The semicircular canals are generally described as a set of mutually perpendicular membranous rings that meet in a dilated expansion called the utricle. The anterior and posterior canals are approximately vertical with each canal at an approximately 45° angle to the midsagittal plane and the lateral canal approximately horizontal, which is why it is also called the horizontal canal. The planes of the canals form an array similar to the corner of a box that is rotated 45° out of the sagittal plane.
The vestibular part of the membranous labyrinth is composed of the utricle and saccule and three roughly mutually perpendicular semicircular canals. The semicircular canals arise from and end in the utricle. One end of each canal has a dilation called an ampulla, which contains the sensory epithelia for rotational acceleration. The utricle and saccule each contain a sensory epithelium for linear acceleration.
The semicircular canals lie in a similarly shaped bony chamber called the bony labyrinth, which also has a cochlea and three semicircular canals. The canals, along with the utricle and saccule and the scala media of the cochlea and the endolymphatic duct form a complex membranous structure called the membranous labyrinth. The scala media is attached to the walls of the bony cochlea and the membranous semicircular canals lie within the bony semicircular canals. The utricle and saccule lie in the vestibule of the bony labyrinth.
The membranous labyrinth is filled with an ionic solution called endolymph and surrounded by another ionic fluid called perilymph. The different ionic compositions of the two fluids are important for the correct operation of the hair cells. Both fluids are approximately the same density and viscosity as water.
The vestibular parts of the membranous labyrinth, the saccule and utricle and the three semicircular canals are suspended in the bony labyrinth by thin connective tissue trabeculae, so that the membranous labyrinth is floating in the bony labyrinth, but retrained from large movements within the bony chamber. The arrangement is like a water-filled balloon floating in a substantially larger pool of water, held in place by many thin elastic bands.
The cross-sectional radius of the membranous semicircular canal is about one-third that of the bony labyrinth and it is modestly flattened so that the canal has an elliptical cross-section. The elliptical cross-section means that the canal can expand in volume readily, simply by assuming a more circular cross-section. A circle has a greater area than an ellipse with the same circumference.
At one end of each canal, just before it meets the utricle, the membranous semicircular canal dilates into a bulbous ending called the ampulla of the canal. The ampulla has about three times the radial cross-section of the remainder of the canal. The ampulla has a transverse ridge, called the crista ampullae, that contains sensory hair cells oriented so as to respond head movements in the plane of the canal. Surmounting the crista ampullae and separating the ampulla into two spaces is a gelatinous membrane called the cupula.