INTRODUCTION:-

It is a median left between the two thalami. It represents the cavity of the diencephalon. It is lined by ependyma.

COMMUNICATION:-

ANTEROSUPERIORLY:- On each side, it communicates with the lateral ventricle through the interventricular foramen. This Foramen is anteriorly bounded by a column of fornix and posteriorly by the tubercle of the thalamus.
POSTEROINFERIORLY:- It communicates with fourth ventricle through the cerebral aqueduct.

RESSCES:-

These are extensions of the cavity.
  • Supraspinal
  • Pineal
  • Infundibulum
  • Optic.

BOUNDARIES:-

1. ANTERIOR WALL:-

It is bounded by
  • Lamina terminals.
  • Anterior commissure.
  • The anterior column of the fornix.

2. POSTERIOR WALL:-

It is bounded by
  • Pineal body.
  • Posterior commissure.
  • Cerebral aqueduct.

ROOF:-

  • It is formed by the ependymal lining under the surface of the choroid of the third ventricle.
  • The choroid plexus of third ventricle projects downwards from the roof.
  • At the junction of the roof with the anterior and the lateral wall, there is the interventricular foramen.

FLOOR:-

It is formed by hypothalamic structure:-
  • Optic chiasma.
  • Tubercinerium .
  • Infundibulum.
  • Mammillary body.
  • Posterior perforated substance.
  • Tegmentum of the midbrain.