114 THE INDIVIDUAL ORGANISM 



the pulsations of the air. This membrane separates the canal of the 

 external ear from the cavity of the middle ear. The air pressure in the 

 latter is kept in equilibrium with that of the outer air by the eustachian 

 tube, which connects the middle ear with the pharynx. 



Vibrations of the tympanic membrane are transmitted across the 

 middle ear to the inner ear by three small bones — the malleus, incus, 

 and stapes. The last of these fits against a membrane-filled opening, the 

 oval window, through which it sets into vibratory motion the fluid peri- 

 lymph that surrounds and in part fills the structures of the inner ear. A 

 second membrane-filled opening, the round window, makes the motion 

 of the perilymph possible by bulging out when the stapes presses the 

 oval window in and springing back when the pressure is relaxed. 



The pulsations in the perilymph stimulate auditory receptor neurons 

 located within a coiled structure called the cochlea, from its resemblance 

 to a snail shell. Essentially the cochlea consists of three closely applied 

 tubes. The central one of these contains the elongated organ of Corti in 

 which lie the auditory receptors. It ends blindly at the tip of the cochlea 

 but at the base is narrowly connected to a fluid-filled reservoir, the 

 sacculus; the fluid endolymph which fills the sacculus and tube is entirely 

 separated from the surrounding perilymph. The two outer tubes of the 

 cochlea are really the two arms of a perilymph-filled loop that starts at 

 the oval window and ends at the round window. The organ of Corti is 

 separated from the second half of this loop only by a thin membrane. 

 Vibrations in any part of this membrane stimulate the receptors at that 

 point, and since the width of the membrane diminishes gradually from 

 the base to the tip of the cochlea, sounds of different wave lengths (pitch) 

 cause different parts of the membrane to vibrate. The nerve impulses 

 sent to the brain from receptors located along the organ of Corti thus 

 correspond to different frequencies of vibration. 



The impulses from the auditory receptors travel over the auditory 

 (8th cranial) nerves to the medulla. Thence they pass to the auditory 

 center of the midbrain and on by way of the thalamus to the cerebral 

 cortex, where they result in sound perception. 



Static sensations. The part of the ear that gives rise to static sensa- 

 tions consists of three semicircular canals in the inner ear. One is horizon- 

 tal, the other two are vertical, and each is situated in a plane approxi- 

 mately at right angles to the planes of the other two. The two vertical 

 canals stand at an angle of about 45 degrees with the median plane of the 

 body. 



The ends of all three canals open into a sac called the utricidus, which is 

 narrowly connected with the sacculus mentioned above. The endolymph, 

 that fills the middle tube of the cochlea and the sacculus, also fills the 

 utriculus and the semicircular canals. 



