Bone, cartilage and the axial skeleton

What holds you up

Press the back of your neck and run your finger down your back. You feel a row of hard bumps. They are part of your skeleton. The skeleton is the frame of the body.

The skeletal system is a framework of bones and a few cartilages. Bone and cartilage are specialised connective tissues.

• Bone has a very hard matrix, because it holds calcium salts.
• Cartilage has a slightly pliable matrix, because it holds chondroitin salts.

The skeleton of an adult human has 206 bones and a few cartilages. It works in these ways.

• It supports the body and gives it shape.
• It protects soft organs. The skull protects the brain, the vertebral column protects the spinal cord, and the rib cage protects the heart and the lungs.
• It gives a point of attachment to the muscles. Muscles pull on bones to move the body.
• The red bone marrow inside some bones makes the red blood cells.

The skeleton has two main divisions. The axial skeleton lies along the main axis of the body. The appendicular skeleton is the limbs and the girdles that join them to the axis. The axial skeleton has 80 bones. This lesson is about it.

The skull

The skull has two sets of bones, the cranial and the facial. Together they make 22 bones.

• Cranial bones: 8. They form the hard outer covering of the brain, the cranium.
• Facial bones: 14. They form the front part of the skull.

Two more kinds of bone are counted with the skull.

• A single U shaped bone, the hyoid, lies at the base of the buccal cavity. It is included in the skull.
• Each middle ear holds three tiny bones, the malleus, the incus and the stapes. They are the ear ossicles. Two ears hold six.

The skull joins the top of the vertebral column by two occipital condyles. So it is called a dicondylic skull.

The vertebral column

The vertebral column lies along the back and forms the main frame of the trunk. NCERT counts 26 units, called vertebrae. Each vertebra has a hollow in the middle, the neural canal, through which the spinal cord passes.

The first vertebra is the atlas. It articulates with the occipital condyles. Almost all mammals, human beings included, have seven cervical vertebrae.

A note on the count. Many anatomy books count the sacrum as five fused vertebrae and the coccyx as four. They give 33 vertebrae in all. NCERT counts each fused part as one unit, so it gives 26. This course follows NCERT, and a NEET paper follows it too.

The vertebral column protects the spinal cord. It supports the head, and it is the place where the ribs and the muscles of the back are attached.

The sternum and the ribs

The sternum is a flat bone in the middle of the front of the chest. There are 12 pairs of ribs. A rib is a thin flat bone. It joins the vertebral column at the back, and it joins the sternum at the front. At its back end it has two articulation surfaces, so it is called bicephalic.

• The first seven pairs are the true ribs. They join the thoracic vertebrae at the back and the sternum at the front, by hyaline cartilage.
• The 8th, 9th and 10th pairs do not reach the sternum. They join the seventh rib by hyaline cartilage. They are the vertebrochondral, or false, ribs.
• The 11th and 12th pairs are not joined at the front. They are the floating ribs.

The thoracic vertebrae, the ribs and the sternum together make the rib cage.

Adding up the axial skeleton

You should find that pairs 1 to 7 are true ribs, pairs 8 to 10 are false ribs and pairs 11 and 12 are floating ribs.

Try it in Python

The program adds the parts of the axial skeleton.

Practise with new numbers

Common mistakes

• Counting 33 vertebrae in an NCERT question. NCERT gives 26 units.
• Saying that the skull has 29 bones when the question asks for cranial and facial bones. Those two sets make 22. The hyoid and the ear ossicles are extra.
• Mixing the false and the floating ribs. The false ribs join the seventh rib. The floating ribs are free.
• Giving eight cervical vertebrae. There are seven.
• Saying that the skull has one occipital condyle. It has two.

Remember these

• Skeleton: 206 bones. Axial: 80 bones. Bone has calcium salts, and cartilage has chondroitin salts.
• Skull: 8 cranial and 14 facial bones, plus the hyoid and 6 ear ossicles. Two occipital condyles.
• Vertebral column: 7 cervical, 12 thoracic, 5 lumbar, 1 sacral and 1 coccygeal unit. The atlas is the first.
• Ribs: 12 pairs. True: 7 pairs. False: 3 pairs. Floating: 2 pairs. Sternum: 1.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many bones are in the axial skeleton, and which parts make it?

2. How many cranial and how many facial bones are there? What are the ear ossicles?

3. Give the number of vertebrae in each region of the vertebral column.

4. Which ribs are true, which are false and which are floating?

5. What is the neural canal, and what passes through it?

References

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 6, Bone Tissue and the Skeletal System, and chapter 7, The Axial Skeleton.
• NCERT (2023). Biology, Textbook for Class XI, chapter 17.