Fig. 144—A touch corpuscle highly magnified. (See text.)
The largest of the simple forms of sense organs are bodies visible to the naked eye and called, from their discoverer Pacini, the Pacinian corpuscles. They lie along the course of nerves in many parts of the body, and have the general form of grains of wheat. (See Practical Work.) The Pacinian corpuscles are composed of connective tissue[pg 343] arranged in separate layers around a narrow central cavity called the core (Fig. 145). Within the core is the termination of a large nerve fiber. These corpuscles are found in the connective tissue beneath the skin, along tendons, around joints, and among the organs of the abdominal cavity.
Fig. 145—Pacinian corpuscle, magnified. A. Medullated nerve fiber. B. Axis cylinder terminating in small bulb at C. D. Concentric layers of connective tissue. E. Inner bulb.
The simple forms of sense organs have a more or less general distribution over the body, and are concerned in the production of at least three special sensations. These are touch, temperature, and the muscular sensation.
Touch, or feeling, is perhaps the simplest of the sensations. The sense organs employed are the touch corpuscles, and the external stimulus is some form of pressure or impact. Pressure applied to the skin, by acting on the fiber terminations in the corpuscles, starts the impulses that give rise to the sensation. The touch corpuscles render the fiber terminations so sensitive that the slightest pressure is able to arouse sensations of touch. It is found that a change of pressure, rather than pressure that is constant, is the active stimulus. That all parts of the skin are not equally sensitive to pressure, and that the mind does not interpret equally well the sensations from different parts, are facts easily demonstrated by experiment. (See Practical Work.)
The Temperature Sensation.—Temperature sensations,[pg 344] like those of touch, are limited almost entirely to the skin. They are of two kinds, and are designated as heat sensations and as cold sensations. Whether the sense organs for temperature are different from those of touch is not known. It is known, however, that the same corpuscles do not respond alike to heat, cold, and pressure.
A Change of Temperature, rather than any specific degree of heat or cold, is the active temperature stimulus. The sensation of warmth is obtained when the temperature of the skin is being raised, and of cold when it is being lowered. This explains why in going into a hallway from a heated room one receives a sensation of cold, while in coming into the same hallway from the outside air he receives a sensation of warmth. It is for the same reason that we are able to distinguish only the relative, not the actual, temperature of bodies.
Muscular Sensations.—These are sensations produced by impulses arising at the muscles. Such impulses originate at the fiber terminations which are found in both the muscles and their tendons. By muscular sensations one is conscious of the location of a contracting muscle and of the degree of its tension. They also make it possible to judge of the weight of objects.