The anterior median fissure (ventral or ventromedian fissure) contains a 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 extends along the entire length of the medulla oblongata

Medulla oblongata

The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

: It ends at the lower border of the pons in a small triangular expansion, termed the foramen cecum

Foramen cecum (medulla oblongata)

The anterior median fissure of the medulla oblongata contains a fold of pia mater, and extends along the entire length of the medulla oblongata: it ends at the lower border of the pons in a small triangular expansion, termed the foramen cecum ....

.

Its lower part is interrupted by bundles of fibers that cross obliquely from one side to the other, and constitute the pyramidal decussation.

Some fibers, termed the anterior external arcuate fibers

Anterior external arcuate fibers

The anterior external arcuate fibers vary as to their prominence in different cases: in some they form an almost continuous layer covering the pyramid and olivary body, while in others they are barely visible on the surface.They arise from the cells of the gracile and cuneate nuclei, and passing...

, emerge from the fissure above this decussation and curve lateralward and upward over the surface of the medulla oblongata to join the inferior peduncle.