1338 A MANUAL OF ANATOMY 



The medulla oblongata, or bulb, lies on the under aspect of the 

 cerebellum, in the median line, occupying the vallecula which 

 separates the two cerebellar hemispheres. The surface exposed is 

 the ventral surface, which presents (i) the anterior median fissure, 

 crossed at its lower part by the decussation of the pyramids ; 

 (2) the pyramid, on either side of this fissure ; and (3) the olivary 

 body, external to each pyramid. 



The hemispheres of the cerebellum lie one on either side of the 

 medulla oblongata, and they conceal from view the posterior parts 

 of the cerebral hemispheres, and the posterior part of the great 

 longitudmal fissure. They are characterized by the laminated 

 arrangement of their nervous matter, the laminae being curved and 

 separated from each other by fissures. Posteriorly the hemispheres 

 are separated from each other by the posterior notch. When the 

 medulla oblongata is raised, and the cerebellar hemispheres slightly 

 separated from each other, the vallecula is fully exposed, and the 

 inferior vermis is seen lying deeply in it, with the sulcus valleculce 

 on either side of it. 



The pons Varolii forms a prominent elevation above the medulla 

 oblongata, the surface exposed being the ventral surface. In the 

 median line this surface presents a longitudinal groove, which is 

 occupied by the basilar artery. On either side of the pons the 

 middle peduncle of the cerebellum is seen, passing outwards and 

 backwards into the cerebellar hemisphere. 



The temporal lobes of the cerebrum are situated in front of the 

 cerebellar hemispheres, and are conspicuous by their prominence. 

 Each terminates anteriorly in a projecting extremity, called the 

 temporal pole. 



The stem of the fissure of Sylvius lies immediately in front of the 

 temporal lobe, and is occupied by the middle cerebral artery. At 

 the inner end of the stem of the fissure of Sylvius is the depression 

 known as the vallecula Sylvii. 



The frontal lobe of the cerebrum lies in front of the stem of the 

 fissure of Sylvius. The exposed part is the orbital surface, which 

 is separated from its fellow of the opposite side by the great longi- 

 tudinal fissure. Each orbital surface presents a straight fissure, 

 called the olfactory sulcus, which is situated near the great longi- 

 tudinal fissure, and is parallel to it. This sulcus is occupied by the 

 olfactory tract and olfactory bulb. 



The crura cerebri, right and left, appear at the upper border of 

 the pons Varolii, and soon diverge from each other as they pass 

 forwards and upwards to sink into the cerebral hemispheres. 



The optic tract of each side winds round the outer and ventral 

 aspects of the corresponding crus cerebri. Its course is forwards 

 and inwards towards its fellow of the opposite side. 



The optic commissure, or chiasma, connects the two optic tracts 

 after their convergence. 



The optic nerves, right and left, leave the front of the com- 

 missure, and pass forwards and outwards to the optic foramina. 



