The Internal Ear, Or Labyrinth, consists primarily of chambers and tubes hollowed out in the temporal bone. The middle chamber, called the vestibule (V, Fig. 95), has an opening, the oval foramen, o, in its outer side, into which the inner end of the stapes, or stirrup-bone, fits. Behind, the vestibule opens into three semicircular canals, one of which is shown at B, Fig. 95; and in front into a spirally coiled tube, S, the cochlea. In these bony chambers and tubes lie membranous chambers and tubes, in which the fibres of the auditory nerve (A, Fig. 95) end. All the labyrinth chamber outside these membranous parts is occupied by a watery liquid, known as perilymph; the membranous chambers are filled with a similar liquid, the endolymph.

When sound-waves of the air make the tympanic membrane vibrate, it shakes the tympanic bones; the stapes then shakes the liquids in the labyrinth, and sets up vibrations in them, which excite the endings of the auditory nerve. The stimulated auditory nerve then conveys a nervous impulse to the brain-centre of hearing and excites it, and a sensation of sound results.

Touch, Or The Pressure Sense

Many sensory nerves end in the skin, and through it we get several kinds of sensation; touch proper, heat and cold, and pain; and we can with more or less accuracy localize them on the surface of the body. The interior of the mouth possesses also these sensibilities. Through touch proper we recognize pressure or traction exerted on the skin, and the force of the pressure; the softness or hardness, roughness or smoothness, of the body producing it; and the form of this, when not too large to be felt all over.

Of what does the internal ear primarily consist? What is the vestibule?

What is found on the outer side of the vestibule?

Into what does the vestibule open behind ? In front? What lie in the bony cavities of the labyrinth? What is the perilymph? The endolymph?

What happens when sound-waves set the tympanic membrane in vibration?

The delicacy of the tactile sense varies on different parts of the skin; it is greatest on the forehead and temples, where a weight of 3/100 of a grain can be felt.