THE CEREBRAL HEMISPHERES 



671 



The olfactory bulb (l^ulhus olfactorius) is an oval enlarsomont which curves 

 upward in front of the frontal pole of th(> hemisphere. Its convex superficial face 

 fits into the ethmoidal fossa and rec(>ives numerous olfactory nerve-fibers through 

 the cribriform plate; hence it is very difficult to remove the bulb intact. It con- 

 tains a considerable 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 pole of the hemisphere and is connected 

 with the olfactory peduncle. 



The gray niatttM- of tlio bulb is external and is thickest on the convox anterior surface. The 

 l)osterior surface consists to a large extent of fibers which 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 sulwtance which aris(>s in the olfactory !)ulb and extends back to be continued 

 by the olfactory tracts. 



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



/ liter ventric iilar forame n 

 Gyrus fornicatus 

 Suhlinibir Jissurc (tint, part) 



Transverse- fissure 



/ Calloso-uiarginal fissure 



Subluiihic fiissure (post, parf) 



Entomaniiiiai fissure 



Marginal fissure 



Sulcus rhinalis anterior 



Ectoniarqinal fissure 



Hinpocatnpal fissure o ; i • ;■ , ■ 



' ' 'J bulcus rlnnatis posterior 



Fk;. 509. — Mf.dio-vkntral Aspect of Right Cerebral Hemisphere of Horse. 

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

 factorium; T.op., optic tract; Co., chiasma opticum; G.s., subcallosal gyrus; Co., corpus callosum; <S.p., septum 

 jjellucidum; B\ fornix; G.c, callosal gyrus; T, cut surface of thalamus; F' , fimbria; G.d., gyrus dentatus; 

 L p., pyriform lobe; G.h., hiijpocamijal 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 off from the trigonum 

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

 is short, ill defined, and fiat; it contains the canal which o]iens 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 iielow the genu of the corpus callosum. 



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

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

 l)y 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 gyrus or peduncle 

 of the corpus callosum. 



