THE RHOMBENCEPHALON 773 
THE RHOMBENCEPHALON 
THE MEDULLA OBLONGATA 
The medulla oblongata (Figs. 629, 632, 635) lies on the basilar part of the 
occipital bone. It is quadrilateral in outline, but much wider in front than behind, 
and compressed dorso-ventrally. Its length, measured from the root of the first 
cervical nerve to the pons, is about two inches (ca. 5 em.). 
Its ventral surface is convex in the transverse direction, and presents a ventral 
median fissure (Fissura mediana ventralis), which is continuous behind with the 
similar fissure of the spinal cord. The posterior part of the fissure is faintly marked, 
but in front it becomes deeper and ends in a small depression (Foramen cecum) 
behind the central part of a transverse band, the corpus trapezoideum. On either 
side of the fissure is a rounded tract, the pyramid (Pyramis), which is bounded 
laterally by a faint groove (Sulcus intermedius ventralis). ~The pyramids join 
the pons in front; behind they become narrower and disappear into the substance 
of the medulla, in which their fibers intercross, forming the decussation of the 
pyramids (Decussatio pyramidum).! The superficial origin of the oculomotor 
nerve (N. oculomotorius) is just lateral to the anterior end of the pyramid. The 
corpus trapezoideum is a transverse band which extends across the surface imme- 
diately behind the pons. It is crossed by the pyramids, which cut off a small 
central part. The lateral part extends out to the roots of the seventh and eighth 
nerves on either side. Behind the lateral part of the corpus trapezoideum there 
is a variably developed rounded eminence, the tuberculum faciale. The root- 
fibers of the hypoglossal nerve form an oblique linear series lateral to the posterior 
part of the pyramid. 
The dorsal surface is largely concealed by the cerebellum and forms the greater 
part of the floor of the fourth ventricle. The dorsal median fissure (Fissura 
mediana dorsalis), the direct continuation of the corresponding groove of the spinal 
cord, extends forward to about the middle of the surface. Here the restiform 
bodies, which constitute the lips of the fissure, diverge to form the lateral boun- 
daries of a triangular depression; this is the posterior part of the rhomboid fossa or 
floor of the fourth ventricle of the brain. The dorso-lateral groove winds outward 
and forward to the lateral aspect of the medulla, where it presents the roots of the 
glosso-pharyngeal, vagus, and accessory nerves. Lateral to it is a distinct oval 
eminence on the anterior part of the lateral column, termed the tuberculum cin- 
ereum. The central canal of the cord is continued in the posterior part of the 
medulla, inclines dorsally, and opens in the posterior angle of the fourth ventricle. 
Hence it is customary to distinguish a closed and an open part of the medulla. 
The dorsal aspect of the latter, which is concealed at present, will be considered 
later in the description of the fourth ventricle. 
The lateral surface is narrow behind, wider and rounded in front. From it 
the root-fibers of the ninth, tenth, and eleventh cranial nerves arise in a linear 
series, and alongside of it the spinal part of the eleventh nerve passes forward to 
join the medullary root. Close inspection reveals the presence of strize which 
eurve ventrally and backward from the surface of the restiform body toward the 
hypoglossal root-fibers; these are the external arcuate fibers (Fibre arcuate 
extern). The recess between the lateral aspect of the medulla and the cerebellum 
is occupied by an irregular mass of villous projections of the pia mater, containing 
tufts of vessels; this is the chorioid plexus of the fourth ventricle, and is the lateral 
edge of the tela chorioidea of the ventricle. On raising the chorioid plexus, it is 
seen that the tela chorioidea is attached to the dorsal aspect of the medulla, and 
reinforces here the wall of the fourth ventricle; also that the restiform body ter- 
‘The decussation varies superficially in different specimens. In some there is a distinct 
superficial crossing of fibers so that the median fissure is practically effaced at this point. 
