THE CEREBRAL HEMISPHERES 929 



olfactory nerves (fila olfactoria) from the nasal mucous membrane. The olfactory 

 tract (tractus olfactorius) is a band of white substance, of prismatic outline on 

 section, its apical ridge fitting into the olfactory fissure. Toward its root region 

 it is somewhat narrowed. 



The medial and lateral olfactory gyres are also termed the medial and lateral 

 roots of the tract, and diverge in the region of the trigonum. The olfactory 

 tubercle (tubercidum olfactorium) is best seen if the bulb and tract be lifted away 

 from the olfactory fissure; the tubercle appears as a small pyramidal elevation, 

 its apex buried in the olfactory fissure, its irregularly triangular base forming 

 the trigonum olfactorium, a small gray area frontad of the anterior perforated 

 substance. This area is marked by ridge-iike elevations which appear like 

 radiating roots of the tract, and named, according to their position, the medial, 

 intermediate, and lateral roots, striae or gyres. The lateral olfactory stria or gyre is 

 continuous with the limen insulae in the depths of the basisylvian cleft, and thence 

 passes to the uncus to end in the gyrus ambiens and gyms semilunaris. The sharp 

 turn made at the limen insulae is called the angulus lateralis. The medial olfactory 

 stria or gyre, a narrow gyral band, proceeds mesad and merges with the adjacent 

 cerebral surface; its extension on the mesal surface is known as the parolfactory 

 area (Broca) limited frontad by the anterior parolfactory sulcus (sulcus parol- 

 factorius anterior) and separated from the subcallosal gyre by the posterior par- 

 olfactory sulcus (sulcus par olf actor i^ls posterior). 



The intermediate stria is not always very distinct; when present it may be traced 

 from the proximal end of the olfactory tract for a short distance over the gray 

 field of the trigonum, to plunge into the gray of the anterior perforated substance. 



POSTOLFACTORY DIVISION. The anterior perforated substance or space (pre- 

 perforatum) occupies an irregular quadrate field between the olfactory trigone 

 and the optic chiasm and tract. A more or less marked groove (sulcus parolfac- 

 torius posterior), which is identical with the fissura prima (His) of the embryo, 

 separates the trigonum from the anterior perforated substance. Its frontal 

 part, much perforated, is of a darker color than the hind portion; the latter is 

 distinguished by the name of Broca's diagonal band. This courses obliquely 

 laterad along the optic tract toward the uncus; mesally the bands of the two sides 

 converge, frontad of the lamina terminalis, and proceed toward the rostrum of the 

 corpus callosum as narrow fields which taper to curve around the genu and con- 

 tinue in the indusium of the corpus callosum as the striae longitudinales. The 

 narrow field seen on the mesal aspect frontad of the lamina terminalis and anterior 

 commissure is known as the gyrus subcallosus (formerly peduncle of the corpus 

 callosum). The continuity of the various parts may be understood by reference 

 to Figs. 680 and 681. 



The cortical and central part of the rhinencephalon comprise: 



1. The hippocampus. 1 



2. The uncus. 



3. Gyrus dentatus. 

 Cortical J "^ F as dl a cinerea. 



5. Indusium, medial jind lateral longitudinal striae upon the corpus 



callosum. 



6. Gyri Andreae Retzii. 



7. Gyri subcallosi. 



8. Fornix and fimbria. 



Central ^' Corpus albicans and albicantiothalamic tract. 

 I 10. Part of anterior commissure (precommissure). 

 ill. Part of septum pellucidum. 



1 Not to be confounded with the hippocampal gyre of the pallium. 

 59 



