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 limitantes) 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 (Strize acusticze) winds over the anterior end of the restiform body to 
the superficial origin of the cochlear nerve. 
The lateral wall is formed by the restiform body and the anterior peduncle 
of the cerebellum. 
The roof (Tegmen ventriculi quarti) is formed in its middle part by the vermis 
Fic. 636.—Brarn Stem AND Basat GanGura oF Horse; Ricutr View. 
F.a., External arcuate fibers; C.r., corpus restiforme; P, pyramid; 7’, corpus trapezoideum; B. p., middle pe- 
duncle of cerebellum; P. c., cerebral peduncle; S, sulcus lateralis; T.t., tractus transversus; 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 medullare 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 (Tzenia 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 (Aperture 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 
1 Formerly termed the valve of Vieussens. 
