THE MESENCEPHALON 



663 



The lateral wall is fornu'tl by the restiform body and the anterior peduncle 

 of the cerel)elhun. 



The roof is formed in its middle part by the vermis of the cerebellum, covered 

 by the epithelium before mentioned. There is conmionly a dorsal recess (Recessus 

 tecti s. fastigium) between the extremities of the vermis. The anterior part of the 

 roof is formed by a thin lamina of white substance, the anterior medullary velum 

 (or valve of Vieussens), which extends backward from the corpora (iuadrij2;emina, 

 and is attached on either side to the anterior peduncles of the cerebellum. Its 

 anterior part is relatively thick and contains the decussation of the fibers of the 

 trochlear nerves. Posteriorly it blends with the white matter of the cerebellum. 

 A thin lamina of white matter, the posterior medullary velum, backed by pia mater, 

 completes the roof posteriorly. Aft(M- removal of the cere])ellum the line of 

 attachment (Taenia ventriculi quarti) to the medulla is seen; it begins centrally 

 over the opening of the central canal, runs forward on the inner face of the restiform 

 body, and turns outward behind the brachium pontis. The thick part which 

 stretches over the posterior angle of the ventricle is termed the obex. The ]ios- 

 terior part of the ventricle forms three recesses, of which two are lateral and the 

 third median and posterior. The lateral recesses communicate with the sub- 



FiG. 505. — Brain Stem and Basal Ganglia of Horse, Right View. 

 F.a., External arcuate fibers; C.r., corpus restiforme; P, pyramid; T, corpus trapezoideum; B./)., middle 

 peduncle of cerebellum; P.c, cerebral jjeduncle; S, sulcus lateralis; T.t., tractus transversus; L, trigonum lem- 

 nisci; C. a., corpus quad, ant.; r.;;., cor|)us quad, post.; 5, commissure of C. p.," G, corpus geniculatuni internum; 

 T.O., olfactory peduncle; B.u., olfactory bulb. 



arachnoid space by distinct openings (Apertura^ laterales). The layer of pia 

 which strengthens the roof here is named the tela chorioideaof the fourth ventricle. 

 It is triangular in outline and closely adherent to the velum. It forms three 

 fringed masses which contain vascular convolutions and are designated the median 

 and lateral chorioid plexuses of the fourth ventricle. They appear to lie within 

 the ventricle, but are really excluded from the cavity by the epithelial lining, which 

 they invaginate. 



THE MESENCEPHALON 



The mesencephalon or mid-brain connects the rhombencephalon with the 

 fore-brain. In the undissected brain it is covered dorsally by the cerebral hemi- 

 spheres. It consists of a dorsal part, the corpora quadrigemina, and a larger ventral 

 part, the cerebral peduncles, which are visible on the l)ase of the brain. It is tra- 

 versed longitudinally by a narrow canal, the cerel)ral aqueduct, which connects the 

 fourth ventricle with the third. 



The corpora quadrigemina' are four rounded eminences which lie under the 

 posterior part of the cerel)ral hemis]iheres. They consist of two pairs, separated 

 by a transverse groove. The anterior pair (( 'oUiculi nasales) are larger and much 



' In the new nomemclature the term lamina quadrigemina i.s appHed to the dorsal mass of 

 the mid-brain, and the four eminences which it bears are the corpora qua<lrigemina. 



