Internal Sense Organs
Most of the sense organs thus far considered belong to the category of the exteroceptors, that is, sense organs adequate to receive stimuli arising outside of the organism. There are, however, sensations resulting in part at least from stimuli originating within the animal body, whose receptors are little known, but which nevertheless are functionally active. No doubt many of the generalized sensations, such as fatigue, nausea, “feeling fit” or “run down,” lack specific sense organs, being the result of the general tone of several organs working together. The problematical receptors dealing with internal stimuli have been divided into proprioceptors, “located in the deeper regions of the body, stimulated only indirectly by environmental forces, but excited by processes within the organism itself” (Mitchell), and interoceptors, “located in the linings of the digestive system, and stimulated by conditions in this system.”
These sense organs are closely related to pressure of some sort, acting (1) through muscles, tendons, and joints, as a “muscular sense,” by means of which an idea of the weight of objects is gained, and the relative position of different parts of the body is realized; (2) through internal nocireceptors, that give rise to warning rheumatic or visceral pains; or (3) through the semicircular canal apparatus, already described in connection with the ear, to secure equilibration and orientation in space.
The weight of any object is estimated by lifting, or “hefting” it, that is, testing by muscular resistance the pull of gravity which it exerts. For example it takes twice as much muscular effort to keep a ten-pound ball from falling when it has been lifted into the air as is necessary to sustain a five-pound ball in the same way. The difference in weight of the two balls is less accurately determined when they are simply placed in succession upon the hand while it rests upon a solid support without muscular exertion.
The kinaesthetic sense organs involved in the process of ascertaining the weight of objects are located not only in the muscles, but also in the tendons and joints. They are quite distinct from cutaneous sense organs of pressure that obviously supplement them in giving information about the character of ponderous objects.
Appetite, hunger, and thirst are three distinct sensations associated with the digestive apparatus, for which adequate sense organs have not yet been discovered.
Appetite is a pleasant sensation, not to be confused with hunger, which is an unpleasant sensation. It has been described as the “memory of food enjoyment,” brought about by internal changes in conjunction with such external stimuli as the sight, odor, or taste of food.
Hunger, which is sometimes erroneously referred to muscular contraction of the walls of an empty stomach, is the result of nutritional poverty in the blood, while the sensation of thirst, referred to the mucous lining of the throat, is due to an increase in the salt content of the blood, that impels the animal to replace if possible loss of water from the body. If hunger and thirst were pleasant sensations we would not be so concerned to banish them by food and drink.