Answer: The foramen magnum is the large hole on the ventral (bottom) side of the skull where the medulla, arteries, and the spinal cord exits down towards the body.
The brain is able to communicate with the rest of the body using a series of nerves that are bundled together in the spinal cord, the part of the central nervous system that extends from the ventral side of the brain. All motor output functions, such as moving the muscles in your legs or arms, pass down through the spinal cord. Autonomic functions, such as the involuntary increase of heart rate in a scary situation (the sympathetic nervous system response, or “fight, flight, or fright”) also communicate through extensions of nerves out of the posterior end of the brain. From the other end, sensory inputs from the skin, muscles, and internal organs send signals up into the brain via the spinal cord.
However, the brain itself is protected by the skull, a series of interconnected plates of bones. For neurons in the brain to communicate with structures outside of the brain, their axons must be able to project out from the skull. On the other hand, sensory neurons need to send axonal projections into the brain.
Much of this communication is done by the bundles of axons which pass through small holes in the skull called foramen (foramina is the plural form.) The number and size of these foramina differ slightly between individuals and across a person's age. Each individual foramen is either named based on the specific bone of the cranium that it runs through, the major nerve that is carried, or a word that describes the appearance of the foramen.
The ventral most plate of the skull is a curved trapezoidal-shaped bone called the occipital bone. The bone itself has several foramina. The largest foramen in this bone is called the foramen magnum. The foramen magnum is oval shaped, approximately 3.5 cm long anterior to posterior, and 3 cm wide lateral. It is the largest of the foramen, and very obvious anatomically.
In addition to the brain stem and spinal cord, other anatomically significant organs pass through the foramen magnum. The accessory nerve, cranial nerve 11, passes through the foramen magnum. The accessory nerve only has motor function, and innervates the sternocleidomastoid and trapezius muscles. Several blood vessels also pass through the foramen magnum, including the vertebral arteries which supply blood to the posterior part of the brain, cerebellum, and the upper spinal cord. The posterior and anterior spinal arteries also pass through the foramen magnum, both of which send blood through to the cells of the spinal cord.