Sensitivity to changes occurring inside the body through receptors located in the internal organs.A great deal of activity is constantly taking place in the interior of our bodies, but relatively little of it comes to our attention. We occasionally notice the beating of our hearts or the churning of our intestines, but few people become aware of the movements that occur in their liver and spleen. It usually takes an illness or upset to make us realize that we have these organs at all, and even then we may not be able to localize the sensations accurately.Relatively little is known about the organic senses, since the receptors are virtually inaccessible to experimentation. Anatomical studies indicate that the number of sensory fibers embedded in some of our visceral organs is only about one tenth the number of motor fibers leading to them. There are some pressure and pain endings in the internal organs, but they are unevenly distributed. The liver, spleen, kidneys, and intestines can be squeezed, cut, or cauterized under local anesthetic. Yet these organs can at times give rise to the most intense pain. Intestinal colic is extremely distressing to babies, and there are few pains more excruciating than those caused by gallstones and kidney stones.The most extensively studied organ is the stomach. It is sensitive to warm and cold substances, alcohol and other chemicals, and spices such as mustard and peppermint. When it is empty, its walls rub together and we experience the pangs associated with hunger. After a particularly heavy meal we may experience a feeling of pressure due to distention. But we are hardly ever aware of this organ in its ordinary state.Many specialists believe the sensations we receive from our visceral organs are almost always a combination of pain, pressure, and other senses, such as hot and cold. The reason is that these sensations usually involve gross areas of the body containing many kinds of receptors. A typical example is nausea, a complex reaction made up of general discomfort in the stomach region, muscular weakness, and various circulatory disturbances including sweating, paling, “goose flesh,” and chills.Organic sensitivity has been found to decrease with age. This accounts for the fact that coronary thrombosis, appendicitis, and other painful disorders of the internal organs may be experienced by older people without acute discomfort.