aspect, the nasal bones attach to cartilaginous structures that make up the frame of the external nose.
The lacrimal bones are small, delicate bones that form the medial aspect of the orbital socket. Each lacrimal bone includes a deep channel called the lacrimal fossa, which houses a structure known as the lacrimal sac. This structure provides a passageway for tears to drain from the eyes into the nasal cavity.
The palatine bones are L-shaped structures that form the posterior aspect of the hard palate. Together with the inferior portions of the maxillary bones, they form the roof of the oral cavity. The vomer (which literally means “plow”) is a thin, aptly named plow-shaped structure that provides the framework for the nasal septum. Finally, the paired inferior nasal conchae are narrow, slightly bowed structures that form the lateral walls of the nasal cavity. The superior and middle nasal conchae are subdivisions of the ethmoid bone.
■The Thoracic Cage
To be technically and anatomically correct, the thorax refers exclusively to the chest and the bony substructures that serve to protect the chest, hence the name thoracic cage. Particular divisions of the thoracic cage include the thoracic vertebrae posteriorly, the sternum and costal cartilage anteriorly, and the ribs laterally.
The function of the ribs is two-fold: first and most obvious is protection of the thoracic cavity. Second, they serve as an attachment site for the intercostal muscles – the muscles of respiration. Acting as a hinge, the ribs are both extremely durable and flexible – and their movement can increase or decrease the volume of the thoracic cage. While the ribs can bend and cushion blows, sudden and/or severe force can cause rib fractures.
Amazingly, because they are securely fastened to connective tissue, a fractured rib can heal without the placement of a cast or splint. However, compound rib fractures can propel fragments of bone into the thoracic cavity, possibly piercing vital internal organs.
Cooperatively, the thoracic cage provides two main functions:
•Guards the heart, lungs, major blood vessels, and thymus located in the thoracic cavity
•Provides a platform or scaffold for the fixture of the skeletal muscles involved in breathing, movements of the pectoral girdle and limbs, and secure positioning of the vertebral column.
The sternum, commonly referred to as the breast bone, is positioned in the anterior midline of the chest. Technically, it’s a composite structure consisting of three fused bones14:
•the manubrium
•body
•xiphoid process
The manubrium is the broadest and most superior segment of the sternum; triangular in shape, it resembles a knight’s shield. The manubrium articulates with the first pair of ribs laterally and the clavicles superiorly. The central depression of the manubrium (which is easily palpated), is located between the articulations of the clavicles and is referred to as the jugular notch.
This is an important anatomical landmark, as it designates the point at which the left common carotid artery branches from the aorta. Inferior to the manubrium is the second segment of the sternum, called the sternal body. The horizontal groove between the manubrium and the body of the sternum is referred to as the sternal angle; another important anatomical landmark, it designates the level of the second ribs. This landmark serves as an essential tool in physical examination and is utilized as a reference point for listening to sounds made by the aortic and pulmonic heart valves.
Figure 4-9 The thoracic cage.
The xiphoid process shapes the inferior border of the sternum and serves as an attachment station for some of the abdominal muscles. It is worth mentioning that ossification of the sternum is not complete until at least the age of 25, with the xiphoid process generally being the last section to fuse. This makes proper hand placement during cardiopulmonary resuscitation (CPR) rather crucial, and therefore is strongly emphasized during CPR training.
The ribs, or costae, are long, rounded bones that form the bell-shaped sides of the thoracic cage.15 Each of the twelve pairs of ribs attach posteriorly to the twelve thoracic vertebrae. From their posterior point of attachment, the ribs curve slightly inferiorly and spread anteriorly to reach the anterior body wall.
The superior seven pairs of ribs attach directly to the sternum by individual cartilaginous annexes called costal cartilages. Because they attach directly to the sternum, the first seven ribs are called true ribs. The inferior, or lower, five pairs of ribs are referred to as false ribs, since they do not attach directly to the sternum; rib pairs 8, 9, and 10 (the vertebrochondral ribs) attach directly to the cartilaginous extension of rib pair number 7 before they make their way to the sternum. The last two pairs of ribs, ribs 11 and 12, are referred to as vertebral ribs, or simply, floating ribs, considering that they have no connection to the sternum or any other rib.
An important anatomical characteristic to note is that the pronounced furrow that runs along the inferior border of each individual rib (called the costal groove) designates the pathway of nerves and blood vessels.
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