Page 357 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 342

342 The central nervous system

The blood supply of the medulla is derived from the vertebral arteries
directly and from their posterior inferior cerebellar branches.

Clinical features

The medulla contains the respiratory, cardiac and vasomotor centres— the
‘vital centres’. The respiratory centre is particularly vulnerable to compres-
sion, injury or poliomyelitis with consequent respiratory failure.

The pons

External features (Fig. 240)
The pons lies between the medulla and the midbrain and is connected to
the cerebellum by the middle cerebellar peduncles. It is 1in (25mm) in
length and 1.5in (38mm) in width. Its ventral surface presents a shallow
median groove and numerous transverse ridges, which are continuous lat-
erally with the middle cerebellar peduncle. The dorsal surface of the pons
forms the upper part of the floor of the IVth ventricle. Its junction with the
medulla is marked close to the ventral midline by the emergence of the VIth
cranial nerves and, in the angle between the pons and the cerebellum, by
the VIIth and VIIIth nerves. Both the motor and sensory roots of V leave the
lateral part of the pons near its upper border.

Internal structure

The pons consists for the most part of a number of cell masses (the pontine
nuclei), scattered amongst the long ascending and descending pathways
and the decussating pontocerebellar fibres, the pontine tegmentum (the
pontine component of the reticular formation) and the central connections
of the Vth, VIth and VIIth cranial nerves.
Atypical cross-section through the pons is shown in Fig. 243.
The blood supply of the pons is derived from the basilar artery (Fig. 240),
formed by the junction of the two vertebral arteries, by way of a number of
small pontine branches.

The cerebellum

External features (Fig. 240)
The cerebellum is the largest part of the hind-brain and occupies most of the
posterior cranial fossa. It is made up of two lateral cerebellar hemispheres and
a median vermis. Inferiorly, the vermis is clearly separated from the two
hemispheres and lies at the bottom of a deep cleft, the vallecula; superiorly, it
is only marked off from the hemispheres as a low median elevation. Asmall
ventral portion of the hemisphere lying on the middle cerebellar peduncle
is almost completely separated from the rest of the cerebellum as the floccu-