Skeletal Systems of Organisms
Senior 1 · Biology
Skeletal Systems of Organisms
Lesson 4 of 5
25 min
Hold your arm out beside your leg and compare them. Each has one long bone at the top, two bones in the middle, a cluster of small bones at the wrist or ankle, and then fingers or toes. Yet your arm can throw a ball, write and thread a needle, while your leg carries your whole weight up the hills of Rwanda. The difference lies in how each limb is attached to the body, and in the size of its bones.
Definition
Limb girdle: a ring of bones that attaches a pair of limbs to the axial skeleton.
Pectoral girdle (shoulder girdle): the two clavicles (collarbones) and two scapulae (shoulder blades), which attach the arms.
Pelvic girdle (hip girdle): the two hip bones, each made of three bones fused together (ilium, ischium and pubis), joined to the sacrum at the back and to each other at the front. It attaches the legs.
Figure 1: The appendicular skeleton (orange): the pectoral girdle and arms, and the pelvic girdle and legs.
| Part of limb | Arm | Leg |
|---|---|---|
| girdle | pectoral girdle: clavicle and scapula | pelvic girdle: hip bone |
| upper limb: 1 bone | humerus | femur (the longest, strongest bone in the body) |
| lower limb: 2 bones | radius (thumb side) and ulna | tibia (shinbone, carries the weight) and fibula |
| at the joint in between | (none) | patella (kneecap), in front of the knee |
| wrist or ankle | 8 carpals | 7 tarsals |
| palm or foot | 5 metacarpals | 5 metatarsals |
| digits | 14 phalanges (fingers) | 14 phalanges (toes) |
Worked example 1: working from the diagram
Use Figure 1 and the table. (a) How many bones are in one hand, from the wrist to the fingertips? (b) What percentage of the adult skeleton's 206 bones are in the two hands?
Answer: 27 bones in each hand; about a quarter of all your bones are in your hands. So many small bones, with joints between them, are what make fine movements such as writing possible.
| Feature | Pectoral girdle (shoulder) | Pelvic girdle (hip) |
|---|---|---|
| Bones | 2 clavicles and 2 scapulae | 2 hip bones, each of 3 fused bones |
| Attachment to the axial skeleton | loose: joined to the sternum only by the clavicles; the scapulae are held by muscles | firm: joined to the sacrum by strong ligaments |
| Socket for the limb | shallow (on the scapula) | deep (on the hip bone) |
| Main job | freedom of movement for the arms | strength: carries the weight of the upper body to the legs |
| Other role | anchors the muscles of the arms and chest | supports and protects the organs of the lower abdomen |
This is a trade-off. The shoulder's shallow socket lets the arm swing in a full circle, but it is also why the shoulder is the joint most easily dislocated. The hip's deep socket is very stable, but its movement is more limited.
Definition
Joint: a place where two or more bones meet.
Ball-and-socket joint: a joint in which the rounded end of one bone fits into a cup-shaped hollow in another, allowing movement in all directions and rotation. Examples: shoulder, hip.
Hinge joint: a joint that allows movement in one plane only, bending and straightening like a door on its hinges. Examples: elbow, knee, the joints between finger bones.
Figure 2: Two freely movable joints. The ends of the bones are covered with smooth cartilage, which reduces friction.
At the shoulder, the ball-shaped head of the humerus fits the shallow socket of the scapula; at the hip, the head of the femur sits in the deep socket of the hip bone. Elbows and knees are hinge joints. Not all joints move: the sutures of the skull are fixed, and the joints between vertebrae move only slightly. In Senior Two you will study the full structure of movable joints and how muscles work across them.
Worked example 2: classifying a joint by testing its movement
Decide whether each joint is a ball-and-socket or a hinge joint: (a) the knee, (b) the shoulder, (c) the middle joint of your index finger.
Answer: (a) hinge, (b) ball-and-socket, (c) hinge.
Warning
Common mistakes. (1) Mixing up radius and ulna: the radius is on the thumb side; remember "the radius runs to the thumb". (2) Mixing up tibia and fibula: the tibia is the thick shinbone that carries the weight; the thin fibula is on the outer side. (3) Calling the shoulder or hip a hinge joint: they move in all directions, so they are ball-and-socket joints. (4) Thinking the clavicle is part of the axial skeleton: it belongs to the pectoral girdle.
Joints (FuseSchool). Watch for the three main kinds of joint and for examples of ball-and-socket and hinge joints. The details of joint structure are for Senior Two.
Tip
Memory hook: "1, 2, many, many, many." Both limbs follow the same plan: 1 upper bone, 2 lower bones, then many small bones in the wrist or ankle, palm or foot, and digits. Learn the arm, then swap in the leg names: humerus → femur; radius and ulna → tibia and fibula; carpals → tarsals; metacarpals → metatarsals.
Practice: limb girdles, limbs and joints
Key points
Sources and credits
Written by Ganzaa: Figures 1 and 2, both tables, the worked examples and the practice questions.
