How to sort any bone into the axial or the appendicular skeleton, with counts that add up

1. What is the axial skeleton, and what is the appendicular?

The open textbook chapter on the divisions of the skeletal system defines the axial skeleton as the vertical, central axis of the body: all the bones of the head, neck, chest and back. The appendicular skeleton includes all the bones of the upper and lower limbs, plus the bones that attach each limb to the axial skeleton.

The two divisions also differ in what they do. The chapter says the axial skeleton protects the brain, the spinal cord, the heart and the lungs, and gives attachment to the muscles that move the head, neck and back, as well as to muscles that act across the shoulder and hip joints. The appendicular skeleton carries the limbs and anchors them to that axis.

The same chapter adds a contrast between the two halves of the skeleton as a whole. The lower part is specialized for stability during walking or running, while the upper part has greater mobility, which lets you lift and carry objects or turn your head and trunk.

The osteology appendix of Explorations, an open biological anthropology textbook, describes the same split in three parts per division: the skull, the vertebral column and the thoracic cage on one side; the pectoral girdle, the pelvic girdle and the bones of the limbs on the other.

2. How many bones in the axial skeleton: the count of 80

The textbook chapter gives 80 bones for the adult axial skeleton and builds the total region by region:

For the first row, Explorations divides the skull into the mandible and the cranium, and the cranium into the neurocranium, which houses the brain, and the facial skeleton. All of it is axial.

Two other rows need care. The vertebral column counts 26 because the sacrum and the coccyx each count as one bone, even though each forms from several fused vertebrae. Our comparison of cervical vs thoracic vs lumbar vertebrae covers the 24 that stay separate. The thoracic cage counts 25 because the 12 pairs make 24 rib bones, plus the sternum.

The chapter adds that younger people have more bones, because some fuse during childhood and adolescence. These totals are for the adult.

3. The appendicular skeleton bones, limb by limb

The 126 make sense once you count one side of the body. The textbook gives 30 bones for each upper limb, from the humerus to the phalanges, and 30 for each lower limb, from the femur to the phalanges. Then come the girdles, which attach the limbs to the axial skeleton.

Explorations names the bones of each girdle. The pectoral girdle is the clavicle and the scapula, two bones per side. For the lower limb, the hip bone, or os coxa, is the bone that belongs to each side; it forms from three bones, the ilium, the ischium and the pubis, that fuse during growth.

So one side carries 30 + 2 for the upper limb and 30 + 1 for the lower limb, 63 bones in all. Double it and you get 126. Our comparison of the pectoral vs pelvic girdle explains why the shoulder girdle is built for movement and the hip girdle for bearing weight.

4. Sort any bone in four questions

This method is ours, built on the definitions above. Ask the questions in order and stop at the first yes:

1. Is it in the head or the neck? The skull, the hyoid, the ear ossicles and the cervical vertebrae are axial.
2. Is it in the vertebral column or the wall of the chest? Every vertebra, the sacrum, the coccyx, the ribs and the sternum are axial.
3. Does it attach a limb to the trunk? Then it is a girdle bone, and girdle bones are appendicular: the clavicle, the scapula and the hip bone.
4. Is it in an arm, a forearm, a hand, a thigh, a leg or a foot? Then it is appendicular.

The order matters. Question 3 comes before question 4 because the girdle bones sit on the trunk, and a student who only asks "is it in a limb?" puts the scapula on the wrong side.

Try it on three bones. The patella: no to questions 1 to 3, yes to 4, so appendicular. The sternum: yes to question 2, axial. The clavicle: no to 1 and 2, yes to 3, appendicular. The hip bone follows the same path as the clavicle, even though it sits right next to the sacrum.

5. The difference between axial and appendicular skeleton

This table puts the two divisions side by side:

Watch the last row. Being on the trunk does not make a bone axial: the clavicle, the scapula and the hip bone sit on the trunk but belong to a girdle, so they are appendicular.

6. Borderline cases among the axial skeleton bones

A few bones look like they could go either way. Here is where the textbook count puts them:

• The hyoid sits in the neck, apart from the skull. Explorations notes that it is the only bone of the skeleton that does not articulate with another bone; it hangs in a sling of muscles. It still counts as axial, among the seven bones associated with the head. Our atlas page on the hyoid bone shows where it sits.
• The ear ossicles are six tiny bones, three per middle ear, and they are axial too. Forget them and your axial total comes out at 74.
• The sacrum sits at the back of the pelvis, between the two hip bones, yet the textbook counts it in the vertebral column, which makes it axial. Explorations describes the pelvic girdle as the two os coxae together with the sacrum, so some courses draw the line differently. For the 80 and 126 totals, keep the sacrum axial.
• The ribs count as 24 bones, not 12. Count pairs instead of bones and the axial total falls 12 short.

If your course places the sacrum in the pelvic girdle, the two totals shift by one but the 206 stays the same. Say which convention you use when the question asks for numbers.

Before the exam

Rebuild the first table from memory and check that it adds up to 80, then run the four questions on a mixed set of bones or photos. Osteo+ can name the bone in a photo, so you can check its name before you decide which division it belongs to.

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