By Elizabeth A. Murray, Mount St. Joseph University
When it comes to the thoracic vertebrae, it is their companion—the ribs and the sternum—that form the complex known as the rib cage. The sternum is generally counted as one bone among the standard number of 206 bones in the human body, but it has subunits. What are they?
The Superior Portion of the Sternum
The superior portion of the sternum is the manubrium. It is followed by a bladelike body of the sternum, which antique books often called the gladiolus, named after the blade carried by gladiators. The inferior tip of the sternum is called the xiphoid process and is cartilaginous in young people but typically ossifies with age.
At the superior edge of the manubrium is the suprasternal notch, also called the jugular notch. It feels deeper than it actually is on the manubrium, because of the clavicles on either side. These join the manubrium at its paired clavicular notches, which are just lateral to the suprasternal notch. The articulation where the manubrium meets the body of the sternum often projects slightly anteriorly and is called the sternal angle.
The Sternal Angle
The sternal angle is an important anatomical landmark: it’s where the aorta begins and ends its arch from the heart before descending through the chest as the thoracic aorta. It is also where the trachea, or windpipe, bifurcates into a pair of bronchi that enter each lung. It often surprises people when they learn that the trachea ends that far superiorly. And if one sent a horizontal plane straight back from the sternal angle, it would intersect the T4–T5 intervertebral disc.
But the importance of the sternal angle to the rib cage is that this is where the costal cartilage of the second rib attaches to the sternum. For this reason, paramedics and other health professionals can use the sternal angle to quickly locate the second rib, and from there, more readily figure out and assess other ribs.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
The Costal Arch
The first rib attaches by its costal cartilage to the manubrium, the second rib attaches to the sternum at the sternal angle between the manubrium in the body, and the costal cartilages of ribs three through seven attach to the body of the sternum. Remember, a true rib has its own cartilage connecting it to the sternum. These sternocostal joints are gliding or planar synovial joints.
The costal cartilages of ribs eight, nine, and 10 link up to the seventh costal cartilage to reach the sternum, and in so doing form the surface anatomy landmark called the costal arch. If one asks a little kid or someone thin to suck in their belly, they can see the costal arch formed by the ascending cartilages of ribs eight through 10 reaching the cartilage of rib seven, in their collective path to the sternum.
Costal Cartilages
It is worth mentioning that the costal cartilages tend to ossify with age. This is helpful in assessing the age of an unknown skeleton at the morgue, which can be done visually from dried bones or by using X-rays in a body that is not skeletonized. There is no precise method to determine age from this, but when ossification of the costal cartilages is seen, it usually indicates an older person.
In addition, as costal cartilages turn to bone, someone doing lifesaving cardiopulmonary resuscitation, or CPR, on an older person may hear fractures occurring during chest compressions.
It is interesting to note how the ribs attach posteriorly to the vertebrae and how this relates to the intercostal nerves. Ribs don’t just attach in one place to their respective vertebra; they attach in two places, which facilitates stability and movement. The head of a rib attaches to the body of a single thoracic vertebra, or it may span across the bodies of two thoracic vertebrae. In the superior and inferior portions of the rib cage, the rib head will attach to a single thoracic vertebral body, and the midrange ribs will attach to costal facets on two adjacent vertebrae, along with the intervening disc between them.
Regardless, the tubercle of each rib attaches to the transverse process of the thoracic vertebra it corresponds to—so the tubercle of rib seven attaches to the transverse process of the T7 vertebra, even though the rib’s head attaches to both T6 and T7. These so-called costovertebral joints are gliding synovial articulations, so are freely movable.
The Intercostal Nerves
In addition, there are intervertebral foramina between adjacent vertebrae, and out of those come spinal nerves that almost immediately branch into dorsal and ventral rami for distribution to tissues. In the thoracic region, the ventral rami of those spinal nerves are called intercostal nerves and assume a simple pattern that hugs the inferomedial side of the ribs, running in the costal grooves on the deep surfaces of the ribs.
These intercostal nerves have sensory components that govern segmental patches of skin on the thorax, which we learned are called dermatome patterns. But the intercostal nerves also supply the muscles of the rib cage, as well as muscles of the anterolateral abdominal wall.
The ventral ramus of the first intercostal nerve, T1, also participates in the brachial plexus that governs the sensory and motor supply of the upper limb.
Common Questions about the Anatomy of the Sternum
The superior portion of the sternum is the manubrium. It is followed by a bladelike body of the sternum, which antique books often called the gladiolus, named after the blade carried by gladiators.
The sternal angle is an important anatomical landmark: it’s where the aorta begins and ends its arch from the heart before descending through the chest as the thoracic aorta. It is also where the trachea, or windpipe, bifurcates into a pair of bronchi that enter each lung.
The costal cartilages tend to ossify with age. This is helpful in assessing the age of an unknown skeleton at the morgue, which can be done visually from dried bones or by using X-rays in a body that is not skeletonized.
