Gross Anatomy 11 



The cerebellar hemisphere represents only a part of the much 

 larger hemisphere of man. 



The cerebellum consists morphologically of three lobes, 

 anterior, middle, and posterior, the fissures separating them 

 being the sulcus primarius and sulcus praepyramidalis re- 

 spectively (see Fig. 3). The anterior lobe consists of four 

 transverse lobules, but only the last of these can be seen 

 from the dorsal surface. The posterior lobe is composed of 

 a median part and the lateral floccular lobes, the median 

 part being made up of pyramis, uvula, and nodulus (named 

 dorso-ventrally), of which only the pyramis and the large 

 uvula behind it appear from above. The middle lobe like- 

 wise consists of median and lateral portions, the lateral ones 

 being the hemispheres, all of which are divided by transverse 

 grooves into narrow gyri. 



The cerebellum may be removed by carefully cutting 

 through the peduncles which attach it to the medulla ob- 

 longata at each side. When this is done the cerebellum is 

 found to constitute only a very narrow transverse strip of the 

 actual roof of the ventricle below it. The greater part of 

 the ventricle is roofed by two membranes attached to the 

 cerebellum along this strip and extending forward (velum 

 medullare anterius or anterior medullary velum) and back- 

 ward (velum medullare posterius or posterior medullary 

 velum) to the extremities of the widely open fourth ventricle. 

 The posterior medullary velum contains greatly convoluted 

 groups of blood vessels which project into the ventricle, 

 pushing the velum before them — these vessels forming the 

 chorioid plexus of the fourth ventricle. 



The trochlear nerve (I\') decussates in the anterior edge 

 of the anterior medullary velum and may be seen emerging 

 round the postero-lateral aspect of the midbrain. 



The middle cerebellar peduncle, which has been cut 

 through, extends straight ventrally and forms a thick trans- 

 verse mass of fibres crossing the most anterior part of the 



