2. They supply the basis for the higher mental activities, as perception, memory, thought, imagination, and emotion.
Intelligent action is impossible without a knowledge both of the bodily organs and of the body's surroundings. Protection and the regulation of the work of an organ necessitate a knowledge of its condition, while the adapting and adjusting of the body to its surroundings require a knowledge of what those surroundings are. The dependence of all the higher forms of mental activity upon sensations is recognized by psychologists and is easily[pg 341] demonstrated by a study of the manner in which we acquire knowledge. "Without sensation there can be no thought."
Steps in the Production of Sensations.—The steps in the production of sensations are not essentially different from those in the production of reflex action. First of all, external stimuli act upon the fiber terminations in the sense organs, or elsewhere, starting impulses in the neurons. These pass into the central nervous system and there excite neurons which in turn discharge impulses into the cerebrum. The result is to arouse an activity of the mind—a sensation. The steps in the production of any special sensation naturally involve the following parts:
1. A sense organ where the terminations of the neurons are acted upon by the stimulus.
2. A chain of neurons which connect the sense organ with the brain.
3. The part of the cerebrum which produces the sensation.
Sense Organs.—The sense organs are not parts of the afferent neurons, but are structures of various kinds, in which the neurons terminate. Their function is to enable the sensation stimuli to start the impulses. By directing, concentrating, or controlling the stimuli, the sense organs enable them to act to the best advantage upon the neurons. When it is recognized that such widely different forces as light waves, sound waves, heat, pressure, and odors are enabled by them to stimulate neurons, the importance of these organs becomes apparent. As would naturally be inferred, the construction of any sense organ has particular reference to the nature of the stimulus which it is to receive. This is most apparent in the sense organs of sight and hearing.
[pg 342]Simple Forms of Sense Organs.—The simplest form of a sense organ (if such it may be called) is one found among the various tissues. It consists of the terminal branches of nerve fibers which spread over a small area of cells, as a network or plexus. Such endings are numerous in the skin and muscles.
Next in order of complexity are the so-called end-bulbs. These consist of rounded, or elongated, connective tissue capsules, within which the nerve fibers terminate. On the inside the fibers lose their sheaths and divide into branches, which wind through the capsule. End-bulbs are abundant in the lining membrane of the eye, and are found also in the skin of the lips and in the tissues around the joints.
Slightly more complex than the end-bulbs are the touch corpuscles. These are elongated bulb-like bodies, having a length of about one three-hundredth of an inch, and occupying the papillæ of the skin (Fig. 144). They are composed mainly of connective tissue. Each corpuscle receives the termination of one or more nerve fibers. These, on entering, lose the medullary sheath and separate into a number of branches that penetrate the corpuscle in different directions.