Chiari malformations are a group of conditions where the cerebellum of the brain extends down into the spinal canal and results in the blockage of the normal flow of cerebrospinal fluid.
The cerebellum, or "little brain", is divided into two hemispheres which are separated by a narrow worm-like structure in the middle called the vermis.
Directly beneath the hemispheres are two smaller lobes called cerebellar tonsils.
Normally, the cerebellum and cerebellar tonsils sit right above the foramen magnum which is the opening at the base of the skull through which the spinal cord passes through.
The cerebellum is contained within the posterior fossa or floor of the back of the skull, which also contains the medulla oblongata.
The cerebellum controls balance, posture, and helps to initiate as well as fine-tune voluntary motor activity - think about the fancy finger work of a piano player or the vocal cords of a singer - that’s the cerebellum in action.
The medulla, on the other hand, controls autonomic functions such as breathing and blood pressure, as well as reflexes such as coughing, vomiting, gagging, and swallowing.
Now, there’s also a set of four interconnected cavities in the brain called ventricles, each of which create and circulate cerebrospinal fluid.
Highest up, are two C-shaped lateral ventricles that lie deep in each cerebral hemisphere.
The two lateral ventricles drain their cerebrospinal fluid into the third ventricle, which is a narrow, funnel-shaped, cavity at the center of the brain.
The third ventricle makes a bit more cerebrospinal fluid and then sends all of the cerebrospinal fluid to the fourth ventricle via the cerebral aqueduct.