Human Anatomy - Zinoviy Mirchuk, Maryna Mirchuk 2007

Muscles of the Body Parts
Muscles of the Trunk

The chest Muscles are arranged in several layers. One group of muscles originates on The thoracic cage, extends toward the Pectoral Girdle and the free upper limb to set them in motion, while the second group comprises the intrinsic chest muscles, which are part of the Respiratory Muscles. The pectoralis major Muscle is located on the anterior wall of the chest; when contracted, it pulls the arm downward and adducts it toward the trunk. The serratus anterior muscle lies laterally against the chest. Upon contraction, it pulls the scapula forward and laterally away from the median plane while also rotating it. The intercostal muscles—the intrinsic Muscles of the chest—lie within the intercostal spaces. The external intercostal muscles elevate the Ribs, thereby increasing the volume of the thoracic cavity, whereas the internal intercostal muscles depress the ribs, decreasing its volume.

The Abdominal muscles are situated between the lower margin of the thoracic cage and a line level with the inguinal folds, extending from the anterior superior iliac spine to the pubic tubercle. They form the muscular foundation of the abdominal wall. The lateral walls of the abdomen are formed by the external and internal oblique muscles and the transversus abdominis muscle. The external oblique muscle originates from the 5th–12th ribs, courses downward, and transitions into a broad tendon known as an aponeurosis. The lower, thickened edge of the aponeurosis is folded inward and secured to form a trough between the anterior superior iliac spine and the pubic tubercle, thus creating the inguinal (Poupart's) ligament. The internal oblique muscle lies deep to the external oblique, and the transversus abdominis muscle lies deeper still.

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Fig. 35 Muscles of the Trunk: 1 - trapezius muscle; 2 - deltoid muscle; 3 - pectoralis major muscle; 4 - serratus anterior muscle; 5 - latissimus dorsi muscle; 6 - intercostal muscles; 7 - rectus abdominis muscle; 8 - external oblique muscle of the abdomen; 9 - pyramidalis muscle.

Lateral to the anterior median line, enclosed within the tendinous sheaths of the oblique and transverse abdominal muscles, lies the rectus abdominis muscle. It originates from the pubic crest and Pubic Symphysis, extends upward, and inserts into the xiphoid process of the Sternum and the external surface of the cartilages of the 7th, 6th, and 5th ribs. The muscle bundles are interrupted by 3 to 4 tendinous intersections that are firmly fused with the anterior lamina of the rectus sheath. When contracted, the rectus abdominis muscles flex the thoracic cage and bend the spine, and—with a fixed Thorax—they elevate the pelvis. The right and left muscles of the anterolateral abdominal wall are united by tendinous fibers to form the linea alba (white line of the abdomen), a fibrous band stretching from the xiphoid process to the pubic symphysis. Approximately at the midpoint of this line lies the umbilicus—a scar formed after the healing of the umbilical cord, which was severed in the newborn.

Running parallel and superior to the inguinal ligament on the anterior abdominal wall is a slit-like passage known as the Inguinal Canal (40–50 mm in length), through which the Spermatic Cord passes in males and the round ligament of the Uterus in females. The inguinal canal has four walls: the anterior wall is formed by the aponeurosis of the external oblique muscle; the posterior wall, by the transversalis fascia; the inferior wall, by the gutter of the inguinal ligament; and the superior wall, by the lower margins of the internal oblique and transversus abdominis muscles. Superior to the pubic tubercle lies the superficial inguinal ring, which has an oval shape. Approximately at the midpoint of the inguinal ligament, on the posterior surface of the abdominal wall, is the deep inguinal ring, which appears as a funnel-shaped depression in the transversalis fascia.

The abdominal muscles also include the quadratus lumborum muscle, which participates in forming the posterior wall of the abdomen. It originates from the iliac crest and the transverse processes of the lower lumbar vertebrae and inserts into the lower border of the 12th rib, the body of the 12th thoracic vertebra, and the transverse processes of the upper four lumbar vertebrae.

Fig. 36. Surface Anatomy of the trunk muscles

Fig. 37. Muscles of the back

1 - trapezius muscle; 2 - deltoid muscle;

3 - teres major muscle; 4 - teres minor muscle;

5 - infraspinatus muscle; 6 - rhomboid muscle;

7 - latissimus dorsi muscle; 8 - serratus anterior muscles;

9 - latissimus dorsi muscle; 10 - thoracolumbar fascia;

11 - external oblique muscle of the abdomen; 12 - iliac crest; 13 - gluteus maximus muscle

The muscles of the back (Fig. 37) are divided into superficial and deep groups. The superficial muscles include: the trapezius muscle, the latissimus dorsi muscle, the levator scapulae muscle, and the Major and minor rhomboid muscles. Located in the upper back and posterior neck region, lying directly beneath the Skin, is the flat trapezius muscle, whose broad base faces THE Vertebral Column. Upon contraction, the trapezius muscle moves the scapula, draws it toward the spine, and rotates it around the sagittal axis. When the pectoral girdle is fixed, the contracted trapezius muscles pull the HEAD backward, whereas unilateral contraction tilts the head to the corresponding side while turning the face in the opposite direction.

Situated in the lower back is the latissimus dorsi muscle—the largest muscle in The Human Body. It adducts the arm to the trunk and rotates it medially, lowers an elevated arm, reaches it behind the back with the palm facing outward, extends the shoulder, and assists in shifting the lower ribs upward during Respiration.

The deep back muscles are represented by the splenius capitis and cervicis muscles, the erector spinae muscle, and the interspinales and intertransversarii muscles. The largest and strongest deep muscle of the back is the erector spinae. It consists of the iliocostalis, longissimus, and spinalis muscles. These muscles extend the spine and its individual regions, as well as laterally bend it.

Fig. 38. Surface anatomy of the back muscles