Thk Central Nervous System. 



191 



hp /_ 





5. By dividing the supports of the cerebellum on either side, the 

 entire structure may be removed and the surface exposed, as in Fig. 

 55, for an examination of the structures of the dorsal surface of the 

 rhombencephalon. The posterior medullary velum is removed with the 

 cerebellum, but a small portion of the anterior medullary velum should 

 remain in place. 



(a) The fourth ventricle (ventriculus quartus) is the extensive 

 space enclosed by the rhombencephalon. It is connected 

 forwards with the cerebral aqueduct and backwards with 

 the central canal of the spinal cord. Its roof is formed 

 principally by the anterior and posterior medullary vela, the 

 latter being attached' to the cerebellum close to one another, 

 so that they underlie it. 



(b) The rhomboid fossa (fossa 

 rhomboidea) is the shallow de- 

 pression enclosed by the thick 

 lateral and anterior walls and 

 floor of the ventricle. The 

 middle line shows a shallow 

 depression, the posterior me- 

 dian fissure (fissura mediana 

 posterior), on either side of 

 which the floor is raised into a 

 low ridge, described as the 

 medial eminence (eminentia 

 medialis). The posterior end 

 of the fossa forms with the 

 enclosing wall the somewhat 

 triangular figure described as 

 the calamus scriptorius. 



(c) The lateral supports of the 

 cerebellum, now represented by 

 their cut ends, are formed by 

 fibre-bands passing into the 



cerebellum from adjacent ven- ^ f^°- ^s. The rhombencephalon. 



, -' . Dorsal view, after removal ot the 



tral portions Ol the bram. In cerebellum: b.c, brachium conjunc- 



h- 1 11 I 1 i.1 tivum; b.p., brachium pontis; cl., 



a middle band, the clava; cl.i , inferior colliculus (me- 



brachium pontis, enters the sencephalon); c.r.. restiform body; 



^ .' e.m., medial eminence; f.c, fasciculus 



pons ; an anterior Imnd, the cuneatus; f.g., fasciculus gracilis; 



brachium conjunctivum, enters l^^-f.^,nlV'itls^'t? rhomboid 



the mesencephalon, while a f°ssa; s.m.p posterior median sulcus 



^ . ' . of the medulla; s.l.p., posterior 



posterior connection is estab- lateral sulcus; v.m.a., anterior 



lishedwith the medulla through "^^dullary velum. 



the restiform body. 



(d) The lateral wall of the fossa is formed by a thick ridge of 

 nervous matter, convex on its medial side, the restiform 

 body (corpus restiforme). Its anterior portion bears a large 

 spherical elevation overlying the origin of the acoustic nerve. 



