iROLOGY 





gyrus. It is separated from the medial part of the superior frontal gyrus by the 

 cingulate sulcus, and from the precuneus by the subparietal sulcus. 



The hippocampal gyrus (gyrus hippocampi) is bounded above by the hippocampul 

 fissure, and below by the anterior part of the collateral fissure. Behind, it is con- 

 tinuous superiorly, through the isthmus, with the cingulate gyrus and inferiorly 

 with the lingual gyrus. Running in the substance of the cingulate and hippocampal 

 gyri, and connecting them together, is a tract of arched fibers, named the cingulum 

 (page 843) . The anterior extremity of the hippocampal gyrus is recurved in the 

 form of a hook (uncus), which is separated from the apex of the temporal lobe by 

 a slight fissure, the incisura temporalis. Although superficially continuous with the 

 hippocampal gyrus, the uncus forms morphologically a part of the rhinencephalon. 



The Hippocampal Fissure (fissura hippocampi; dentate fissure) begins immediately 

 behind the splenium of the corpus callosum, and runs forward between the hippo- 

 campal and dentate gyri to end in the uncus. It is a complete fissure (page 819), 

 and gives rise to the prominence of the hippocampus in the inferior cornu of the 

 lateral ventricle. 



Gyrus supracallosus 



Gyrus subcallosus 

 Parolfactory area 



Medial root 



Fascia denlata 

 hippocampi 



Lateral root' /fl^ / Uncus 



Anterior perforated substance J Band of Giacomini 

 FIG. 732. Scheme of rhinencephalon. 



Rhinencephalon (Fig. 732). The rhinencephalon comprises the olfactory lobe, 

 the uncus, the subcallosal and supracallosal gyri, the fascia dentata hippocampi, 

 the septum pellucidum, the fornix, and the hippocampus. 



1. The Olfactory Lobe (lobus olfactorius) is situated under the inferior or orbital 

 surface of the frontal lobe. In many vertebrates it constitutes a well-marked 

 portion of the hemisphere and contains an extension of the lateral ventricle; but 

 in man and some other mammals it is rudimentary. It consists of the olfactory 

 bulb and tract, the olfactory trigone, the parolfactory area of Broca, and the anterior 

 perforated substance. 



(a) The olfactory bulb (bulbus olfactorius) is an oval, reddish-gray mass which 

 rests on the cribriform plate of the ethmoid and forms the anterior expanded 

 extremity of the olfactory tract. Its under surface receives the olfactory nerves, 

 which pass upward through the cribriform plate from the olfactory region of the 

 nasal cavity. Its minute structure is described on page 848. 



(&) The olfactory tract (tractus olfactorius} is a narrow white band, triangular 

 on coronal section, the apex being directed upward. It lies in the olfactory sulcus 

 on the inferior surface of the frontal lobe, and divides posteriorly into two striae, 

 a medial and a lateral. The lateral stria is directed across the lateral part of the 



