The anterior median fissure of the spinal cord has an average depth of about 3 mm, but this is increased in the lower part of the medulla spinalis.

It contains a double fold of pia mater

Pia mater

Pia mater often referred to as simply the pia, is the delicate innermost layer of the meninges, the membranes surrounding the brain and spinal cord. The word finds its roots in Latin, meaning literally "tender mother." The other two meningeal membranes are the dura mater and the arachnoid mater....

, and its floor is formed by a transverse band of white substance, the anterior white commissure

Anterior white commissure

The anterior white commissure, also known as the alba anterior medullae spinalis, is a bundle of nerve fibers which cross the midline of the spinal cord just anterior to the gray commissure...

, which is perforated by blood vessels on their way to or from the central part of the medulla spinalis.