Thorax



Thorax



1 Introduction


The thorax lies between the neck and abdomen, encasing the great vessels, heart, and lungs, and provides a conduit for structures passing between the head and neck superiorly and the abdomen, pelvis, and lower limbs inferiorly. Functionally, the thorax and its encased visceral structures are involved in the following:



The sternum, ribs (12 pairs), and thoracic vertebrae (12) encircle the thoracic contents and provide a stable thoracic cage that both protects the visceral structures of the thorax and offers assistance with breathing. Because of the lower extent of the rib cage, the thorax also offers protection for some of the abdominal viscera, including the liver and gallbladder on the right side, the stomach and spleen on the left side, and the adrenal (suprarenal) glands and upper poles of the kidneys on both sides.


The superior thoracic aperture (the anatomical thoracic inlet) conveys large vessels, important nerves, the thoracic lymphatic duct, the trachea, and the esophagus between the neck and thorax. Clinicians often refer to “thoracic outlet syndrome,” which describes symptoms associated with compression of the brachial plexus as it passes over the first rib (specifically, the T1 ventral ramus). Technically, this is a misnomer because these nerves are not exiting the superior thoracic aperture (thoracic inlet). The inferior thoracic aperture (the anatomical thoracic outlet) conveys the inferior vena cava (IVC), aorta, esophagus, nerves, and thoracic lymphatic duct between the thorax and the abdominal cavity. Additionally, the thorax contains two pleural cavities laterally and a central “middle space” called the mediastinum, which is divided as follows (Fig. 3-1):





2 Surface Anatomy


Key Landmarks


Key surface landmarks for thoracic structures include the following (Fig. 3-2):





Planes of Reference


In addition to the sternal angle of Louis, physicians often use other imaginary planes of reference to assist in locating underlying visceral structures of clinical importance. Important vertical planes of reference include the following (Fig. 3-3):





3 Thoracic Wall


Thoracic Cage


The thoracic cage, which is part of the axial skeleton, includes the thoracic vertebrae, the midline sternum, the 12 pairs of ribs (each with a head, neck, tubercle, and body; floating ribs 11 and 12 are short and do not have a neck or tubercle), and the costal cartilages (Fig. 3-4). This bony framework provides the scaffolding for attachment of the chest wall muscles and the pectoral girdle, which includes the clavicle and scapula and forms the attachment of the upper limb to the thoracic cage at the shoulder joint (Table 3-1).




Rib fractures can be a painful injury (we must continue to breathe) but are less common in children because their thoracic wall is still fairly elastic. The weakest part of the rib is the angle.



Joints of Thoracic Cage


Joints of the thoracic cage include articulations between the ribs and the sternum and thoracic vertebrae and between the sternum and the clavicle and are summarized in Figure 3-5 and Table 3-2.





Muscles of Anterior Thoracic Wall


The musculature of the anterior thoracic wall include several muscles that attach to the thoracic cage but that actually are muscles that act on the upper limb (Fig. 3-6). These muscles are as follows (for a review, see Chapter 7):




The true anterior thoracic wall muscles fill the intercostal spaces or support the ribs, act on the ribs (elevate or depress the ribs), and keep the intercostal spaces rigid, thereby preventing them from bulging out during expiration and being drawn in during inspiration (Fig. 3-6 and Table 3-3). Note that the external intercostal muscles are replaced by the anterior intercostal membrane at the costochondral junction anteriorly, and that the internal intercostal muscles extend posteriorly to the angle and then are replaced by the posterior intercostal membrane. The innermost intercostal muscles lie deep to the internal intercostals and extend from the midclavicular line to about the angles of the ribs posteriorly.





Intercostal Vessels and Nerves


The intercostal neurovascular bundles (vein, artery, and nerve) lie inferior to each rib, running in the costal groove deep to the internal intercostal muscles (Fig. 3-7 and Table 3-4). The veins largely correspond to the arteries and drain into the azygos system of veins or the internal thoracic veins. The intercostal arteries form an anastomotic loop between the internal thoracic artery (branches of anterior intercostal arteries arise here) and the thoracic aorta posteriorly. Posterior intercostal arteries arise from the aorta, except for the first two, which arise from the supreme intercostal artery, a branch of the costocervical trunk of the subclavian artery.




The intercostal nerves are the ventral rami of the first 11 thoracic spinal nerves. The 12th thoracic nerve gives rise to the subcostal nerve, which courses inferior to the 12th rib. The nerves give rise to lateral and anterior cutaneous branches and branches innervating the intercostal muscles (Fig. 3-7).



Female Breast


The female breast extends from approximately the second to the sixth ribs and from the sternum medially to the midaxillary line laterally. Mammary tissue is composed of compound tubuloacinar glands organized into about 15 to 20 lobes, which are supported and separated from each other by fibrous connective tissue septae (the suspensory ligaments of Cooper) and fat. Each lobe is divided in lobules of secretory acini and their ducts. Features of the breast include the following (Fig. 3-8):




• Breast: fatty tissue containing glands that produce milk; lies in the superficial fascia above the retromammary space, which lies above the deep pectoral fascia enveloping the pectoralis major muscle.


• Areola: circular pigmented skin surrounding the nipple; it contains modified sebaceous and sweat glands (glands of Montgomery) that lubricate the nipple and keep it supple.


• Nipple: site of opening for the lactiferous ducts; usually lies at about the level of the fourth intercostal space.


• Axillary tail (of Spence): extension of mammary tissue superolaterally toward the axilla.


• Lymphatic system: lymph is drained from breast tissues; about 75% of lymphatic drainage is to the axillary lymph nodes (Fig. 3-9; see also Fig. 7-11), and the remainder drains to infraclavicular, pectoral, or parasternal nodes.



The primary arterial supply to the breast includes the following:



The venous drainage (Fig. 3-9) largely parallels the arterial supply, finally draining into the internal thoracic, axillary, and adjacent intercostal veins.







4 Pleura and Lungs


Pleural Spaces (Cavities)


The thorax is divided into the following three compartments:



The lungs lie within the pleural cavity (right and left) (Fig. 3-10). This “potential space” is between the investing visceral pleura, which closely envelops each lung, and the parietal pleura, which reflects off each lung and lines the inner aspect of the thoracic wall, the superior surface of the diaphragm, and the sides of the pericardial sac (Table 3-5). Normally, the pleural cavity contains a small amount of serous fluid, which lubricates the surfaces and reduces friction during respiration. The parietal pleura is richly innervated with afferent fibers that course in the somatic intercostal nerves and, over the surface of the diaphragm, the phrenic nerve (C3-C5); the visceral pleura has few, if any, pain fibers.




Clinically, it is important for physicians to be able to “visualize” the extent of the lungs and pleural cavities topographically on the surface of their patients (Fig. 3-10). The lungs lie adjacent to the parietal pleura inferiorly to the sixth costal cartilage. (Note the presence of the cardiac notch on the left side.) Beyond this point, the lungs do not occupy the full extent of the pleural cavity during quiet respiration. These points are important to know if one needs access to the pleural cavity without injuring the lungs (Table 3-6), such as to drain inflammatory exudate (pleural effusion), hemorrhage into the cavity (hemothorax), or air (pneumothorax). In quiet respiration, the lung margins reside two ribs above the extent of the pleural cavity at the midclavicular, midaxillary, and midscapular lines.




The Lungs


The paired lungs are invested in the visceral pleura and are attached to mediastinal structures (trachea and heart) at their hilum. Each lung possesses the following surfaces:



The right lung has three lobes and is slightly larger than the left lung, which has two lobes. Both lungs are composed of spongy and elastic tissue, which readily expands and contracts to conform to the internal contours of the thoracic cage (Fig. 3-11 and Table 3-7).





The lung’s parenchyma is supplied by several small bronchial arteries that arise from the proximal portion of the descending thoracic aorta. Usually, one small right bronchial artery and a pair of left bronchial arteries (superior and inferior) can be found on the posterior aspect of the main bronchi. Although much of this blood returns to the heart via the pulmonary veins, some also collects into small bronchial veins that drain into the azygos system of veins (see Fig. 3-25).


The lymphatic drainage of both lungs is to pulmonary (intrapulmonary) and bronchopulmonary (hilar) nodes (i.e., from distal sites to the proximal hilum). Lymph then drains into tracheobronchial nodes at the tracheal bifurcation and into right and left paratracheal nodes (Fig. 3-12).



As visceral structures, the lungs are innervated by the autonomic nervous system. Sympathetic bronchodilator fibers, which relax smooth muscle, arise from upper thoracic spinal cord segments. Parasympathetic bronchoconstrictor fibers, which contract smooth muscle and increase mucus secretion, arise from the vagus nerve.



Respiration


During quiet inspiration the contraction of the diaphragm alone accounts for most of the decrease in intrapleural pressure, allowing air to expand the lungs. Active inspiration occurs when the diaphragm and intercostal muscles together increase the diameter of the thoracic wall, decreasing intrapleural pressure even more. Although the first rib is stationary, ribs 2 to 6 tend to increase the anteroposterior diameter of the chest wall, and the lower ribs mainly increase the transverse diameter. Accessory muscles of inspiration that attach to the thoracic cage may also assist in very deep inspiration.


During quiet expiration the elastic recoil of the lungs and thoracic cage expel the air. In forced expiration the abdominal muscles contract and, by compressing the abdominal viscera superiorly, raise the intraabdominal pressure and force the diaphragm upward. Having the “wind knocked out of you” shows how forceful this maneuver can be.




Jun 16, 2016 | Posted by in ANATOMY | Comments Off on Thorax
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