THE CEBEBIIAL HEMISPHERES 



787 



mrt of 7h "^^ttf ' te^ed the fimbria, which is the prolongation of the greater 

 part of the crus of the fornix mto 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 limbria. 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, i his 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-inarginal fissure 



Siibliminc fissure (post. parC) 



Entomartjinal fissure 



Marginal fissure 



Sulcus rhinnlis anterior 



Ertomarginal fissure 



Hippocampal fissure 



Sulcus rhinalis posterior 



Fig. 640. — Medio-ventral Aspect of Right Cerebral Hemisphere of Horse. 

 The olfactory bulb is cut off. T.ol., Olfactory tract; -l.p., area parolfactoria; Tr.O., trigonum olfactorium; 

 Tr.op., optic tract; Co., cfiiagma opticum; G.s., subcallosal gyrus; C.c, corpus callosum; S.p., septum pellucidum; F, 

 fornix; F', fimbria; G.c, callosal gyrus; T, 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 stride, 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 



' From the morphological point of view other structures should be included, but in descriptive 

 anatomy it is usual to limit the apphoation of the term to the parts enumerated above. 



