The interthalamic adhesion 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.
In mammals other than humans, it is a large structure. In humans it is only about one centimetre long, though In females it is larger by about 50%. Sometimes it is in two parts and 20% to 30% of the time it is absent.
Its absence is seen to be inconsequential.
The interthalamic adhesion contains nerve cells and nerve fibers; a few of the latter may cross the middle line, but most of them pass toward the middle line and then curve laterally on the same side. It is still uncertain whether the interthalamic adhesion contains fibers that cross the mid-line 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.