One of the most unique aspects of the human skull is the bend in the skull base and how it affects the position of the foramen magnum. The foramen magnum is the large hole in the posterior fossa of the skull through which the brainstem connects to the spinal cord.

The posterior fossa was so named because it is the rear-most compartment in most mammals. As upright posture evolved in hominids and humans the foramen magnum slowly migrated forward toward the front of the head eventually ending up beneath the center of it. The posterior fossa subsquently ended up on the bottom. Thus, because of the bend in the base of the cranium and the foward migration of the foramen magnum, the posterior fossa is more like a basement than a rear compartment.

The bend in the floor of the cranium and the position of the posterior fossa at the bottom of the cranial vault puts the brainstem in a precarious postion suspended directly over the foramem magnum. It’s precarious because it makes the brainstem susceptible to cerebellar tonsilar ectopia (CTE)and pressure conus type conditions where the cerebellum or other parts of the brainstem sink toward the foramen magnum. The consequences of (CTE) and pressure conus type conditions can be potentially catastrophic. In addition to compressing the brainstem they also block venous outflow from the brain.

What makes the brainstem’s position so precarious is that the brain isn’t attached to anything inside the cranial vault. Instead it simply floats, suspended in a water jacket that contains cerebrospinal fluid, called CSF. The only attachment point is at the tail end of the cord called the filum terminale, which connects to the tailbone called the coccyx.

Lastly, certain types of abnormal conditons of the spine, such as scoliosis, predispose the brain to pressure conus and CTE type conditions due to downward traction from the filum terminale, which is called a tethered cord syndrome. When the brainstem sinks toward the foramen it compresses and blocks venous outlets in the base of the vault which can cause a backup of venous blood and CSF.