The inner ear has two major parts concerned with orientation:  the semicircular canals and the otolith organs.  [Figure 3-3]  The  semicircular canals detect angular acceleration of the body,  while the otolith organs detect linear acceleration and gravity.  The semicircular canals consist of three tubes at approximate  right angles to each other, each located on one of three axes:  pitch, roll, or yaw as illustrated in  Figure 3-4 . Each canal is  filled with a fluid called endolymph fluid. In the center of  the canal is the cupola, a gelatinous structure that rests upon  sensory hairs located at the end of the vestibular nerves. It  is the movement of these hairs within the fluid that causes  sensations of motion. Because of the friction between the fluid and the canal, it  may take about 15–20 seconds for the fluid in the ear canal  to reach the same speed as the canal’s motion. To illustrate what happens during a turn, visualize the  aircraft in straight-and-level flight. With no acceleration of  the aircraft, the hair cells are upright, and the body senses  that no turn has occurred. Therefore, the position of the hair  cells and the actual sensation correspond. Placing the aircraft into a turn puts the semicircular canal and  its fluid into motion, with the fluid within the semicircular  canal lagging behind the accelerated canal walls.  [Figure 3-5] This lag creates a relative movement of the fluid within the  canal. The canal wall and the cupula move in the opposite  direction from the motion of the fluid. The brain interprets the movement of the hairs to be a turn in  the same direction as the canal wall. The body correctly senses  that a turn is being made. If the turn continues at a constant  rate for several seconds or longer, the motion of the fluid in  Ears Saccule  Semicircular canals Utricle  Cochlea  Tubular ducts containing endolymph Ampullae  Semicircular canals Endolymph fluid Cupola  Filaments of hair cells Hair cells Vestibular nerve Ampulla of a semicircular canal Cupola  Sensory hairs Otolith organ Vestibular nerve The motion sensing system is  located in each inner ear in the  approximate position shown.  YAW  ROLL YAW  PITCH  PITCH  ROLL Eardrum  Eustachian tube Bone  Ear canal Bone Eardrum The semicircular tubes are  arranged at approximately  right angles to each other, in  the roll, pitch, and yaw axes.  Figure 3-4.  Angular acceleration and the semicircular tubes. Figure 3-3.  Inner ear orientation. 3-4