The interthalamic adhesion (also known as the massa intermedia, intermediate mass or middle commissure) is a flattened band of tissue that connects both parts of the thalamus at their medial surfaces.

The medial surfaces form the upper part of the lateral wall to the third ventricle.

It is still uncertain whether the interthalamic adhesion contains fibers that cross the midline – and for this reason, it is inappropriate to call it a commissure.

The interthalamic adhesion is notably enlarged in patients with the type II Arnold–Chiari malformation.

[3] This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)