The velum interpositum is a small membrane containing a potential space just above and anterior to the pineal gland which can become enlarged to form a cavum veli interpositi. 

Gross anatomy

The velum interpositum is formed by an invagination of pia mater forming a triangular membrane the apex of which points anteriorly.

The velum interpositum is often said to be the medial part of the choroidal fissure, but this is incorrect. It is important to understand that the choroidal fissure is merely a C-shaped slit in the medial wall of the cerebral hemisphere; extending from the interventricular foramen of Monroe, to around the thalamus and cerebral peduncle as far as the uncus of the temporal lobe. The convexity of the C shape is contained by the body and crus (pillar) of the fornix, the fimbria and the hippocampus. The concavity of the C shaped slit is contained by the thalamus (upper and posterior surfaces) and the tail of the caudate nucleus. The slit of the choroid fissure is where pia mater and ependyma are in direct contact with each other and evaginate into the lateral ventricles as choroid plexus¹. Opening the choroidal fissure between the fornix and the thalamus will lead into the velum interpositum. In other words, the velum interpositum is like a room in a house and the choroid fissure is merely the threshold of the doorway into the room.

Boundaries

• superiorly: the body of the fornix and hippocampal commissure (psalterium) reaching as far forward as the foramen of Monro

• inferiorly: the internal cerebral veins and tela choroidea of the third ventricle

• inferolaterally: the thalamus

• posteriorly: the narrow base of the triangle abuts the splenium of the corpus callosum ²

It varies in shape from person to person, sometimes interposed between the internal cerebral veins and splenium, and depending on whether or not there is a cavum vergae (in which case the columns of the fornices are displaced inferiorly, narrowing the velum interpositum). 

When somewhat distended by fluid it forms a small triangular (in axial section) space and is referred to as a cavum velum interpositum. If larger and exterting mass effect it is known as a cavum velum interpositum cyst. 

The space between the two leaves of pia contains the medial posterior choroidal arteries ³.