The Inner Ear.--The inner ear is one of the most complicated, as well as one of the most delicate, organs of the body. Deep within the temporal bone there are found two parts, one of which is called, collectively, the _semicircular canal region_, the other the _cochlea_, or organ of hearing. It has been discovered by experimenting with fish, in which the semicircular canal region forms the chief part of the ear, that this region has to do with the equilibrium or balancing of the body. We gain in part our knowledge of our position and movements in space by means of the _semicircular canals_. That part of the ear which receives sound waves is known as the _cochlea_, or snail shell, because of its shape. This very complicated organ is lined with sensory cells provided with cilia. The cavity of the cochlea is filled with a fluid. It is believed that somewhat as a stone thrown into water causes ripples to emanate from the spot where it strikes, so sound waves are transmitted by means of the fluid filling the cavity to the sensory cells of the cochlea (collectively known as the _organ of Corti_) and thence to the brain by means of the auditory nerve. The Character of Sound.--When vibrations which are received by the ear follow each other at regular intervals, the sound is said to be musical. If the vibrations come irregularly, we call the sound a noise. If the vibrations come slowly, the pitch of the sound is low; if they come rapidly, the pitch is high. The ear is able to perceive as low as thirty vibrations per second and as high as almost thirty thousand. The ear can be trained to recognize sounds which are unnoticed in untrained ears. [Illustration: Longitudinal section through the eye.] The Eye.--The eye or organ of vision is an almost spherical body which fits into a socket of bone, the _orbit_. A stalklike structure, the _optic nerve_, connects the eye with the brain. Free movement is obtained by means of six little muscles which are attached to the outer coat, the _eyeball_, and to the bony socket around the eye.