The cerebellar tonsil is analogous to a rounded lobule on the undersurface of each cerebellar hemisphere, continuous medially with the uvula of the Cerebellar vermis

Cerebellar vermis

The cerebellar vermis is located in the medial, cortico-nuclear zone of the cerebellum, residing in the posterior fossa of the cranium. The primary fissure in the vermis curves ventrolaterally to the superior surface of the cerebellum, dividing it into anterior and posterior lobes....

 and superiorly by the flocculonodular lobe

Flocculonodular lobe

The flocculonodular lobe is a lobe of the cerebellum consisting of the nodule and the flocculus.It is closely associated with the vestibulocerebellum. This lobe is involved in the maintenance of equilibrium.-External links:...

 (source: Biology Online) Synonyms include: tonsilla cerebelli, amygdala cerebelli, the latter of which is not to be confused with the cerebral tonsils or amydgala nuclei located deep within the medial tempeoral lobes of the cerebral cortex. The flocculonodular lobe of the cerebellum which can also be confused for the cerebellar tonsils, is one of three lobes that make up the overall composition of the cerebellum. The cerebellum consists of three anatomical and functional lobes: anterior lobe, posterior lobe, and flocculonodular lobe.

Elongation of the cerebellar tonsils can, due to pressure, lead to this portion of the cerebellum to slip or be pushed through the foramen magnum of the skull resulting in a condition known as chiari malformation (CM).

Pathology

A Type I Arnold-Chiari malformation

Arnold-Chiari malformation

Arnold–Chiari malformation, or often simply Chiari malformation, is a malformation of the brain. It consists of a downward displacement of the cerebellar tonsils through the foramen magnum , sometimes causing non-communicating hydrocephalus as a result of obstruction of cerebrospinal fluid outflow...

 is a congenital anomaly of 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,...

 in which the cerebellar tonsils are elongated and pushed down through the opening of the base of the skull

Human skull

The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...

 (see foramen magnum), blocking the flow of cerebrospinal fluid

Cerebrospinal fluid

Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...

(CSF). Also called tonsillar herniation. Although congenital, CM can also be induced due to physical head trauma.