and the diameter can be manipulated by three different muscle types: circular, radial, and transverse. Circular musculature wraps around the circumference of the cylinder, radial musculature extends from the center of the cylinder towards the surface, and transverse musculature arrange in parallel and perpendicular sheets crossing the diameter of the cylinder.

Within the cylinder lies fluid, most often water. The fluid is resistant to changes in volume. Contraction of circular, radial or transverse muscles increases the pressure within