Medical Terminology for the Axial Skeleton - dummies

Medical Terminology for the Axial Skeleton

By Beverley Henderson, Jennifer Lee Dorsey

Think of the word axis when you think about medical terminology for the axial skeleton. The bones that make up this particular part of the skeleton tend to encircle important organs or rotate in an axial motion.

Cranium

The bones of the cranium protect the brain. The bones of the skull include the frontal bone, which forms the forehead and bony sockets that contain the eyes. The parietal bone forms the roof and upper sides of the skull. Two temporal bones form the lower sides and base. The mastoid process is a small round part of the temporal bone behind the ear.

The occipital bone forms the back and base of the skull and joins the parietal and temporal bones, forming a suture of cranial bones. The occipital bone has an opening called the foramen magnum through which the spinal cord passes.

The sphenoid bone extends behind the eyes and forms part of the base of the skull. It joins the frontal, occipital, and ethmoid bones and serves as an anchor to hold these bones together. The ethmoid bone is a thin delicate bone, supporting the nasal cavity and forms part of the orbits of the eyes.

Be careful when working with a newborn cranium, as the cranial bones of a newborn are not completely joined. There are gaps of unossified tissue in the skull, called the soft spot or fontanelle. The lines where the bones of the skull join are called cranial sutures.

Facial bones

All the facial bones except one are joined together. Only the mandible, or lower jaw bone, is capable of moving, which is necessary for chewing and speaking. Other facial bones include the nasal bones, and the maxillary bones. Two large bones compose the upper jaw. Both the mandible and maxilla contain sockets called alveoli, in which the teeth are embedded.

The mandible joins the skull at the temporal bone, forming the lengthily named temporomandibular joint. The zygoma or zygomatic bones form the cheek. Together, these bones create a sort of mega-bone that makes up the upper portion of your face.

Vertebral column

The vertebral column, or spinal column, is composed of 26 bone segments called vertebrae, which are arranged in five divisions: cervical, thoracic, lumbar, sacrum, and coccyx (tailbone).

The first seven vertebrae are called the cervical vertebrae (C1-C7). These vertebrae do not join with the ribs. The first cervical vertebra, C1, articulates with the occipital bone of the skull at the back of neck. It supports the head and allows it to move forward and back. The second cervical vertebra, C2, acts as a pivot, allowing head to turn from side to side, extend, and flex.

The second division consists of 12 thoracic vertebrae (T1-T12). These vertebrae join with the 12 pairs of ribs. The third division consists of five lumbar vertebrae (L1-L5). They are the strongest and largest of the back bones. The sacrum is a slightly curved triangular bone, composed of five separate segments, or sacral bones, that gradually become fused. The coccyx is the tailbone. It is also a fused bone, formed from four small coccygeal bones.

A vertebra is composed of a disk-shaped portion called the vertebral body, which is the solid anterior portion. A lamina is a part of the posterior portion of a vertebra. Spinous processes, thoracic processes, and transverse processes are little wing-like projections that project or extend from each vertebra. The foramen is the opening in the middle of each vertebra that the spinal cord passes through.

Between the body of one vertebra and the bodies of vertebrae lying beneath are vertebral disks, which help to provide flexibility and cushion shock to the vertebral column.

Thorax

The thorax starts with the clavicle, or the collarbone, connecting the sternum to each shoulder. The scapula is the shoulder blade, consisting of two flat triangular bones, one on each back side of the thorax. The scapulae extend to join with the clavicle at the acromion.

The sternum is the breastbone. The uppermost part of the sternum joins to the sides of the clavicle and ribs, whereas the other, narrowed portion is attached to the diaphragm. The lower portion of the sternum is the xiphoid process, the small, mobile bone tag on the very end of the sternum. This is the thing you would feel for when performing CPR.

The 12 pairs of ribs are close neighbors with the sternum. The first seven pairs join the sternum anteriorly (at the chest) by attachments of costal cartilage. Ribs 1–7 are called true ribs. Ribs 8–12 are called false ribs. The false ribs join with the vertebral column in the back, but join the 7th rib anteriorly and do not attach to the sternum.

Ribs 11 and 12 are called floating ribs because they are completely free at their anterior end.

Pelvis

The pelvic girdle or hip bone is a large bone that supports the trunk of the body and joins with the femur and sacrum. The adult pelvic bone is composed of three pairs of fused bone: the ilium, the ischium, and the pubis.

The ilium is the uppermost and largest portion. The connection between the iliac bones and the sacrum is so firm that they are often referred to as one bone, the sacroiliac. The iliac crests are found on both the anterior and posterior portions of the pelvis. They are filled with red bone marrow and serve as an attachment for abdominal wall muscles.

The ischium is the posterior portion of the pelvis. The ischium and the muscles attached to it are what you sit on.

The pubis is the anterior portion containing two parts that are joined by way of a disk. This area of fusion is called the pubic symphysis. The region within the bone formed by the pelvic girdle is called the pelvic cavity. The rectum, sigmoid colon, bladder, and female reproductive organs are contained in this cavity.