There is one last component of the axial skeleton we did not cover last lab: the thoracic cage, also called the rib cage. The thoracic cage surrounds and protects the heart and lungs in the thoracic cavity. It consists of the ribs, the sternum, and the thoracic vertebrae, to which the ribs articulate.
We examined the thoracic vertebrae last lab, so here we will only examine the ribs and sternum.
There are twelve pairs of ribs. The number is the same in both males and females. Each pair articulates with a different thoracic vertebra on the posterior side of the body. The most superior rib is designated rib 1 and it articulates with the T1 thoracic vertebrae. The rib below that is rib 2, and it connects to the T2 thoracic vertebra, and so on. Ten of the twelve ribs connect to strips of hyaline cartilage on the anterior side of the body. The cartilage strips are called costal cartilage (“costal” is the anatomical adjective that refers to the rib) and connect on their other end to the sternum.
On an individual rib, one end has various processes, facets, and bumps. This is the end that articulates with the vertebra. The other end is blunt and smooth. This is the end that connects to costal cartilage (unless it is a floating rib. See below.)
Ribs 1-7 are called the true ribs. Each true rib connects to its own strip of costal cartilage, which in turn connects to the sternum. Ribs 8-12 are called the false ribs. Ribs 8, 9, and 10 do connect to costal cartilage, but the costal cartilage of each of these ribs connects to the costal cartilage of the rib above it, rather than directly to the sternum. Ribs 11 and 12 do not have any costal cartilage connected to them at all, and in addition to being grouped in the false ribs, these two are also called floating ribs, to reflect that fact.
The sternum has three parts. The manubrium, at the superior end of the sternum, and wider than the rest of the bone, provides articulation points for the clavicles and for the costal cartilage extending from rib 1. The central, thin body provides articulation points for costal cartilage from ribs 2 through 7. The xiphoid process which hangs down at the inferior end of the process (“xiphoid” is from the Greek for sword), starts out as cartilage, and does not typically ossify into bone until an individual is about 40 years old.
Lab 7 eXERCISES 7.1
- The instructor will provide you with a single rib from the human body. On that individual rib, identify which end is the head, and which is the anterior end.
- On one of the intact skeletons in the lab, identify all the following components of the thoracic cage:
the true ribs the false ribs the floating ribs
costal cartilage sternum xiphoid process
manubrium sternal body