THE ORGANS OF RESPONSE: MUSCLES AND GLANDS
Introduction. --In order to complete our sketch of the mechanisms involved in human response, it remains for us to study the effectors. So far we have studied receptors or sense organs and their stimuli, and the system of conductors stretching out between receptors and the effectors, or acting organs. The motor neurones of the cord and brain end directly in skeletal muscle or indirectly (through the intermediation of a sympathetic post-ganglionic neurone, page 157) in the smooth muscles of the body and the glands. Our sketch would be incomplete if we failed to get a good working notion of action in skeletal muscles, smooth muscles, and glands. In the sketch which follows we have omitted all details and have summarized only the most important features of such action, and the features with which psychology has most to do. We can study the effector system under three general divisions: I, the striped muscles; II, the smooth muscles; III, the glands.
Structure of the Striped Muscles. --The skeletal or striped muscles constitute the principal mass of the body as a whole. Each muscle is more or less of an organic whole, which can assume various shapes and sizes. The morphological unit of a muscle, however, is a muscle fiber or muscle cell. Each muscle consists of a large number of thread-like cells which usually lie parallel to the long axis of the muscle. At one or both ends, the muscle tapers down and forms a junction with a tendon. The tendons in turn are attached to the bone. The fibers of the muscles are grouped into larger and smaller bundles, each bundle being bound with connective tissue. A sheath, or perimysium, surrounds the muscle as a whole.
The individual muscle fibers vary greatly in diameter and length. They are rarely longer than 36 mm., and the diameter varies from 0.1 to 0.01 mm. The fibers are cylindrical in shape.