A Civic Biology, Presented in Problems

by George W. Hunter

Middle Ear.—The middle ear in man is a cavity inclosed by the temporal bone, and separated from the outer ear by the tympanic membrane. A little tube called the Eustachian tube connects the inner ear with the mouth cavity. By allowing air to enter from the mouth, the air pressure is equalized on the ear drum. For this reason, we open the mouth at the time of a heavy concussion and thus prevent the rupture of the delicate tympanic membrane. Placed directly against the tympanic membrane and connecting it with the inner ear is a chain of three tiny bones, the smallest bones of the body. The outermost is called the hammer; the next the anvil; the third the stirrup. All three bones are so called from their resemblances in shape to the articles for which they are named. These bones are held in place by very small muscles which are delicately adjusted so as to tighten or relax the membranes guarding the middle and inner ear.

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.

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.

The wall of the eyeball is made up of three coats. An outer tough white coat, of connective tissue, is called the sclerotic coat. Under the sclerotic coat, in front, the eye bulges outward a little. Here the outer coat is continuous with a transparent tough layer called the cornea. A second coat, the choroid, is supplied with blood vessels and cells which bear pigments. It is a part of this coat which we see through the cornea as the colored part of the eye (the iris). In the center of the iris is a small circular hole (the pupil). The iris is under the control of muscles, and may be adjusted to varying amounts of light, the hole becoming larger in dim light, and smaller in bright light. The inmost layer of the eye is called the retina. This is, perhaps, the most delicate layer in the entire body. Despite the fact that the retina is less than 1/80 of an inch in thickness, there are several layers of cells in its composition. The optic nerve enters the eye from behind and spreads out to form the surface of the retina. Its finest fibers are ultimately connected with numerous elongated cells which are stimulated by light. The retina is dark purple in color, this color being caused by a layer of cells next to the choroid coat. This accounts for the black appearance of the pupil of the eye, when we look through the pupil into the darkened space within the eyeball. The retina acts as the sensitized plate in the camera, for on it are received the impressions which are transformed and sent to the brain as sensations of sight. The eye, like the camera, has a lens. This lens is formed of transparent, elastic material. It is found directly behind the iris and is attached to the choroid coat by means of delicate ligaments. In front of the lens is a small cavity filled with a watery fluid, the aqueous humor, while behind it is the main cavity of the eye, filled with a transparent, almost jellylike, vitreous humor. The lens itself is elastic. This circumstance permits of a change of form and, in consequence, a change of focus upon the retina of the lens. By means of this change in form, or accommodation, we are able to distinguish between near and distant objects.