3: The Thorax

imageChapter 3The Thorax

1 Skeleton and Divisions

The thorax, or chest region, is a hollow body cavity enclosed by a thoracic bony and muscular wall. Contained within the thoracic cavity are the major organs of circulation and respiration.

The thoracic cavity extends superiorly from the dome of the thoracic diaphragm to the cervical region just above the first rib. The diaphragm separates the abdominal cavity below from the thoracic cavity above.

THORACIC SKELETON

The skeleton of the thorax consists of (1) a midline sternum, (2) 12 pairs of ribs and associated costal cartilages, and (3) 12 thoracic vertebrae (Figure 3-1). The first ribs, sternum, and first thoracic vertebra comprise the thoracic inlet.

Thoracic Vertebrae

In addition to all of the features attributable to a typical vertebra described in Chapter 2, the thoracic vertebrae exhibit the following unique features: (1) the body is heart-shaped, (2) the spinous processes are long and slender, and (3) the bodies and transverse processes have facets for articulation with ribs (Figures 3-5 and 3-6).

Figure 3-7 demonstrates the articulation of a rib with a thoracic vertebra. Except for ribs 1, 10, 11, and 12, the head of each rib articulates with the body of its own vertebra and that of the vertebra above. The facets on the vertebral bodies are really demifacets, and in the articulated spine the demifacet below and the one above make a complete facet. Ribs 1, 10, 11, and 12 articulate with only their own vertebrae. The tubercles of the ribs articulate with the transverse processes of each thoracic vertebra.

2 The Thoracic or Chest Wall

SURFACE FEATURES

The Breast

The breast, or mammary gland, arises as modified sweat gland tissue within the superficial fascia of the anterior chest wall and is covered by skin (Figure 3-8). In postpubertal women the breast is a large organ capable of lactation. In men and children it is rudimentary.

In women the breast overlies the pectoralis major muscles at the level of ribs 2 to 6. An axillary tail passes laterally and superiorly to the axillary region (armpit). The functional component of the breast consists of 15 to 20 lobules of glandular tissue lying within accumulated fat of the superficial fascia. The glandular ducts empty to the surface through the nipple. Surrounding the nipple is an areola containing areolar glands and ranging from light pink to dark brown. Suspensory ligaments (of Cooper) support the breast by anchoring it to underlying deep fascia. Arterial supply comes from mammary branches of the axillary artery, the internal thoracic artery, and intercostal arteries.

Knowledge of lymphatic drainage is extremely important because malignancies of the breast may spread along lymphatic routes. Lymphatics radiate out from the nipple and communicate with lymph nodes of various regions. The breast is divided into quadrants. The two lateral quadrants drain superiorly to nodes of the axilla. The two medial quadrants drain to the axillary nodes, the anterior chest wall, and the interior abdominal wall, and they may even drain across the midline to the opposite breast.

Skeletal Landmarks

The following important landmarks in the chest are used to locate underlying structures (Figure 3-9):

The surface anatomy of the heart and lungs is covered in the descriptions of these organs in subsequent sections.

MUSCLES

Extrinsic Thoracic Muscles

The superficial muscles covering the chest wall are actually muscles belonging to other regions and are described in sections that discuss these regions. Muscles of the upper limb (see Chapter 9), which originate from the thoracic skeleton, are the pectoralis major and minor, serratus anterior, latissimus dorsi, rhomboid major and minor, levator scapu-lae, and trapezius. Muscles of the anterior abdominal wall (see Chapter 4), which attach to the thoracic skeleton, are the rectus abdominis, external oblique, internal oblique, and transversus abdominis. Posteriorly, erector spinae and muscles of the back (see Chapter 2) attach to the thoracic skeleton.

Intercostal Muscles

The intercostal muscles of the thorax are involved with the mechanics of breathing. They run from (1) rib to rib, (2) sternum to rib, and (3) vertebra to rib (Figure 3-10 and Table 3-1). The external intercostal muscles pass from rib to rib in an anteroinferior direction (in the same direction as the external abdominal oblique muscle) and elevate the ribs during inspiration. The internal intercostal muscle passes from rib to rib, perpendicular to the external intercostal muscle, and depresses the ribs during expiration. The innermost intercostal muscles run in same direction as the internal intercostal muscles, but the two layers are separated by the intercostal nerves and vessels. The innermost layer is subdivided into the subcostal and transversus thoracis. They likely aid the internal intercostals in depressing the ribs. Figure 3-11 shows that the muscles are not continuous sheets, and in some areas the muscle is replaced by thin membranous tendon. The intercostal muscles are supplied by intercostal nerves.

Diaphragm

The diaphragm is the most important muscle of respiration and its attachments are described in Chapter 4, pages 96 and 97. On contraction, the diaphragm pulls the central tendon inferiorly to increase the vertical dimension of the thorax during inspiration. It is supplied by the right and left phrenic nerves (anterior rami of C3, C4, and C5).

The serratus posterior superior and inferior are thin, flat muscles on the posterior thoracic wall (see Figure 2-15). The superior muscle runs from the lower cervical and upper thoracic vertebral spines downward and laterally to the upper ribs. It elevates the ribs during inspiration. The inferior muscle arises from the upper lumbar and lower thoracic vertebral spines and passes upward and laterally to insert into the lower ribs. It depresses or stabilizes the lower ribs.

INTERCOSTAL BLOOD VESSELS AND NERVES

The intercostal vessels and nerves run between the ribs, under the shelter of the subcostal groove of the more superior rib (see Figures 3-10 and 3-11). The vessels and nerves run between the internal and innermost intercostal muscles.

3 The Pleural Cavities and Lungs

PLEURA

The right and left pleural cavities are completely enclosed spaces within the thorax that contain the right and left lungs (Figure 3-12). Like the peritoneal cavity, the pleural cavity is lined with serous membrane, or pleura.

Parietal pleura lines the inner aspect of the pleural cavity. It is subdivided as follows: Costal pleura lines the inner aspect of the rib cage, diaphragmatic pleura lines the superior aspect of the diaphragm, mediastinal pleura covers the mediastinum, and cervical pleura bulges up into the neck as the cupola. Where parietal pleura reflects from the mediastinum and diaphragm onto the thoracic wall is important, and the areas of reflection may be plotted based on knowledge of surface anatomy.

Visceral pleura lines the lungs, following all the contours and fissures intimately. Both visceral pleura and parietal pleura are continuous at the root of the lung. It is helpful to imagine the lung bud in the developing embryo growing into an empty pleural cavity lined with parietal pleura. As the lung invades the cavity, it carries with it parietal pleura, which eventually covers the lung as visceral pleura.

Pleural recesses are areas where there is a space between reflected layers of pleura. During quiet inspiration the recesses are not filled with lung tissue. There is a costodiaphragmatic recess laterally and inferiorly and a costomediastinal recess medially and anteriorly.

LUNGS

The lungs are two spongy organs resembling inverted, blunted cones cut in half. They are housed in the right and left pleural cavities. The lungs consist of small, air-filled chambers, or alveoli, where exchange of gases (oxygen [O2] and carbon dioxide [CO2] takes place with the circulatory system. In turn the alveoli are supported by elastic tissue, which tends to collapse and shrink the lung (elastic recoil) during respiratory expiration.

BRONCHI AND BRONCHIAL TREE

The trachea consists of approximately 20 U-shaped, incomplete cartilaginous rings strung together with fibroelastic tissue (Figure 3-15). The posterior deficient portion is covered with fibrous tissue and involuntary muscle. Approximately half the course of the trachea is within the neck. It continues inferiorly from the larynx in the neck, lying anterior to and paralleling the course of the esophagus. Applied laterally to the trachea are the two lobes of the thyroid gland, connected by an isthmus that crosses the second or third tracheal ring (see Chapter 5).

The trachea descends and enters the inlet of the thorax, passing deep to the sternum, where it is covered by the thymus gland. At vertebral level T5 (sternal angle), the trachea bifurcates into a right and left primary bronchus. The last tracheal ring features the carina, a midline cartilaginous ridge that separates the lumens of the primary bronchi. It is seen during a bronchoscopic examination.

Bronchi and Bronchioles

The right and left primary bronchi are approximately half the diameter of the trachea. The right bronchus differs from the left, however, in that it is slightly wider, shorter, and in a more direct line with the trachea.

Right Bronchial Tree

The right primary bronchus divides into three secondary bronchi, or lobar bronchi, one for each of the three lobes. The secondary bronchus to the upper lobe is given off before the primary bronchus plunges through the hilus. Within the right lung the primary bronchus divides to supply secondary bronchi to the middle and lower lobes.

Each lobe of the right lung is further subdivided functionally into bronchopulmonary segments. Each secondary bronchus breaks up into tertiary or segmental bronchi to supply 10 bronchopulmonary segments. Each segment can function independently of the others, and the thoracic surgeon makes use of this fact when considering the removal of diseased portions of lung tissue without affecting the function of remaining healthy segments.

Within the bronchopulmonary segments the tertiary bronchi continue to divide, decrease in size, and lose their cartilaginous support. When this occurs, the tubes are called bronchioles and their walls are supported only by a relative increase in smooth muscle thickness. Spasm of this smooth muscle decreases airflow, a condition known as asthma.

The bronchioles finally end as terminal bronchioles, from which ductules lead off to blind sacs, or alveoli, which are one cell thick. Surrounding each alveolus is a capillary network fed by arterioles branching from the pulm/>

Jan 5, 2015 | Posted by in General Dentistry | Comments Off on 3: The Thorax
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