What substance is found within each semicircular canal?

The substance found within each semicircular canal is endolymph fluid. This specialized fluid is important for the function of the vestibular system, which is responsible for helping the body maintain balance and spatial orientation. The semicircular canals are part of the inner ear and play a crucial role in detecting rotational movements. Endolymph is crucial in this process because its movement within the canals creates a sensory stimulus that the hair cells within these structures detect. This detection allows the body to understand its position and motion in three-dimensional space, contributing to equilibrium and balance.

The other options do not pertain to the function of the semicircular canals. Perilymph fluid is found in the spaces surrounding the semicircular canals rather than inside them. Aqueous humor is related to the eye and helps to maintain intraocular pressure, while a semi-solid gel does not accurately describe the fluid dynamics necessary for the operation of the semicircular canals. Thus, the presence of endolymph fluid is vital for the proper functioning of the body's balance system.