In addition to the diaphragm and external inter-costals, other accessory muscles assist inspiration, in-cluding the sternocleidomastoid, scalenes, serratus anterior, pectoralis minor, pectoralis major, and upper trapezius. The activity of the external intercostals is responsible for bringing approximately 25% of the volume of air into the lungs. The ribs articulate with the vertebrae and sternum in a way that the antero-posterior and transverse diameters are increased when they are lifted up by this muscle. They are innervated by nerves arising from thoracic segments 1 to 12. The external intercostal muscles that originate in the upper rib, with insertion in the lower rib, with the fibers running anteriorly and inferiorly, help elevate the ribs. The diaphragm is responsible for 75% of air movement into the lungs. When the di-aphragm contracts, the central tendon is pulled down-ward into the abdominal cavity, increasing the diame-ter of the thoracic cavity in the superior-inferior direction. The di-aphragm is innervated by the phrenic nerve that arises from cervical segments 3, 4, and 5. Its origin is along the walls of the thorax and in-sertion is into a centrally placed tendon. The diaphragm is a dome-shaped, skeletal muscle, projecting into the thoracic cavity. The major muscle of in- spiration is the diaphragm. Spasmodic inspiratory movement of the diaphragm produces the characteristic sound known as hiccupping.Inspiration is an active process where the muscles of respiration (see Figure 10.10 and Table 1) contract to increase the thoracic volume. Small holes in the membranous portion of the diaphragm sometimes allow abnormal accumulations of fluid or air to move from the abdominal cavity (where pressure is positive during inspiration) into the pleural spaces of the chest (where pressure is negative during inspiration). The diaphragm is pierced by many structures, notably the esophagus, aorta, and inferior vena cava, and is occasionally subject to herniation (rupture). The diaphragm is also important in expulsive actions- e.g., coughing, sneezing, vomiting, crying, and expelling feces, urine, and, in parturition, the fetus. Relaxation of the diaphragm and the natural elasticity of lung tissue and the thoracic cage produce expiration. Contraction of the diaphragm increases the internal height of the thoracic cavity, thus lowering its internal pressure and causing inspiration of air. The muscles of the diaphragm arise from the lower part of the sternum (breastbone), the lower six ribs, and the lumbar (loin) vertebrae of the spine and are attached to a central membranous tendon. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.
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