ANATOMY AND RELATIONSHIPS OF
THE PITUITARY GLAND
The pituitary
gland is reddish gray and ovoid,
measuring about 12 mm transversely, 8 mm in its anterior-posterior diameter,
and 6 mm in its vertical dimension. It weighs approximately 500 mg in men and
600 mg in women. It is contiguous with the end of the infundibulum and is
situated in the hypophysial fossa of the sphenoid bone. A circular fold of dura
mater, the diaphragma sellae, forms the roof of this fossa. In turn, the floor
of the hypophysial fossa forms part of the roof of the sphenoid sinus. The
diaphragma sellae is pierced by a small central aperture through which the
pituitary stalk passes, and it separates the anterior part of the upper surface
of the gland from the optic chiasm. The hypophysis is bound on each side by the
cavernous sinuses and the structures that they contain. Inferiorly, it is
separated from the floor of the fossa by a large, partially vacuolated venous
sinus, which communicates freely with the circular sinus. The meninges blend
with the capsule of the gland and cannot be identified as separate layers of the
fossa. However, the subarachnoid space often extends a variable distance into
the sella, particularly anteriorly, and may be referred to as a “partially
empty sella” when seen on magnetic resonance imaging (MRI) (see Plate 1-12). In
some cases of subarachnoid hemorrhage, the dorsal third of the gland may be
covered with blood that has extended down into this space.
The hypothalamus is an important relation of the pituitary gland, both
anatomically and functionally. This designation refers to the structures
contained in the anterior part of the floor of the third ventricle and to those
comprising the lateral wall of the third ventricle below and in front of the
hypothalamic sulcus. The mamillary bodies are two round, white, pea-sized
masses located side by side below the gray matter of the floor of the third
ventricle in front of the posterior perforated substance. They form the posterior
limits of the hypothalamus. At certain sites at the base of the brain, the
arachnoid is separated from the pia mater by wide intervals that communicate
freely with one another; these are called subarachnoid cisterns. As the
arachnoid extends across between the two temporal lobes, it is separated from
the cerebral peduncles by the interpeduncular cistern. Anteriorly, this space
is continued in front of the optic chiasm as the chiasmatic cistern.
Space-occupying lesions distort these cisterns. The optic chiasm is an
extremely important superior relation of the pituitary gland. It is a flat,
somewhat quadrilateral bundle of optic nerve fibers situated at the junction of
the anterior wall of the third ventricle with its floor. Its anterolateral
angles are contiguous with the optic
nerves, and its posterolateral angles are contiguous with the optic tracts. The
lamina terminalis, which represents the cephalic end of the primitive neural
tube, forms a thin layer of gray matter stretching from the upper surface of
the chiasm to the rostrum of the corpus callosum. Inferiorly, the chiasm rests
on the diaphragma sellae just behind the optic groove of the sphenoid bone. A
small recess of the third ventricle, called the optic recess, passes
downward and forward over its upper surface
as far as the lamina terminalis. A more distant relationship is the pineal
gland, which is a small, conical, reddish-gray body lying below the splenium of
the corpus callosum. Rarely, ectopic pineal tissue occurs in the floor of the
third ventricle and gives rise to tumors of that region. Compression of
neighboring cranial nerves, other than the optic nerves, may occur if there is
extensive cavernous sinus extension
from a pituitary neoplasm (see Plate
1-24).
