus rectus (Fig. 26) forms the medial border of this 

 surface. It is separated from the medial orbital gyrus by the 

 sulcus olfactorius in which lie the olfactory bulb and tract. Over 

 on the medial surface it forms a part of the marginal gyrus and 

 it joins the superior frontal at the frontal pole. Posteriorly, the 

 gyrus rectus is separated from the parolfactory area (of Broca) 

 by a slight furrow, the anterior parolfactory sulcus. 



The medial orbital gyrus lies between the sulcus of the same 

 name and the sulcus olfactorius (Fig. 26). It extends from -the 

 frontal pole to the anterior perforated substance and the island. 

 The anterior and posterior orbital gyri lie within the H-shaped 

 orbital sulcus separated from each other by the transverse orbital 

 sulcus. The former is continuous with the frontal gyri at the 

 superciliary border; the latter is only partially separated, behind, 

 from the island by the anterior circular sulcus ; the posterior orbital 

 gyrus is likewise continuous with the posterior end of the lateral 

 orbital gyrus and with the orbital portion of the inferior frontal. 

 The lateral orbital gyrus, which is a distinct gyrus only when the 

 lateral orbital sulcus is long, is situated external to the H-shaped 

 sulcus. It is continuous with both middle and inferior frontal 

 gyri at the superciliary border of the hemisphere. 



The Island (of Reil), Inferior Surface (Fig. 26). If the an- 

 terior part of the temporal lobe be removed, the under surface 

 of the island (insula) is brought into view. The circular sulcus 



