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The centers of the lower thoracic cord lie in the gray matter of the seventh, eighth, ninth, tenth, eleventh and twelfth thoracic and the first lumbar segments. In part the cells of the uppermost of these centers extend into the sixth and fifth thoracic segments, and in part the cells of the lowermost of them may extend below the level of the second lumbar segment. The gray matter of the lower thoracic spinal cord is rather small in extent. The posterior horns are long and rather slender. The lateral horns are well marked. The dorsal nucleus (Clarke’s column) is a conspicuous cell mass in the root of the posterior horn. The anterior horn is small. The anterior horn includes the cell columns of the mesial groups. Their axons leave the cord at its anterior roots, and pass without relay to the following muscles: Longissimus dorsi, multifidus spinae, accessories, latissimus dorsi, quadratus lumborum, pyramidalis, cremaster, psoas major and minor, the corresponding intercoastal and interchondral muscles, and the abdominal muscles. The spinal centers controlling the activities and the nutrition of these muscles are influenced by impulses from the following sources: I. Somatic and visceral sensory impulses affect the tone of the muscles through reflex action. The contraction of the abdominal muscles in the presence of inflammation involving the abdominal viscera is a familiar example of this reaction. Less well recognized is the contraction of the deeper spinal muscles under the same conditions. II. Descending impulses from the medullary centers affect these muscles. This reaction is noticed in the respiratory movements, and in the movements of vomiting, etc. III. Descending impulses from the cerebellum, vestibular nuclei, pontine centers, olive, etc., assist in maintaining the tone of these muscles, and in coordinating their action. This seems to have special reference to the needs of the body in maintaining the equilibrium of the erect position of the body. IV. Descending impulses from the precentral cerebral convolutions bring these muscles under a certain amount of volitional control. This control is not nearly absolulte. V. Descending impulses from the red nucleus are carried by way of the rubro-spinal tract. In this way the movements of the lower thoracic groups of somatic muscles are brought under the control of the instinctive and emotional states. The abdominal muscles are peculiarly subject to a loss of their normal tone. This lack of tone may be due to a number of causes already recognized, such as excessive fat, pregnancy, tumors which greatly increase the size of the abdomen, etc. |