Egypt Secondary 3 Biology, Week 1: The appendicular skeleton: girdles, arms and legs

Support in plants and the human skeleton

Week 1 · Chapter 1 Support and Movement in Living Organisms (A): objectives, support in plants, the human skeleton: axial skeleton (vertebral column, skull, thoracic cage) and appendicular skeleton (girdles and limbs) · 3 lessons

The appendicular skeleton: girdles, arms and legs

Biology Sec 3 Term 1, Chapter 1: pp. 5–10

GoalName the bones of the shoulder girdle, arm, hip girdle and leg, and compare the arm with the leg bone for bone.

New words

pectoral girdle · the shoulder girdle: two halves, each a scapula and a clavicle, joining the arms to the axispelvic girdle · the hip girdle: two halves meeting at the front, joining the legs to the vertebral columnhumerus · the bone of the upper arm; its partner in the leg is the femur, the thigh bonephalanges · the small bones of the fingers and toes; 3 in each, 2 in the thumb and big toe

Explanation

Each limb joins the axis through a girdle. The pectoral girdle has two halves. Each half has a triangular scapula at the back and a thin clavicle in front. The scapula has a socket, the glenoid cavity, where the head of the humerus fits to make the shoulder joint. The arm is the humerus (upper arm), the radius and ulna (forearm; the radius can turn round the fixed ulna), 8 carpals in the wrist, 5 metacarpals in the palm and the finger bones, the phalanges: 3 in each finger and 2 in the thumb. The pelvic girdle has two halves that meet in front at the pubic symphysis. Each half is made of an ilium, an ischium and a pubis, which meet at a deep socket called the acetabulum, where the head of the femur fits to make the hip joint. The leg is the femur (thigh), the tibia (inner) and fibula (outer) in the lower leg, the patella (kneecap) in front of the knee, 7 tarsals in the ankle (the largest forms the heel), 5 metatarsals in the instep and the toe bones: 3 phalanges in each toe and 2 in the big toe. Arm and leg follow the same plan, which is why they can be compared bone for bone.

Part	Arm	Leg
Girdle	scapula and clavicle	ilium, ischium and pubis
Upper long bone	humerus	femur
Two lower bones	radius and ulna	tibia and fibula
Kneecap	none	patella
Wrist or ankle	8 carpals	7 tarsals
Palm or instep	5 metacarpals	5 metatarsals
Finger or toe bones	14 phalanges	14 phalanges

Worked examples

Bones in one hand. Each of the 4 fingers has 3 phalanges, and the thumb has 2, so the phalanges number 4 × 3 + 2 = 14. With 8 carpals and 5 metacarpals the hand and wrist have 8 + 5 + 14 = 27 bones. A foot follows the same pattern: 7 + 5 + 14 = 26 bones.

Bones in a whole limb. Arm: humerus 1, radius and ulna 2, hand and wrist 27, so 1 + 2 + 27 = 30. Leg: femur 1, patella 1, tibia and fibula 2, foot and ankle 26, so 1 + 1 + 2 + 26 = 30. Both limbs have 30 bones.

Joint	Rounded head of	Socket	Socket belongs to
Shoulder	humerus	glenoid cavity	scapula
Hip	femur	acetabulum	ilium, ischium and pubis

Two ball-and-socket joints, side by side. The shoulder joint and the hip joint have the same plan: a rounded head of a long bone fits into a socket of the girdle.

Class activity

Measure and compare, with a grown-up. With a tape measure, measure in centimetres the length from your shoulder to your elbow and from your hip to your knee. Which is longer? Name the long bone inside each length. Then find your own collarbone and kneecap with your fingers, and write which girdle or limb each belongs to. Do not press hard on any bone.

Practice

Which two bones form the forearm, and which one can turn round the other?

Name the bones that meet at the acetabulum.

How many phalanges are in one thumb and one finger together?

Which leg bone matches the humerus of the arm, and which two leg bones match the radius and ulna?