Why Your Knee Bends One Way but Your Shoulder Swings in a Circle

The adult human skeleton has 206 bones and a few cartilages. It supports the body, protects organs such as the brain and heart, and works with muscles to produce movement at joints.

This part covers the axial skeleton, the appendicular skeleton, types of joints, and disorders of muscles and bones.

The axial skeleton has 80 bones along the body's main axis — skull, vertebral column, sternum and ribs — and the 12 pairs of ribs are grouped by how they reach the sternum: 7 true pairs directly, 3 false pairs indirectly, and 2 floating pairs not at all.

Skull — 29 bones:

- Cranial bones — 8, forming the protective cranium
- Facial bones — 14, forming the front of the skull
- Hyoid — 1, a U-shaped bone at the base of the buccal cavity
- Ear ossicles — 6, three in each middle ear: malleus, incus and stapes

The skull joins the vertebral column at two occipital condyles, so it is dicondylic.

Vertebral column — 26 vertebrae:

- Cervical 7, thoracic 12, lumbar 5, sacral 1 (fused), coccygeal 1 (fused)
- The first vertebra, the atlas, articulates with the occipital condyles

Sternum — 1, a flat bone on the front midline of the chest.

Ribs — 12 pairs:

- True ribs — 1st to 7th pairs; attached to thoracic vertebrae and directly to the sternum by hyaline cartilage
- False ribs — 8th, 9th and 10th pairs; joined to the 7th rib's cartilage instead of the sternum
- Floating ribs — 11th and 12th pairs; not joined at the front

Worked example — total the axial skeleton.

$$29+26+1+2\times 12=80\text{ bones}$$

An everyday example. Pressing along the middle of your chest, you can feel the flat sternum with ribs curving away on either side.

The substance. Each rib is bicephalic, with two articulation surfaces at its back end.

The appendicular skeleton has 126 bones — 30 in each of the four limbs, 2 in each half of the pectoral girdle, and 1 coxal bone in each half of the pelvic girdle.

Fore limb — 30 bones each:

- Humerus — upper arm
- Radius and ulna — forearm
- Carpals — 8, metacarpals — 5, phalanges — 14

Hind limb — 30 bones each:

- Femur — thigh; the longest bone
- Tibia and fibula — lower leg
- Tarsals — 7, metatarsals — 5, phalanges — 14
- Patella — a cup-shaped bone covering the knee

Pectoral girdle — each half has a clavicle and a scapula:

- Scapula — a flat triangular bone whose glenoid cavity holds the head of the humerus
- Clavicle — the collar bone

Pelvic girdle — two coxal bones, each formed by fusion of the ilium, ischium and pubis; the acetabulum receives the femur, and the two halves meet at the pubic symphysis.

Worked example — total the appendicular skeleton.

$$4\times 30+2\times 2+2\times 1=126\text{ bones}$$

An everyday example. Tapping the front of your knee, you can feel the patella slide as the knee bends.

The substance. **Axial and appendicular bones together give $80+126=206$**, the full adult skeleton.

Joints are grouped by how much they move: fibrous joints allow no movement, cartilaginous joints allow limited movement, and synovial joints, with a fluid-filled cavity, allow considerable movement and include ball-and-socket, hinge, pivot, gliding and saddle joints.

Three structural types:

- Fibrous joints — no movement; dense fibrous tissue joins bones, as in the sutures between flat skull bones
- Cartilaginous joints — limited movement; bones joined by cartilage, as between adjacent vertebrae
- Synovial joints — considerable movement; a fluid-filled synovial cavity between the articulating surfaces

Synovial joint types:

- Ball-and-socket — between the humerus and pectoral girdle; movement in many directions
- Hinge — the knee joint; movement in one plane
- Pivot — between the atlas and axis; rotation
- Gliding — between the carpals; bones slide over each other
- Saddle — between the carpal and metacarpal of the thumb

An everyday example. A bowler swinging an arm in a full circle uses the ball-and-socket joint at the shoulder.

The substance. Skull sutures are joints too — a joint does not have to move.

Myasthenia gravis is an autoimmune attack on the neuromuscular junction, tetany is rapid muscle spasm from low calcium, muscular dystrophy is inherited, progressive muscle degeneration, arthritis is joint inflammation, osteoporosis is age-related loss of bone mass, and gout is joint inflammation from uric acid crystals.

Muscle disorders:

- Myasthenia gravis — an autoimmune disorder affecting the neuromuscular junction, causing fatigue, weakening and paralysis of skeletal muscle
- Tetany — rapid spasms in muscle due to **low Ca${}^{2+}$ in body fluid
- Muscular dystrophy — progressive degeneration of skeletal muscle, mostly due to a genetic disorder

Bone and joint disorders:

- Arthritis — inflammation of joints
- Osteoporosis — an age-related disorder with decreased bone mass and a higher chance of fractures; low oestrogen is a common cause
- Gout — joint inflammation due to accumulation of uric acid crystals

An everyday example. Doctors often advise older people to get enough calcium, vitamin D and weight-bearing exercise to slow the bone loss of osteoporosis.

The substance. Gout links back to excretion** — uric acid, a nitrogenous waste, can crystallise in joints when it builds up in blood.