THE CEREBRAL HEMISPHERES 



671 



The olfactory bulb (liulbus olfactorius) is an oval enlargement which curves 

 upward in front of tlie 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 c()nsid(>ral)le cavity (Ventriculus bulbi olfactorii) which is connected with 

 the lateral ventricle by a small canal in the middle olfactory tract. The deep 

 face is largely in contact with the frontal jiole of the hemisphere and is connected 

 with the olfactory peduncle. 



The gmy matter of the bulb is external and is thickest on the convex anterior surface. The 

 posterior surface consists to a large extent of fibers wliich are the axones of the mitral cells of the 

 deep layer of the gray substance and go to form the peduncle and stria>. 



The olfactory peduncle (Tractus olfactorius) is a very short but wide band of 

 white substance which arises in the olfactory bulb and extends l)ack to be continued 

 ))y the olfactory tracts. 



The olfactory tracts or striae (Striie olfactorii) are three in numljer. The 



InkriTntriculdr foramen 

 GyriiH fornicatus 

 Suhlimhic fissure (ant. pari) 



Transverse fissure 

 I Calloso-marginal fissure 

 ^^ J Suhlimhic fissure (post. parC) 



Entomarginal fissure 



Margiual fissure 



Sulcus rhinnlis (interior 



Ectoiiiarginal fissure 



Hippocampal fissure ^^^^^^^^^ ^j^.^^^^.^ ^^^^^^ .^^ 



Fig. 509. — Medio-ve.vtrai. Aspect of Right Cerebr.vl Hemisphere of Horse. 

 The olfactory bulb is cut off. T.oL, Olfactory peduncle; A.p., area parolfactoria ; Tr.O., trigonum ol- 

 factoriuiii; T.op., optic tract; Co., chiasma opticuni; G.s., subcallosal gyrus; C.c, corpus callosum; S.p., septum 

 pellucidum; F, fornix; G.c, callo.sal gyrus; T, cut surface of thalainu.s; F' , fimbria; G.d., gyrus dentatus; 

 L p., pyriform lobe; G.h., hippocampal gyrus. 



external tract (Stria lateralis) is much the largest and most distinct. It passes 

 backward, upward, and outward, widens out and joins the pyriform lobe. It is 

 clearly defined dorsally by the sulcus rhinalis and is marked ofi" from the trigonum 

 olfactorium by the sulcus arcuatus. The intermediate tract (Stria intermedia) 

 is short, ill defined, and flat; it contains the canal which opens into the anterior 

 horn of the lateral ventricle. The internal tract (Stria medialis) is small, short, 

 and not well defined; it bends over to the parolfactory area (of Broca) on the inner 

 face of the hemisphere l)elow the genu of the corpus callosum. 



The trigonum olfactorium is the prominent gray area situated in the angle of 

 divergence of the internal and external olfactory strise. It is bounded externally 

 by the external olfactory stria, from which it is defined by the arcuate sulcus. It 

 is continuous with the area parolfactoria on the medial surface; behind the latter 

 a band descends from the rostrum of the corpus callosum and is continuous below 

 with the anterior perforated space; it is termed the subcallosal g3^rus or peduncle 

 of the corpus callosum. 



