Human Anatomy - H. I. Koliadenko 2009

Musculoskeletal System
Skeleton of the Trunk
Structure and Joints of the Thoracic Bones

The Thorax is formed by the Sternum, 12 pairs of Ribs, and 12 thoracic vertebrae.

The sternum, or breastbone, is a flat, elongated bone consisting of three parts (Fig. 24): the manubrium, the body, and the xiphoid process. The manubrium is the upper, expanded part of the sternum. Its upper border features the jugular notch, flanked by the clavicular notches for articulation with the clavicles, below which lie the costal notches for articulation with the ribs. At the junction of the manubrium and the sternal body lies the notch for the II pair of ribs. The lateral margins of the sternal body bear four pairs of costal notches for articulation with the costal cartilages. At the junction of the sternal body with the xiphoid process, there are notches for the VII pair of ribs. The xiphoid process connects to the body of the sternum via a synchondrosis (the sternal angle), which transforms into a synostosis after the age of 30.

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Fig. 24. The sternum:

1 — jugular notch; 2 — clavicular notch; 3 — costal notch for the I rib; 4 — manubrium of sternum; 5 — sternal angle; 6 — costal notch for the II rib; 7 — costal notches for III–V ribs; 8 — costal notches for VI and VII ribs; 9 — xiphoid process; 10 — body of sternum

The ribs (costae) and their articulations with THE Vertebral Column and sternum.

Humans have 12 pairs of ribs. These are long, curved plates consisting of a body and two extremities — anterior and posterior (Figs. 25, 26). The body of a rib has two surfaces — an internal concave surface and an external convex surface — and two borders: an upper blunt border and a lower sharp border. On the inner surface along the lower border runs the costal groove, which houses the intercostal Blood Vessels and nerves. The posterior extremity of the rib features a HEAD, a neck, and a tubercle. The head articulates with the vertebral body, while the tubercle articulates with the articular facet of the transverse processes of the thoracic vertebrae. The JOINTS OF THE head and tubercle share a common axis of rotation passing through both joints, uniting them into a single combined compound joint. The posterior part of the rib bends forward at the transition to the body, forming the angle of the rib. The XI and XII pairs of ribs lack this angle. The anterior extremity of the rib terminates in Cartilage. The ribs are oriented obliquely, causing their posterior border to sit higher than the anterior one.

The upper seven pairs of ribs connect to the sternum via cartilage and are called true ribs. The remaining five pairs (VIII–XII) are false ribs; among these, the VIII–X pairs connect to each other via cartilages to form the costal arch. The XI and XII pairs are free ribs located within the thickness of the Muscles, known as floating ribs.

A distinctive feature of the I rib is that it is the widest, most curved, and lies almost horizontally. Its body presents superior and inferior surfaces. The superior surface features the groove for the Subclavian Artery, preceded by the scalene tubercle, to which the anterior scalene Muscle attaches. The I, XI, and XII ribs articulate with the vertebral column solely via their heads.

Fig. 25. Structure of a rib:

1 — head of rib; 2 — articular facets of the head of the rib; 3 — neck of rib; 4 — tubercle of rib; 5 — costal groove; 6 — sternal end of rib; 7 — body of rib

Fig. 26. Right ribs:

a — first rib; b — second rib; 1 — head of rib; 2 — neck of rib; 3 — tubercle of rib; 4 — body of rib; 5 — groove for subclavian artery; 6 — scalene tubercle

Shape and Ontogeny of the thorax. The thorax resembles a truncated cone with its apex directed upward and its base downward; its transverse diameter exceeds its sagittal diameter. The walls of the thoracic cavity are formed by the sternum, ribs, and thoracic vertebrae, with muscles situated between the ribs. The superior thoracic aperture is bounded by the I rib, the manubrium of the sternum, and the body of the I thoracic vertebra. The inferior thoracic aperture is irregular in shape and is bounded by the body of the XII thoracic vertebra, the XI and XII pairs of ribs, the costal arches, and the xiphoid process of the sternum. The inferior aperture is sealed by the Diaphragm. Passing through the superior aperture into the thoracic cavity are the Esophagus, Trachea, vessels, and nerves. The apices of the Lungs extend above the level of the I rib. The superior thoracic aperture is densely closed by muscles and Connective Tissue. The shape and dimensions of the thorax depend on age, sex, and individual Anatomical Features.

The thorax of a newborn is narrow superiorly and broadened inferiorly, with the costal cartilages joining the sternum at an angle, giving it a pyramidal shape. By the third year of life, the ribs assume an arched configuration, and the thorax becomes cone-shaped. In school-aged children, the thorax is more convex than in adults, and the ribs have a lesser downward inclination. Consequently, children's breathing is rapid and shallow.

Fig. 27. Skeleton of the thorax:

1 — seventh cervical vertebra; 2 — first rib; 3 — clavicle; 4 — scapula; 5 — coracoid process of scapula; 6 — acromion of scapula; 7 — head of humerus; 8 — sternum; 9 — xiphoid process; 10 — costal arch; 11 — X rib; 12 — XII rib; 13 — first lumbar vertebra

In children with weak musculature and weak lungs, the thorax is flat (expiratory) in shape, appearing somewhat sunken. Such children have a particular need for Physical Exercise. In children suffering from Rickets, the sternum projects forward ("Pigeon chest"). In children and adults with a well-developed muscular system, the thorax is cone-shaped (inspiratory). In adolescents, when skeletal development outpaces muscle GROWTH AND DEVELOPMENT, the thorax is predominantly cylindrical, occupying an intermediate position between the flat and cone-shaped variants.

In elderly individuals, due to involutional processes in The Musculoskeletal System, the thorax shortens and descends, thoracic Kyphosis increases, the anteroposterior dimension increases while the transverse dimension decreases, and the curvature of the ribs diminishes, causing them to assume a more oblique orientation (Fig. 27).

The female thorax is slightly smaller than the male thorax and has a more barrel-like shape due to its relatively broader upper portion. (In most quadruped animals, Internal Organs exert downward pressure, and the thorax is compressed laterally.)