In cases of hyrocephalus (most commonly found as a birth defect), a silicone elastomer tube called a shunt is used to drain excess cerebrospinal fluid from the brain and carry it to other parts of the body. This tube goes from the affected area of the brain, connects to a one-way valve which sits outside the skull, but beneath the skin, somewhere behind the ear. It then travels down the neck, and into either the abdominal cavity (most common), the pleural cavity (surrounding the lungs) (alternative), or into the atrium of the heart (quite rare). Enough tubing is left in the area it drains to, so that it can uncoil as the child grows.