The thoracic cage is conical in shape, narrower at the top and wider at the bottom. Think of it as a truncated cone: a broad, rounded base that tapers upward to a smaller opening at the neck. This framework of bone and cartilage houses and protects the heart and lungs while remaining flexible enough to expand with every breath.
Bones That Form the Cage
Three groups of bones create the thoracic cage’s walls. The back wall is formed by 12 thoracic vertebrae (T1 through T12), stacked to make up the upper and mid portion of the spine. The front wall is the sternum, a vertical bone running down the center of the chest. Connecting these two structures are 24 ribs, 12 on each side, that curve horizontally around the torso from spine to breastbone. The ribs attach to the sternum through strips of costal cartilage, which give the cage the flexibility it needs to move during breathing.
The Top and Bottom Openings
The cone shape means the thoracic cage has two openings: a small one at the top and a large one at the bottom. The top opening, called the superior thoracic aperture, is a bony ring bordered by the T1 vertebra in the back, the first pair of ribs on either side, and the upper edge of the sternum (the manubrium) in front. Blood vessels, nerves, the trachea, and the esophagus all pass through this relatively tight space on their way between the neck and chest.
The bottom opening is much wider, framed by the lowest ribs and the bottom of the sternum. Where the left and right lower rib margins meet the sternum, they form the infrasternal angle, which typically falls between 70 and 110 degrees depending on body type. A narrower angle gives the lower chest a more V-shaped look, while a wider angle creates a broader, flatter base.
Cross-Section: Oval, Not Round
If you could slice through an adult’s chest horizontally, the outline would look like an oval (wider side to side than front to back). This is different from what you’d see in a newborn, whose thorax is nearly circular in cross-section. As a child grows and begins walking upright, the chest gradually flattens front to back and widens laterally, transforming from that circular shape into the adult oval. The normal ratio of front-to-back depth compared to side-to-side width in an adult is roughly 1:2.
When that ratio shifts to 1:1, the chest takes on a rounded, barrel-like appearance. This “barrel chest” is associated with chronic lung conditions like COPD, where long-term air trapping causes the lungs to stay hyperinflated, gradually pushing the rib cage outward into a more circular profile.
How the Shape Differs Between Men and Women
Men’s rib cages are about 12% larger in overall volume than women’s. Beyond size, the proportions differ: male chests tend to be wider and shorter, while female chests are narrower and longer. Women’s ribs angle downward more steeply, their sternums sit higher, and the bone itself is thinner, both in the sternum and in the ribs. These differences mean women’s rib cages are generally more flexible under compression, while men’s are stiffer.
How the Shape Changes During Breathing
The thoracic cage isn’t static. Its shape shifts with every breath through two distinct motions. The upper ribs swing upward and forward like the handle of an old water pump, increasing the chest’s front-to-back depth. Meanwhile, the lower ribs lift outward like the handle of a bucket being raised, widening the chest from side to side. Together, these pump-handle and bucket-handle movements expand the cage’s internal volume, drawing air into the lungs. When the muscles relax, the ribs drop back down and the cage returns to its resting dimensions.
Common Shape Variations and Deformities
Not everyone’s thoracic cage follows the standard conical template. Two well-known structural variations involve the sternum and the cartilage around it. In pectus excavatum, sometimes called funnel chest, the sternum and nearby cartilage are displaced inward, creating a visible depression in the center of the chest. It’s the most common chest wall deformity. Pectus carinatum is the opposite: the sternum and cartilage push outward, giving the chest a prominent, ridged appearance sometimes described as pigeon chest.
Other variations are subtler. Differences in the infrasternal angle, the curvature of individual ribs, or the overall proportions of the cage all fall along a normal spectrum. Body composition, posture, age, and underlying health conditions can each influence the cage’s resting shape over time.