The Central Nervous System. 



279 



nected 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 

 depression enclosed by the thick lateral and anterior , walls 

 and floor of the ventricle. The middle line shows a narrow 

 depression, the posterior 



median fissure (fissura 

 mediana posterior) , on either 

 side of which the floor is 

 raised into a low ridge, de- 

 scribed as the medial emi- 

 nence (eminentia media- 

 lis). 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 pass- 

 ing into the cerebellum from 

 adjacent ventral portions of 

 the brain. In each a middle 

 band, the brachium pon- 

 tis, enters the pons; an 

 anterior band, the brach- 

 ium conjunctivum, en- 

 ters the mesencephalon, 

 while a posterior connection 



is established with the medulla through 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. 



Fig. 8.6. The rhombencephalon. 

 Dorsal view, after removal of the cere- 

 bellum: b.c, brachium conjunctivum; 

 b.p., brachium pontis; cl,, clava; cl.i., 

 inferior colliculus (mesencephaltfn) ; c.r., 

 restiform body; a.m., medial eminence; 

 f.c., fasciculus cuneatus; f.g., fasciculus 

 gracilis; f.m.p.," posterior median fissure of 

 the rhomboid fossa; f.f., rhomboid fossa; 

 s.ra.p., posterior median ■ sulcus of the 

 medulla; s.l.p., posterior ' lateral sulcus; 

 v.m.a., ariterioi medullary velum. 



