The two pyramids

Pyramid of medulla oblongata

The anterior or ventral portion of the medulla oblongata is named the pyramid and lies between the anterior median fissure and the antero-lateral sulcus....

 contain the motor fibers that pass from the brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 to 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...

 and medulla spinalis, corticobulbar and corticospinal fibers.

When these pyramidal fibers are traced downward, it is found that some three-fifths or more of them leave the pyramids in successive bundles, and decussate in the anterior median fissure of the medulla oblongata

Anterior median fissure of the medulla oblongata

The anterior median fissure 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...

, forming what is termed the pyramidal decussation or motor decussation.

Having crossed the middle line, they pass down in the posterior part of the lateral funiculus as the lateral cerebrospinal fasciculus.