EPITHALAMUS :-
The
Epithalamus occupies the caudal part of the roof of the diencephalon and
consists of
1.
The
right and left habenular nuclei, each situated beneath the floor of the
corresponding habenular trigone.
2.
The
pineal body or epiphysis cerebri
3.
The habenular commissure
4.
The
posterior commissure .
HABENULAR NUCLEUS :-
1.
The
nucleus lies beneath the floor of the habenular trigone.
2.
The
trigone is a small, depressed triangular area, situated above the superior
colliculus and medial to the pulvinar of the thalamus.
3.
Medially
it is bounded by the stria medullaris thalami and the stalk of the pineal body.
4.
The
habenular nucleus forms a part of the limbic system.
PINEAL BODY :-
1.
The
pineal body is a small, conical organ, projecting backwards and downwards
between the two superior colliculi.
2.
It
is placed below the splenium of the
corpus callosum, but is separated from it by the tela choroidea of the third
ventricle and the contained cerebral veins.
3.
It
consists of a conical body about 8 mm long, and a stalk or peduncle which
divides anteriorly into two laminae separated by the pineal recess of the third
ventricle.
4.
The
superior lamina of the stalk contains the habenular commissure and the inferior
lamina contains the posterior commissure.
Morphological significance :-
In many
reptiles the epiphysis cerebri is represented by a double structure.
1.
The
anterior part (parapineal organ) develops into the pineal or parietal eye.
2.
The
posterior part is glandular in nature.
3.
The
human pineal body represents the persistent posterior glandular part only.
4.
The
parietal eye has disappeared.
STRUCTURE :-
1.
The
pineal gland is composed of two types of cells, pinealocytes and neuroglial
cells, with a rich network of blood vessels and sympathetic fibres.
2.
The
vessels and nerves enter the gland through the connective tissue septa which
partly separate the lobules.
3.
Sympathetic
ganglion cells may be present.
4.
Calcareous
concretions are constantly present in the pineal after the 17th year
of life and may form aggregtions (brain
sand). Spaces or cysts may also be present.
FUNCTIONS :-
1.
The
pineal body has for long been regarded as a vestigial organ of no importance.
2.
Recent
investigations have shown that it is an endocrine gland of great importance.
3.
It
produces hormones that may have an important regulatory influence on many other
endocrine organs ( including the
adenohypophysis , the neurohyphysis, the thyroid , the para thyroids,
adrenal cortex and medulla, and the
gonads).
4.
The
best known hormone is melatonin which causes changes in skin colour in some
species.
5.
The
synthesis and discharge of melatonin is remarkably influenced by exposure of
the animal to light.
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