The Human Body

Sense Organs

Sense Organs, in humans and other animals, faculties by which outside information is received for evaluation and response. This is accomplished by the effect of a particular stimulus on a specialized organ, which then transmits impulses to the brain via a nerve or nerves.

Aristotle classified five senses as follows:

1. Hearing, a sense by which sound waves are perceived by the organ of hearing—the ear—in vertebrate animals. The process of sound perception is called audition. The physical stimulus of auditory sensation is the vibration of some material object. The vibration is transmitted from the object to the ear, under ordinary conditions, by a wave movement of air particles.

2. Sight (vision), is the ability to see the features of objects we look at, such as color, shape, size, details, depth, and contrast. Vision is achieved when the eyes and brain work together to form pictures of the world around us. Vision begins with light rays bouncing off the surface of objects. These reflected light rays enter the eye and are transformed into electrical signals. Millions of signals per second leave the eye via the optic nerve and travel to the visual area of the brain. Brain cells then decode the signals into images, providing us with sight.

3. Smell, a sense by which odors are perceived. The nose, equipped with olfactory nerves, is the special organ of smell. The olfactory nerves also account for differing tastes of substances taken into the mouth, that is, most sensations that appear introspectively as tastes are really smells.

4. Taste, a sense by which four gustatory qualities (sweetness, sourness, saltiness, and bitterness) of a substance are distinguished. Taste is determined by receptors, called taste buds, the number and shape of which may vary greatly between one person and another. In general, women have more taste buds than men. A greater number of taste buds appears to endow a greater sensitivity to sweetness, sourness, saltiness, and bitterness. In humans, the taste buds are located on the surface and sides of the tongue, the roof of the mouth, and the entrance to the pharynx. The mucous membrane lining these areas is invested with tiny projections of papillae, each of which in turn is invested with 200 to 300 taste buds. The papillae located at the back of the tongue, and called circumvallate, are arranged to form a V with the angle pointing backward; they transmit the sensation of bitterness. Those at the tip of the tongue transmit sweetness, whereas saltiness and sourness are transmitted from the papillae on the sides of the tongue. Each flask-shaped taste bud contains an opening at its base through which nerve fibers enter. These fibers transmit impulses directly to the brain. In order for a substance to stimulate these impulses, however, it must be in solution, moistened by the salivary glands. Sensations of taste have been determined to be strongly interrelated with sensations of smell.

5. Touch, a sense by which the body perceives contact with substances. In humans, touch is accomplished by nerve endings in the skin that convey sensations to the brain via nerve fibers. Nerves end in or between the cells of the epidermis, the outer layer of the skin, in all parts of the body. In one complex form of nerve ending, the terminals form tiny swellings, or end bulbs; characteristic of this form are the Pacinian corpuscles found in the sensitive pad of each finger. Touch is the least specialized of the senses, but acuteness can be sharpened by use.