THE CEREBRAL HEMISPHERES 787 
band of white matter, termed the fimbria, which is the prolongation of the greater 
part of the crus of the fornix into this region. The ventricular surface of the 
hippocampus (Fig. 642) is covered with a thin layer of white matter, the alveus, 
which is also derived from the crus of the fornix, and is therefore continuous with 
the fimbria. The two hippocampi are connected at their highest parts by trans- 
verse fibers which constitute the hippocampal commissure. 
The interval between the hippocampus and fimbria on the one hand, and the 
brain stem:on the other, is a lateral continuation of the transverse fissure of the 
brain, and is occupied by a fold of pia mater, the tela chorioidea of the third ven- 
tricle. This fold is triangular in outline and its apex reaches to the interventricular 
foramen. Its base is continuous at the transverse fissure with the pia which covers 
the surface of the brain. Its middle part lies over the epithelial roof of the third 
ventricle, as has been seen (Fig. 637). The lateral borders will be seen on the floor 
Interventricular foramen 
Gyrus fornicatus 
Sublimbic fissure (ant. part) 
Transverse fissure 
Calloso-marginal fissure 
Sublimbic fissure (post. par6) 
Entomarginal fissure 
Marginal fissure 
Sulcus rhinalis anterior Ectomarginal fissure 
Hippocampal fissure Sulcus rhinalis posterior 
Fie. 640.—Mepi0-ventrAL Aspect oF RicgHT CEREBRAL HEMISPHERE OF HORSE. 
The olfactory bulb is cut off. T.ol., Olfactory tract; A.p., area parolfactoria; Tr.o., trigonum olfactorium; 
Tr.op., optic tract; C.o., chiasma opticum; G.s., subcallosal gyrus; C.c., corpus callosum; S.p., septum pellucidum; F, 
fornix; F’, fimbria; G.c., callosal gyrus; 7, cut surface of thalamus; G.d., gyrus dentatus; L.p., piriform lobe; 
G.h., hippocampal gyrus. 
of the lateral ventricles, where they form thick, rounded bands containing convolu- 
tions of blood-vessels, known as the chorioid plexuses of the lateral ventricles (Fig. 
642). 
When the tela is pulled out, one may easily get the impression that the lateral ventricle 
communicates with the exterior by means of the chorioid fissure. Such is not the case, since the 
chorioid plexus is covered by the epithelial lining of the ventricle, which has been torn away. 
The rhinencephalon or olfactory part of the brain comprises the olfactory 
bulb, tract, and striz, the trigonum olfactorium, the area parolfactoria, and the 
piriform lobe.? 
The olfactory bulb (Bulbus olfactorius) is an oval enlargement which curves 
upward in front of the frontal pole of the hemisphere. Its convex superficial face 
fits into the ethmoidal fossa and receives numerous olfactory nerve-fibers through 
the cribriform plate; hence it is very difficult to remove the bulb intact. It con- 
tains a considerable cavity, the ventricle of the olfactory bulb (Ventriculus bulbi 
1 From the morphological point of view other structures should be included, but in descriptive 
anatomy it is usual to limit the application of the term to the parts enumerated above. 
