188 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. The diaphragm is a dome-shaped sheet of muscle and fibrous tissue that separates the thoracic and abdominal cavities. Contraction of the diaphragm enlarges the thoracic cavity, drawing air into the lungs. Posterior Trunk Muscles The superficial, paired trapezius muscles cover a large, kite-shaped expanse of the upper back and neck. The origin of each muscle runs from the occipital bone of the skull, down the nuchal ligament, and attaches to the spinous processes of vertebrae C7–T3. The inser- tion is on the clavicle and scapular spine. Actions of the trapezius muscles include extension and hyper- extension of the head, as well as elevation, depression, and adduction of the scapula. The erector spinae group includes three columns of muscle running most of the length of the spine: the iliocostalis, longissimus, and spinalis. The iliocostalis originates on the ilium and the ribs, and it connects to the adjacent ribs and transverse processes. Longissimus connects the transverse processes of adjacent vertebrae. As the name suggests, spinalis connects the adjacent spinous processes of the vertebrae. Functionally, these muscles serve as the primary extensors of the trunk. They also produce lateral flexion to the same side when contracting unilaterally. The quadratus lumborum also acts as an extensor of the trunk. This muscle originates on the iliac crest and inserts on the upper lumbar vertebrae. These bilateral muscles function like the erector spinae group. SELF CHECK 1. Why does training for gymnasts differ from training for football and other contact sports? 2. What is the purpose of the rectus sheath? 3. Explain the function of the intercostal muscles. Muscles of the Upper Limb The joints of the upper limb include those of the shoulder, elbow, wrist, and fingers. This section includes information about the major muscles that cross the shoulder and elbow joints. Figure 6.19 shows the major muscles of the upper limb, and the table in Figure 6.20 summarizes their locations and functions. which are visible in individuals with well-developed abdominal muscles (Figure 6.18). The rectus abdominis connects the pubic crest to the sternum and ribs 5 through 7. Contraction of both muscles produces flexion of the trunk, and unilateral contraction assists with lateral flexion of the trunk. The paired external oblique muscles are also super- ficial in the abdominal region. Running from rib 512 to the anterior iliac crest and pubis, these muscles form the lateral walls of the abdomen. Working together, the external obliques contribute to trunk flexion. Individually, they produce lateral flexion of the trunk and rotation to the opposite side. Lying deep to the external obliques are the internal obliques. Connecting the iliac crest to ribs 10–12, these muscles are positioned at right angles to the external obliques. Functionally, these muscles produce the same movement as the external obliques. The external and internal intercostal muscles are located between the ribs. Contraction of the external intercostals pulls the ribs outward, enlarging the thoracic cavity for inhalation. Contraction of the internal intercostals pulls the ribs inward, compressing the lungs for forced exhalation. Aleksandr Petrunovskyi/Shutterstock.com Figure 6.18 The paired rectus abdominis muscles are divided into eight muscle bellies by the linea alba, running longitudinally, and bands of connective tissue running laterally.