The inside of the skull (called the intracranial space) is partly separated into two compartments by a tent like structure called the tentorium. The large compartment above the tentorium is called the supratentorial compartment and the compartment below the tentorium is called the infratentorial compartment (supra means above, and infra means below). Most doctors call the infratentorial compartment the Posterior Fossa.

The area above the tentorium contains the two halves of the cerebrum known as cerebral hemispheres and a deep central area called the diencephalon. Below the tentorium is the brainstem and the Cerebellum. Like the cerebrum, the cerebellum is divided into two halves called hemispheres. At the bottom of each cerebellar Hemisphere is the tonsil (these Tonsils are different from those located in the throat).

There are four cavities filled with spinal fluid within the brain called ventricles. Three of these (two lateral ventricles, and the third Ventricle) are located within the cerebral hemispheres while the fourth ventricle is located in the posterior fossa between the brainstem and cerebellum.

Spinal fluid is created by tufts of vascular tissue in each ventricle. With every heartbeat, blood passing in through the tufts (called choroid plexus) is filtered to create a clear colorless fluid that looks like water called cerebral spinal fluid (CSF). The spinal fluid flows down through the ventricles, exits the through the three openings of the fourth ventricle, and flows around the brain and spinal cord.

Side view of the brain showing the cerebral ventricles.

The spinal fluid helps cushion the brain from injury. The space around the surface of the brain and within the spinal canal that contains spinal fluid is partially divided into spinal fluid pockets called cisterns. An important cistern, the cisterna magna, is located at the bottom of the skull. The cisterna magna is important because most of the spinal fluid flowing from the ventricles drains into it on its way to the spinal canal and over the surface of the brain. Crowding of the cisterna magna, such as in the Chiari I malformation, can block the normal flow of spinal fluid. 

Side view of the foramen magnum area. Adapted from BNI Quarterly.

The figure above shows a side view of a normal posterior fossa. The tent like structure above the cerebellum is the tentorium. The clivus (A) and the supraocciput (B) are bones of the skull. If we draw a line starting at the lower end of the clivus to the lower end of the supraoccitput bone (dots on the figure), we are outlining the large opening at the bottom of the skull called the foramen magnum.

We can also see that the brain stem becomes the spinal cord at about the level of the foramen magnum. Within the spinal cord run nerve fibers that control movement and carry sensation from the arms, trunk and legs. The upper bones (vertebrae) of the spine can also be seen. The upper most is the Cervical one vertebrae (C1) and the second one is cervical two (C2).

Back view of the foramen magnum area. Adapted from BNI Quarterly.

The figure above shows the area of the foramen magnum as seen from behind. The cerebellar tonsils at the lower part of each cerebellar hemispheres can be seen. Also visible is the spinal cord. The large arteries seen running through the bones of the spine, called the vertebral arteries, deliver blood to the brain. An important branch of each vertebral artery can be seen as small loops just below each tonsil. These are called the posterior inferior cerebellar arteries (PICA).