The subungual herniation of the cerebellar tonsils and the spinal cord cavitation are strictly speaking two diseases, the first is called subungual herniation of the cerebellar tonsils, and the other is called spinal cord cavitation, which should be described from the anatomical aspect. In general, most of these diseases are caused by a certain deformity of the foramen magnum, which is theoretically a circular thing with a certain volume inside, and when the foramen magnum is deformed, it allows the cerebellar tonsils above it to go down, destroying the normal structure, thus causing a subhypophyseal herniation deformity. The normal spinal cord has a central canal, which is thin but open. When the cerebellar tonsils herniate, it will compress the spinal cord in the foramen magnum, thus causing an imbalance of pressure here, and this imbalance of pressure will prompt the spinal cord to gradually expand and form a barrel-like thing, which is called a cavity on CT or MRI. This is the case of subungual herniation of the cerebellar tonsils, which eventually causes spinal cord cavitation.