The medial surface of the thalamus constitutes the upper part of the lateral wall of the third ventricle, and is connected to the corresponding surface of the opposite thalamus by a flattened gray band, the Interthalamic adhesion (massa intermedia, middle commissure, gray commissure).
In non-human mammals it is a large structure. In humans this mass averages about 1 cm in length in its antero-posterior diameter. It sometimes consists of two parts and occasionally is absent. The interthalamic adhesion is found in 70-80% of humans. It is present more often in females and larger than in males by an average of 53 percent.  When absent in development, no noticeable deficit has been observed.
It 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. In 1889, a Portuguese anatomist by the name of Macedo examined 215 brains, showing that male humans are approximately twice as likely to lack an interthalamic adhesion as are female humans. He anecdotally attributed the finding to a "prevailing feature of people deprived of [the interthalamic adhesion] is to present in their psychical acts a remarkable precipitation, joined to a certain dysharmony between internal and external feelings." It is noted to be enlarged in patients with Chiari II malformations. 
Human brain left dissected midsagittal view
This article incorporates text from a public domain edition of Gray's Anatomy.