Skeletal Systems of Organisms
Senior 1 · Biology
Skeletal Systems of Organisms
Lesson 3 of 5
25 min
You cannot see your bones, but you can feel many of them. Work with a partner. Press your fingertips along the top of your chest to find your collarbones. Run your hand down the middle of your chest to find a flat bone, the breastbone, and count the ribs you can feel at your side. Ask your partner to bend forward: a line of bumps runs down the middle of the back. Open and close your mouth with your fingers on your cheeks: only the lower jaw moves; the rest of your skull stays still.
An adult skeleton has 206 bones. A newborn baby has about 300, but many of them join together (fuse) as the child grows.
Definition
Axial skeleton: the bones along the central axis of the body: the skull, the vertebral column and the ribcage (ribs and sternum). It supports the head and trunk and protects the brain, spinal cord, heart and lungs.
Appendicular skeleton: the bones of the limbs (arms and legs) and the girdles that attach them to the axial skeleton. You will study it in the next lesson.
Definition
Vertebra (plural vertebrae): one of the small bones that make up the vertebral column. Each has a centrum and a neural canal for the spinal cord.
Intervertebral disc: a pad of cartilage between two vertebrae that absorbs shocks and lets the column bend.
Suture: an immovable joint where two skull bones meet, joined by a wavy line of tough tissue.
Ribcage (thoracic cage): the ribs, the sternum (breastbone) and the thoracic vertebrae, which together enclose the chest.
Figure 1: The human skeleton from the front. The axial skeleton (blue) forms the body's central axis; the appendicular skeleton (cream) is the limbs and their girdles.
Of the 206 bones, 80 are in the axial skeleton: 22 in the skull, 6 tiny bones in the middle ears, the hyoid bone in the neck, 26 in the vertebral column and 25 in the ribcage (24 ribs and the sternum). The other 126 are appendicular.
Worked example 1: what share of the skeleton is axial?
Answer: about 39% of the bones are axial and about 61% appendicular. The limbs have so many bones because the hands and feet alone contain more than half of the body's bones.
The skull is made of 22 bones. The eight bones of the cranium form a strong box around the brain. They are joined by wavy, immovable joints called sutures, so the cranium acts as a single hard shell. Fourteen facial bones hold the eyes in their sockets and frame the nose and mouth. The only skull bone that moves is the mandible (lower jaw), which lets you chew and speak.
In a newborn baby the cranial bones are not yet joined: soft areas called fontanelles let the head change shape slightly during birth and leave room for the brain to grow. They close during the first two years. Even an adult skull cannot resist every blow, which is why moto drivers and their passengers in Rwanda must wear a helmet.
The vertebral column (backbone or spine) is a chain of small bones called vertebrae (singular vertebra). Between neighbouring vertebrae are pads of cartilage, the intervertebral discs, which absorb shocks and let the column bend a little. A baby has 33 vertebrae; in an adult, the lowest ones fuse to form the sacrum and coccyx, leaving 26 separate bones. Seen from the side, the column has a gentle S-shaped curve, which makes it work like a spring when you walk, run or carry a load.
Figure 2: The vertebral column from the side, showing its five regions and its S-shaped curve. A disc of cartilage lies between neighbouring vertebrae.
Every vertebra has the same basic plan (Figure 3): a solid centrum that carries weight, and behind it a neural arch around the neural canal. Lined up, the neural canals form a tube that protects the spinal cord. Projections (processes) give attachment to muscles and ribs, and the facets fit against the vertebrae above and below.
Figure 3: A vertebra seen from above. The spinal cord runs through the neural canal of every vertebra.
| Type of vertebra | Number | Where | Special features | Main function |
|---|---|---|---|---|
| Cervical | 7 | neck | small; a hole in each transverse process for an artery. The first (atlas) has no centrum and holds the skull; the second (axis) has a peg on which the atlas turns | support the head; allow nodding and turning the head |
| Thoracic | 12 | chest | long neural spine pointing down; facets where the ribs join | anchor the ribs; support the chest |
| Lumbar | 5 | lower back | largest, thickest centrum; short, broad spine and wide transverse processes for strong back muscles | carry most of the body's weight |
| Sacral | 5 (fused into the sacrum) | between the hips | fused into one triangular bone joined to the hip bones | pass the weight of the body to the pelvic girdle |
| Coccygeal | about 4 (fused into the coccyx) | tail end | very small, fused | attachment for some muscles; the remains of a tail |
Worked example 2: identifying a vertebra from its features
A student is given a vertebra from a goat skeleton. It has a very large, thick centrum, short and broad neural and transverse processes, and no facets for ribs. Which region is it from?
Answer: a lumbar vertebra. The same five regions are found in all mammals, although the numbers of thoracic and lumbar vertebrae differ between species.
The ribcage (thoracic cage) is made of 12 pairs of ribs, the sternum and the 12 thoracic vertebrae. Every rib is joined to a thoracic vertebra at the back. At the front:
The ribcage protects the heart and lungs, and it moves when you breathe: muscles between the ribs lift it up and out when you breathe in, as you saw in Unit 8.
Warning
Common mistakes. (1) Putting the collarbone (clavicle) or shoulder blade in the axial skeleton: they belong to the pectoral girdle, which is appendicular. (2) Writing "the skull is one bone": it is 22 bones, most of them fused. (3) Confusing vertebra (one bone) with vertebrae (more than one). (4) Thinking floating ribs are loose: they are firmly joined to the backbone, just not to the sternum.
The Skeleton (FuseSchool). Watch for the division into axial and appendicular skeletons, and for which organs each part of the axial skeleton protects.
Tip
Memory hook for the vertebrae: "breakfast at 7, lunch at 12, dinner at 5." 7 cervical, 12 thoracic, 5 lumbar, then the sacrum (5 fused) and coccyx (about 4 fused). Ribs follow the thoracic vertebrae: 12 of each.
Practice: the axial skeleton
Key points
Sources and credits
Written by Ganzaa: the activity, Figures 1 and 2, the vertebra table, the worked examples and the practice questions.
