Sternum

The sternum, or breastbone, is a flat bone with three parts: manubrium, body, and xiphoid process. The manubrium is the widest and thickest bone of the sternum. Its upper border is scalloped by a central jugular notch, which can be palpated, and by two clavicular notches that house the clavicles. Its lower border articulates with the upper border of the body at a slight angle, the sternal angle or angle of Louis. This angle, which is easily palpated, is a landmark located between thoracic vertebrae T4 and T5, and is on a level with the second costal cartilages. The bifurcation of the trachea into the right and left main stem bronchi also occurs at the sternal angle. The manubrium and body are joined by fibrocartilage, which may ossify in later life.

The body of the sternum is twice as long as the manubrium. It is a relatively thin bone and can be easily pierced by needles for bone marrow aspirations. The heart is located beneath and to the left of the lower third of the body of the sternum. Although it is attached by cartilage to the ribs, this portion of the sternum is flexible and can be depressed without breaking. This maneuver is used, with care, in closed cardiac massage to artificially circulate blood to the brain and extremities. The lower margin of the body is attached to the xiphoid process by fibrocartilage. This bone is the smallest of the three parts of the sternum and usually fuses with the body of the sternum in later life.

Ribs

A large portion of the bony thoracic cage is formed by 12 ribs located on either side of the sternum. The first seven ribs connect posteriorly with the vertebral column and anteriorly through costal cartilages with the sternum. These are known as the true ribs. The remaining five ribs are known as the false ribs. The first three have their cartilage attached to the cartilage of the rib above. The last two are free or floating ribs. The ribs increase in length from the first to the seventh rib and then decrease to the 12th rib. They also increase in obliquity until the ninth rib and then decrease in obliquity to the 12th rib.

Each rib has a small head and a short neck that articulate with two thoracic vertebrae. The shaft of the rib curves gently from the neck to a sudden sharp bend, the angle of the rib. Fractures often occur at this site. A costal groove is located on the lower border of the shaft of the ribs. This groove houses the intercostal nerves and vessels. Chest tubes and needles are inserted above the ribs to avoid these vessels and nerves. The ribs are separated from each other by the intercostal spaces that contain the intercostal muscles.

Diaphragm

The diaphragm is the principal muscle of respiration. During quiet breathing, the diaphragm contributes approximately two-thirds of the tidal volume in the sitting or standing positions and approximately three-fourths of the tidal volume in the supine position. Approximately two-thirds of the vital capacity in all positions is contributed by the diaphragm.

The diaphragm is a large, dome-shaped muscle that separates the thoracic and abdominal cavities. Its upper surface supports the pericardium (with which it is partially blended), heart, pleurae, and lungs. Its lower surface is almost completely covered by the peritoneum and overlies the liver, kidneys, suprarenal glands, stomach, and spleen (Fig. 3-3). This large muscle can be divided into right and left halves. Each half is made up of three parts: sternal, lumbar, and costal. These three parts insert into the central tendon, which lies just below the heart. The sternal part arises from the back of the xiphoid process and descends to the central tendon. On each side is a small gap, the sternocostal triangle, which is located between the sternal and costal parts. It transmits the superior epigastric vessels and is often the site of diaphragmatic hernias. The costal parts form the right and left domes. They arise from the inner surfaces of the lower four ribs and the lower six costal cartilages, then interdigitate and transverse the abdomen to insert into the anterolateral part of the central tendon, the central part of the diaphragm. The lumbar part arises from the bodies of the upper lumbar vertebrae and extends upward to the central tendon. The central tendon is a thin, strong aponeurosis situated near the center of the muscle, somewhat closer to the front of the body. It resembles a trefoil leaf, with its three divisions or leaflets. The right leaflet is the largest, the middle is the next largest, and the left leaflet is the smallest.

Major vessels traverse the diaphragm through one of three openings (see Fig. 3-3). The vena caval opening is located to the right of the midline in the central tendon and contains branches of the right phrenic nerve and the inferior vena cava. The esophageal opening is located to the left of the midline and contains the esophagus, the vagal nerve trunks, and branches of the gastric vessels. The aortic opening is located in the midline and contains the aorta, the thoracic duct, and sometimes the azygos vein. The diaphragm is also pierced by branches of the left phrenic nerve, small veins, and lymph vessels.

The position of the diaphragm and its range of movement vary with posture, the degree of distention of the stomach, size of the intestines, size of the liver, and obesity. Average movement of the diaphragm in quiet respiration is 12.5 mm on the right and 12 mm on the left. This can increase to a maximum of 30 mm on the right and 28 mm on the left during increased ventilation. An individual’s posture determines the position of the diaphragm. In the supine position, the resting level of the diaphragm rises. The greatest respiratory excursions during normal breathing occur in this position; however, the lung volumes are decreased because of the elevated position of the abdominal organs within the thoracic cavity. In a sitting or upright position, the dome of the diaphragm is pulled down by the abdominal organs, allowing a larger lung volume. For this reason, individuals who are short of breath are more comfortable sitting than reclining. In a side-lying position, the dome of the diaphragm on the lower side rises farther into the thorax than the dome on the upper side (Fig. 3-4). The abdominal organs tend to be displaced forward in a side-lying position, allowing greater excursion of the dome on the lower side. In contrast, the upper side moves little with respiration. On radiograph, the position of the diaphragm can indicate whether the film was taken during inspiration or expiration and may also indicate pathology in the lungs, pleurae, or abdomen.

Each half of the diaphragm is innervated by a separate nerve—the phrenic nerve on that side. Although the halves contract simultaneously, it is possible for half of the muscle to be paralyzed without affecting the other half. Generally, the paralyzed half remains at the normal level during rest. With deep inspiration, however, the paralyzed half is pulled up by the negative pressure in the thorax.

Contraction of the diaphragm increases the thoracic volume vertically and transversely. The central tendon is drawn down by the diaphragm as it contracts. As the dome descends, abdominal organs are pushed forward as far as the abdominal walls will allow. When the dome can descend no farther, the costal fibers of the diaphragm contract to increase the thoracic diameter of the thorax. This occurs because the fibers of the costal part of the diaphragm run vertically from their attachment at the costal margin. Thus contraction of these fibers elevates and everts the ribs (Fig. 3-5). If the diaphragm is in a low position, it will change the angle of pull of the muscle’s costal fibers. Contraction of these fibers creates a horizontal pull, which causes the lateral diameter to become smaller as the ribs are pulled in toward the central tendon.

As the diaphragm descends, it compresses the abdominal organs, increasing intraabdominal pressure. At the same time, the intrathoracic pressure decreases as the lung volume is increased by the descending diaphragm. Inspiratory airflow occurs as a result of this decrease in intrathoracic pressure (see Chapter 4). The pressure gradient between the abdominal and thoracic cavities also facilitates the return of blood to the right side of the heart.

Movement of the diaphragm can be controlled voluntarily to some extent. Vocalists spend years learning to manipulate their diaphragms so they can produce controlled sounds when singing. The diaphragm momentarily ceases movement when a person holds his or her breath. The diaphragm is involuntarily involved in parturition, bearing down in bowel movements, and laughing, crying, and vomiting. Hiccups are spasmodic, sharp contractions of the diaphragm that can indicate disease (e.g., a subphrenic abscess) if they persist.

Intercostals

The external intercostals extend down and forward from the tubercles of the ribs (above) to the costochondral junction of the ribs (below), where they become continuous with the anterior intercostal membrane (Fig. 3-6). This membrane extends the muscle forward to the sternum. There are 11 external intercostal muscles on each side of the sternum. They are thicker posteriorly than anteriorly and thicker than the internal intercostal muscles. They are innervated by the intercostal nerves, and contraction draws the lower rib up and out toward the upper rib. This action increases the volume of the thoracic cavity.

There are 11 internal intercostals on each side. These are considered primarily expiratory in function. The intercartilaginous or parasternal portion of the internal intercostals contracts with the external intercostals during inspiration to help elevate the ribs. In addition to their respiratory functions, the intercostal muscles contract to prevent the intercostal spaces from being drawn in or bulged out during respiratory activity.

Sternocleidomastoid

The sternocleidomastoid (SCM) muscles are strong neck muscles arising from two heads, one from the manubrium and one from the medial part of the clavicle (see Fig. 3-6). These two heads fuse into one muscle mass that inserts behind the ear into the mastoid process. It is innervated by the accessory nerve and the second cervical nerve. There are two of these muscles, one on each side of the neck. When one SCM contracts, it tilts the head toward the shoulder of the same side and rotates the face toward the opposite shoulder. If the two SCM muscles contract together, they pull the head forward into flexion. When the head is fixed, the muscles assist in elevating the sternum, increasing the anteroposterior diameter of the thorax.

The SCMs are the most important accessory muscles of inspiration. Their contractions can be observed in all patients during forced inspiration and in all patients who are dyspneic. These muscles become visually predominant in patients who are chronically dyspneic (see Chapter 5).

Scalenes

The anterior, medial, and posterior scalenes are three separate muscles that are considered a functional unit. They are attached superiorly to the transverse processes of the lower five cervical vertebrae and inferiorly to the upper surface of the first two ribs (see Fig. 3-6). They are innervated by their corresponding cervical spinal nerves. These muscles are primarily supportive neck muscles, but they can assist in respiration through reverse action. When their superior attachment is fixed, the scalenes act as accessory respiratory muscles and elevate the first two ribs during inspiration.

Serratus Anterior

The serratus anterior arises from the outer surfaces of the first eight or nine ribs. It curves backward, forming a sheet of muscle that inserts into the medial border of each scapula. It is innervated by the long thoracic nerve (cervical nerves C5, C6, and C7). There are two of these muscles, one on each side of the body. Normally they assist in forward pushing of the arm (as in punching). When the scapulae are fixed, the serratus anterior muscles act as accessory respiratory muscles and elevate the ribs to which they are attached.

Pectoralis Major

The pectoralis major is a large muscle arising from the clavicle, the sternum, and the cartilages of all the true ribs (see Fig. 3-6). This muscle spreads across the anterior chest and inserts into the intertubercular sulcus of the humerus. It is innervated by the lateral and medial pectoral nerves and cervical nerves C5, C6, C7, C8, and T1. There are two of these muscles, one on each side of the body. This muscle rotates the humerus medially and draws the arm across the chest. During climbing and pull-ups, it draws the arms toward the trunk. During forced inspiration when the arms are fixed, it draws the ribs toward the arms, thereby increasing thoracic diameter.

Pectoralis Minor

The pectoralis minor is a thin muscle originating from the outer surfaces of the third, fourth, and fifth ribs near their cartilages. It inserts into the coracoid process of the scapula. It is innervated by the pectoral nerves (cervical nerves C6, C7, and C8). There are two of these muscles, one on each side of the body. They contract with the serratus anterior to draw the scapulae toward the chest. During deep inspiration, they contract to elevate the ribs to which they are attached.

Trapezius

The trapezius consists of two muscles that form an extensive diamond-shaped sheet extending from the head down the back and out to both shoulders (Fig. 3-7). Its upper belly originates from the external occipital protuberance, curves around the side of the neck, and inserts into the posterior border of the clavicle. The middle part of the muscle arises from a thin diamond-shaped tendinous sheet, the supraspinous ligaments, and the spines of the upper thoracic region; it then runs horizontally and inserts into the spine of the scapula. Its lower belly arises from the supraspinous ligaments and the spines of the lower thoracic region, runs upward, and inserts into the lower border of the spine of the scapula. This large muscle is innervated by the external or spinal part of the accessory nerve and cervical nerves C3 and C4. Its main function is to rotate the scapulae during arm elevation and control gravitational descent of the arms. It also braces the scapulae and raises them, as when shrugging the shoulders. Its ability to stabilize the scapulae makes it an important accessory muscle in respiration. This stabilization enables the serratus anterior and pectoralis minor to elevate the ribs.

Erector Spinae

The erector spinae is a large muscle extending from the sacrum to the skull (see Figure 3-7). It originates from the sacrum, the iliac crest, and the spines of the lower thoracic and lumbar vertebrae. It separates into a lateral iliocostalis, an intermediate longissimus, and a medial spinalis column. This muscle mass inserts into various ribs and vertebral processes all the way up to the skull. It is innervated by the corresponding spinal nerves. These muscles extend, laterally flex, and rotate the vertebral column. They are considered accessory respiratory muscles through their extension of the vertebral column. In deep inspiration, these muscles extend the vertebral column, allowing further elevation of the ribs.

Expiration

Expiration is a passive process that occurs when the intercostals and diaphragm relax. Relaxation of the intercostals and diaphragm allows the ribs to drop to their preinspiratory position and the diaphragm to rise. These activities compress the lungs, which then raises intrathoracic pressure above atmospheric pressure and contributes to air flow out of the lungs.