THE FOURTH VENTRICLE 



779 



and is marked by three longitudinal furrows which converge behind. It is widest 

 and deepest a little in front of its middle. The posterior part narrows to a point at 

 the opening of the central canal of the spinal cord, and on account of its appearance 

 in man it has been termed the calamus scriptorius. The median sulcus (Sulcus 

 medianus) extends the entire length of the floor and is deepest toward the ends. 

 The limiting sulci (Sulci hmitantes) begin on either side of the opening of the central 

 canal and extend forward as the lateral limits of the rhomboid fossa. Just beyond 

 the middle of the fossa they expand into a shallow depression, the anterior fovea 

 (Fovea oralis). On either side of the median sulcus and margined laterally by the 

 limiting sulcus is a slightly rounded column, the eminentia medialis. Opposite the 

 fovea this presents an elongated prominence, the colliculus facialis, so named be- 

 cause it overlies the bend formed by the fibers of origin of the facial nerve. Lateral 

 to the limiting sulcus is a long fusiform elevation, the area acustica, from which a 

 band of fibers (Stride acusticse) winds over the anterior end of the restif orm body to 

 the superficial origin of the cochlear nerve. 



The lateral wall is formed by the restif orm body and the anterior peduncle 

 of the cerebellum. 



The roof (Tegmen ventriculi quarti) is formed in its middle part by the vermis 



S T t 

 Fro, 636. — ^Brain Stem akb Basal Ganglia of Horse; Right View. 

 F.a., External arcuate fibers; Cr., corpus restiforme; P, pyramid; T, corpus trapezoideum; B. p., middle pe- 

 duncle of cerebellum; P. c, cerebral peduncle; 5, sulcus lateralis; T.^., tractus tranaversua; L, trigonum lemnisci; C.a., 

 corpus quad, ant.; C.p., corpus quad, post.; B, commissure of C.p.; G, corpus geniculatum mediale; T.o., olfactory 

 peduncle; B.o., olfactory bulb. 



of the cerebellum, covered by the epithelium before mentioned. There is com- 

 monly a dorsal recess (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 (Velum meduUare anterius),^ which extends backward from the 

 corpora quadrigemina, 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 

 (Velum medullare posterius), backed by pia mater, completes the roof posteriorly 

 After removal of the cerebellum 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 posterior part of the ventricle forms three 

 recesses, of which two are lateral and the third median and posterior. The lateral 

 recesses (Recessus lateralis) communicate with the subarachnoid space by lateral 

 apertures (Aperturse laterales). The layer of pia mater which actually forms the 

 roof here is named the tela chorioidea of the fourth ventricle. It is triangular in 

 outline and closely adherent to the velum. It forms three fringed masses which 

 ' Formerly termed the valve of Vieussens. 



