Bones can be broadly divided into two categories: axial and appendicular. In this section, we'll take a look at the axial bones, so named because they form the axis of the body. Axial bones are associated with the central nervous system and protect delicate organs such as your heart and brain.
- The cranium. Though that coconut on top of your neck feels like one big unit with a jaw attached, the cranium is actually comprised of 22 interlocking cranial and facial bones. These cranial plates and oddly shaped bones are held together by joints, though these joints (quite wisely) don't allow for movement (except for the mandible, or jawbone). Deep in your ear is the smallest bone in your body, the stirrup. It's about the size of a grain of rice.
- The spine. Your spine (also known as the vertebral column) is comprised of 33 specialized bones called vertebrae. These vertebrae provide form for the rest of the body and protect the spinal cord. Starting from the head and moving downward, the first seven vertebrae are cervical vertebrae, which keep your beautiful cranium from rolling down the street every time you come to a sudden stop. They also allow you to nod "yes" or shake your head "no." Next are the 12 thoracic vertebrae, forming the back of your rib cage. Below the thoracic vertebrae are the lumbar vertebrae, which bear much of the body's load. Most back muscles are connected to these workhorses. Below these is the sacrum, which actually begins in childhood as five different vertebrae, but over time fuses into one unit. Below this is another single unit that begins life in several pieces, the coccyx (tailbone).
- The sternum. The sternum, or breastbone, is front and center in its role as organ-protector. It defends your heart, lungs and portions of your major arteries from external forces. Like the coccyx or sacrum, the sternum starts off as different sections that fuse over time into one unified piece. The sternum provides stability to the ribs that are attached to it.
- The ribs. These flat bones form a protective shield around your internal organs. There are 24 ribs, 12 on each side of your body. They come in three different types. From the top, the first seven sets of ribs are true ribs. They connect in the back to the spine and connect in the front to the sternum. Next are the false ribs. These three pairs connect in the back to the spine, but in the front they attach to the seventh true rib, which is the last rib that connects to the sternum. Last are the floating ribs, and these two pairs of ribs are attached to the spine like all the others, but "float" in the front without being attached to the sternum or any other rib.
In the next section, we'll learn about the bones that serve more than they protect: appendicular bones.