Page 377 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 362

362 The central nervous system

form distinct cisterns. The most important of these are: the cisterna magna
between the cerebellum and the dorsum of the medulla; the cisterna pontis
over the ventral surface of the pons, the interpeduncular cistern between the
two cerebral peduncles, the cisterna ambiens between the splenium of the
corpus callosum and the superior surface of the cerebellum (containing
the great cerebral vein and the pineal gland), and the chiasmatic cistern
around the optic chiasma. Re-absorption of C.S.F. is principally by way of
the superior longitudinal and the other dural sinuses, the modified arach-
noid of the arachnoid granulations piercing the dura and bringing the
C.S.F. into direct contact with the sinus mesothelium. Along the superior
sagittal sinus these granulations (or arachnoid villi) clump together to form
the Pacchionian bodies, which produce the pitted erosions readily seen
along the median line of the inner aspect of the skull cap.
About one-fifth of the C.S.F. is absorbed along similar spinal villi or
escapes along the nerve sheaths into the lymphatics. This absorption of
C.S.F. is passive, depending on its hydrostatic pressure being higher than
that of the venous blood.

Clinical features

1◊◊Computerized axial tomography (CT scanning) has quite revolu-
tionized the investigation of intracranial space-occupying lesions (post-
traumatic haematoma, abscess and neoplasms), both by delineating the
lesion itself and by demonstrating displacement of the ventricular system.
Figures 252 and 253 are representative transverse cuts through the skull to
illustrate normal anatomical features; note that the details of the anatomy
of the ventricles are clearly visualized.
2◊◊Magnetic resonance imaging (MRI) is particularly valuable in
producing high-quality images of the central nervous system, although
at present the technique is slower and much more expensive than CT
(Fig. 254).
3◊◊The C.S.F. probably serves several purposes, including the provision of
a protective water-jacket and a regulating mechanism of intracranial pres-
sure with changing cerebral blood flow.
4◊◊The total capacity of the C.S.F. in the adult is about 150ml, of which
some 25ml is contained within the spinal theca; it is normally under a pres-
sure of about 100mm of water (with a range of 80–180) in the lateral hori-
zontal position. The dural theca acts as a simple hydrostatic system, so that
when the patient sits up, the C.S.F. pressure in the lumbar theca rises to
between 350 and 550mm, whereas the ventricular fluid pressure falls to
below atmospheric.
5◊◊Certain parts of the C.S.F. pathway are narrow and easily obstructed.
These sites are the interventricular foramina, the 3rd ventricle, the
aqueduct, the exit foramina of the 4th ventricle and the subarachnoid
space around the midbrain in the tentorial notch. Obstruction to the
system causes increased intracranial pressure and ventricular dilatation
(hydrocephalus).