Muscles of the Trunk

middle scalene, and posterior scalene muscle—which flex, laterally flex, and rotate the head.

They also contribute to deep inhalation.

MUSCLES OF THE TRUNK

There are four pairs of abdominal muscles covering the anterior and lateral abdominal wall.

These include the rectus abdominis, transverse abdominis, internal obliques, and external

obliques. These are the muscles you need to know about:

• External and internal obliques—these direct twisting at the waist and side bending of

the vertebral column. They are seen in the lower ribs to the pelvis.

• Transverse abdominus—these will squeeze the abdomen and are important in

defecation and urination. It is seen from the lower ribs to the pubis.

• Rectus abdominis—these help the body to sit up. It starts in the pubis and ends in the

sternum and lower ribs.

• Quadratus lumborum—this muscle causes side bending. It is seen from the ilium up to

the ribs.

Figure 53 shows the abdominal muscles:

The linea alba is a white, fibrous band that is made from the rectus sheaths that join in the

anterior midline of the body. They enclose the rectus abdominis muscles that extend upward

from the pubis to the sternum. Three transverse bands of collagen fibers are tendinous

intersections that look like six-pack abs when the person does many sit-ups.

The muscles of the chest will facilitate breathing by changing the size of the thoracic cavity.

Inhalation causes the chest to rise and expansion of the lung cavity. Exhalation causes the chest to fall. These are the major muscles of the chest/thorax:

• Diaphragm—this is the main breathing muscle used in inhalation and exhalation. It

originates in the sternum and lower ribs, and ends in the central tendon, causing

compression and expansion of the lung cavity.

• External intercostals—these are between each rib and are secondary muscles involved

in inhalation and exhalation.

• Internal intercostals—between the ribs and are important in forced exhalation.

The diaphragm contracts and relaxes and separates the thoracic and abdominal cavities. At

rest, it is dome-shaped and convex when looking at it from above. It contracts and flattens,

increasing the negative space in the chest cavity, allowing for lung expansion. There are three openings in the diaphragm for the passage of the vena cava, esophagus, and aorta/thoracic

duct.

There are three sets of muscles, called the intercostal muscles, which exist between the ribs

and assist in the breathing process. There are eleven pairs of external intercostal muscles, used

in inspiration. There are also eleven pairs of internal intercostal muscles, used for expiration,

particularly forced expiration. The innermost intercostals are the deepest chest muscles that

act as synergists for the internal intercostal muscles.

There are many muscles of the perineum and pelvic floor. The pelvic floor is a sheet of muscles

that define the bottom of the pelvic cavity. There is a structure called the “pelvic diaphragm,”

which spans the pubis and coccyx. It is made from three muscles: the levator ani muscles

(pubococcygeus and iliococcygeus) and the ischiococcygeus muscle. These support the pelvic

organs and have openings that include the anal canal, vagina (in women), and urethra. Figure

54 illustrates the perineal musculature in females:

The perineum is diamond-shaped and overlies the pelvic diaphragm. It is divided into the

anterior triangle or “urogenital triangle” and the posterior triangle or the “anal triangle,” which

contains the anus. There are deep and superficial layers to these muscles. Women have two

extra muscles: the compressor urethrae and the sphincter urethrovaginalis, which close the

vagina. In men, there is an extra deep transverse perineal muscle that plays a role in

ejaculation.

Superficial muscles in the perineum include the superficial transverse perineal muscle (which

supports the perineal body), the bulbospongiosus (which compresses the urethra), and the

ischiocavernosus (which helps to maintain an erection in males). There are also deep sphincter

muscles: the external urethral sphincter and the external anal sphincter.