Epithalamus

The epithalamus is a dorsal posterior segment of the diencephalon (a segment in the middle of the brain also containing the hypothalamus and the thalamus) which includes the habenula, the stria medullaris and the pineal body. Its function is the connection between the limbic system to other parts of the brain.

Some functions of its components include the secretion of melatonin by the pineal gland.

Details from Gray's anatomy
The Epithalamus comprises the trigonum habenulæ, the pineal body, and the posterior commissure.

Trigonum habenulæ
The trigonum habenulæ is a small depressed triangular area situated in front of the superior colliculus and on the lateral aspect of the posterior part of the taenia thalami.

It contains a group of nerve cells termed the ganglion habenulæ.

Fibers enter it from the stalk of the pineal body, and others, forming what is termed the habenular commissure, pass across the middle line to the corresponding ganglion of the opposite side.

Most of its fibers are, however, directed downward and form a bundle, the fasciculus retroflexus of Meynert, which passes medial to the red nucleus, and, after decussating with the corresponding fasciculus of the opposite side, ends in the interpeduncular ganglion.

Pineal body
The pineal body (corpus pineale; epiphysis) is a small, conical, reddish-gray body which lies in the depression between the superior colliculi.

It is placed beneath the splenium of the corpus callosum, but is separated from this by the tela chorioidea of the third ventricle, the lower layer of which envelops it.

It measures about 8 mm. in length, and its base, directed forward, is attached by a stalk or peduncle of white substance.

The stalk of the pineal body divides anteriorly into two laminæ, a dorsal and a ventral, separated from one another by the pineal recess of the third ventricle.

The ventral lamina is continuous with the posterior commissure; the dorsal lamina is continuous with the habenular commissure and divides into two strands the medullary striæ, which run forward, one on either side, along the junction of the medial and upper surfaces of the thalamus to blend in front with the columns of the fornix.

Posterior commissure
The posterior commissure is a rounded band of white fibers crossing the middle line on the dorsal aspect of the upper end of the cerebral aqueduct.

Its fibers acquire their medullary sheaths early, but their connections have not been definitely determined.

Most of them have their origin in a nucleus, the nucleus of the posterior commissure (nucleus of Darkschewitsch), which lies in the central gray substance of the upper end of the cerebral aqueduct, in front of the nucleus of the oculomotor nerve.

Some are probably derived from the posterior part of the thalamus and from the superior colliculus, while others are believed to be continued downward into the medial longitudinal fasciculus.

(this section only)