This is a complicated structure of bony passages curled on themselves, roughly as in a snail shell, and lined with a delicate membrane. This membrane is, so to speak, floating in fluid. The layer of fluid between it and the bone is called the perilymph, while the two layers of the membrane enclose a similar fluid termed the endolymph. The internal ear or membranous labyrinth may be divided roughly into three chief parts: (1) the cochlea, the true organ of hearing; (2) the semi-circular canals, which control the act of balancing; and (3) the vestibule, or introductory chamber to the semi-circular canals.
The cochlea is a collection of three tubes curled up on themselves in snail-shell fashion.
The central canal of these three is the connecting link by which the sound waves, passed along over the three tiny bones—the malleus, incus, and stapes—finally reach the endings of the main nerve of hearing, the auditory nerve. (See [Plate].)
THE EYE AND ITS WONDERFUL STRUCTURE
The human eye is a hollow globe containing fluids and the crystalline lens. Surrounded by its muscles it lies embedded in a cushion of fat in a conical bony hollow called the orbit. Through an opening in the bones making up the back of the orbit, the optic nerve leads from the back of the eye to the brain.
THE EYELIDS AND
EYE-LASHES
The eyelids are made of layers of muscle and cartilage with an outer surface of skin and an inner surface which is a continuation of the conjunctiva that covers the eyeball. In the edge of the eyelid a series of tiny glands are embedded. The mouths of these open on the margin of the lids. The eye-lashes, whose duty it is to act as a screen, preventing foreign bodies such as dust or other air-born objects getting into the eye, are also inserted in the edge of the lid.