Never Grow a Wishbone Where Your Backbone Should Be
The body has more than 206 bones, all of which are grouped into two main categories. First are those bones aligned with the vertical plane or axis of the body. This is the axial skeleton and includes the skull, vertebral column, and rib cage. The other bones of your body, namely your limbs, make up the appendicular skeleton. Let’s look at the axial skeleton first.
Skull
The skull, or cranium, is composed of numerous small and flattened bones that encase the brain, provide a base upon which the brain sits, and form an attachment point for connection to the vertebral column. The following are the different parts of the skull.
Neurocranium
The skullcap (membranous neurocranium, also called the calvaria) forms the roof of the skull. It is made up of several bones, which at birth are loosely joined together with spaces called fontanelles between them that allow for the rapid growth of the brain and later fuse into a single composite structure.
What is the “soft spot” on a baby’s head?
The soft spot on the top of a newborn baby’s head is actually the largest of several fontanelles (gaps into which the skull bones will later grow).
The skullcap is made up of left and right frontal bones (the forehead), right and left parietal bones (the top and most of the side), and an occipital bone (back lower portion of the skull). Around the area of the ears on the right and left are the temporal bones. These allow the passage of the auditory canal, seven cranial nerves, and major blood vessels.
Endocranium
This is the base on which the brain sits. Imagine the skullcap as the roof and the walls for the brain, and the endocranium as the floor. In human development, these bones first form as cartilage, and are later replaced by bone in the process called endochondral osteogenesis (covered in a later section). As with the skullcap, the base is a composite structure that grows and fuses into a single functional base for the brain.
Facial Skeleton
The final portions of the skull are the bones that make up the jaw and the face. Often referred to in anatomy class as the viscerocranium, it consists of the upper jaws (maxilla), lower jaw (mandible), and the bones of the nose and the palate of the mouth (nasal and palatine bones, respectively). Additional small bones are also present in the eye sockets and the deeper divisions of the nasal cavities.
Vertebral Column
This supporting column for the human body is a highly diverse and composite structure consisting of 33 vertebrae, small bones that interlock to protect the spine while allowing it to bend.
The most superior 7 vertebrae are called the cervical vertebrae and are often designated as C1–C7. Connecting the vertebral column to the occipital condyle, a protuberance at the base of the occipital bone, is the function of C1 (also called the atlas). The next vertebrae, or C2, is called the axis, and it allows the pivoting of the skull on the vertebral column. These are the only 2 named vertebrae in the body.
Anatomy of a Word
superior
Superior in anatomy does not refer to a snide manner, good quality, or excellent performance. It means “closest to the head.” Inferior, then, means farthest from the head.
The next 12 vertebrae are the thoracic vertebrae (T1–T12). Each of these vertebrae projects at about a 45° angle; they form the connection points with the base of each rib.
Lumbar (lower back) vertebrae consist of the next 5 vertebrae (L1–L5), found in the lower portion of the abdominal area.
The remaining 9 vertebrae are actually fused into 2 units, the sacrum and the coccyx. The sacrum consists of 5 of the vertebrae (S1–S5) and functions as a portion of the pelvic girdle, the bones that connect the trunk and the legs. The last 4 small vertebrae are fused together into the structure known as the coccyx, or tailbone.
Rib Cage
Consisting of the ribs, interconnecting cartilages, and the bones of the sternum (breastbone), the rib cage is the protective structure surrounding the vital organs of the thoracic region including the heart and lungs. In addition, the rib cage helps you breathe.
Ribs
These 12 pairs of curved bones reach from the vertebral column and connect to the ventral (front) surface of the thorax (chest) via the sternum. The rib cage as a whole is capable of moving upward (and expanding) each time a person inhales. Likewise, when a person exhales, gravity pulls the rib cage downward (and inward) to its relaxed position.
The first 7 most superior ribs are referred to as the “true” ribs. While the composition of all ribs is identical, these first 7 are attached to the sternum via individual costal cartilages, tough pieces of connective tissue. The next 3 pairs are indirectly attached to the sternum via cartilages that join the costal cartilage of the seventh rib, and therefore do not have their own costal cartilage. The final 2 most inferior ribs (sometimes called “false” ribs) are called “floating ribs” because they are only attached to the vertebral column on the dorsal (back) side of the body, leaving the ventral (front) ends of the ribs unattached.
Sternum
The sternum is a composite flattened plate of bones that defines the ventral surface of the thoracic region of the human body—or, more simply put, your chest. The middle portion of the sternum, and the bulk of its mass, is the body of the sternum. For the superior portion, the manubrium is a broader square-shaped bone that is connected to the body of the sternum. The inferior portion of the sternum is formed by an arrow-shaped bone called the xiphoid process.