First: the skeleton

The adult human skeleton consists of 206 bones, each with a shape and size suited to its job. It is divided into:

• Axial skeleton: the skull, the vertebral column and the thoracic cage (ribs and sternum). It forms the main axis of the body.
• Appendicular skeleton: the pectoral (shoulder) girdle and the upper (fore) limbs, and the pelvic (hip) girdle and the lower (hind) limbs.

The vertebral column is the main axis; at its upper end it bears the skull; it is connected to the thoracic cage and, through the shoulder bones, to the forelimbs, and through the pelvic bones to the hind limbs.

2.1.1 The vertebral column

It consists of 33 vertebrae in five groups that differ in shape according to the region:

Total: 7 + 12 + 5 + 5 + 4 = 33. Only 24 (7 + 12 + 5) are free and movable; 9 are fused (5 + 4), so an adult has effectively 26 separate units (24 + sacrum + coccyx).

Functions of the vertebral column:

• it is the main support of the body; it protects the spinal cord; it carries the skull and allows movement of the head and the upper body; it gives attachment to ribs and to many muscles.
• Cartilaginous discs between the vertebrae (cartilaginous joints) act as shock-absorbers and allow slight bending.

Structure of a typical vertebra. A bony vertebra has:

• an anterior, thick, solid part called the centrum (body);
• two transverse processes projecting laterally from it (for muscle and rib attachment);
• a posterior bony ring, the neural arch, which carries a bony process directed backwards and downwards, the neural spine;
• the neural arch and the back of the centrum enclose the neural canal through which the spinal cord passes and is protected;
• articular surfaces (facets) by which neighbouring vertebrae articulate.

Why are lumbar vertebrae the biggest? They carry the weight of the whole upper body, and so have the largest centra.

2.1.2 The skull (cranium)

The skull is a bony case with two parts:

1. The cranial (posterior) part: 8 flat bones, with serrated (saw-toothed) edges that interlock firmly through fibrous joints to form a cavity that protects the brain. At the base of the skull there is a large opening, the foramen magnum, through which the spinal cord passes to join the brain.
2. The facial (anterior) part: 14 bones that form the face, including the two jaws (the lower jaw — mandible — is the only movable bone of the skull), and containing the sockets/places for the sense organs (eyes, ears, nose).

2.1.3 The thoracic cage

A slightly conical (cone-shaped) cage formed of:

• the sternum (breastbone), an anterior flat bone, pointed at its lower end where it is cartilaginous;
• 12 pairs of ribs, curved bones bent downward. Each rib is joined at the back to the centrum and the transverse process of a thoracic vertebra.
• The first 10 pairs connect (directly or through cartilage) with the sternum in front. The last two pairs are short and do not reach the sternum; they are called floating ribs.

Function: protects the heart and lungs, and its movement helps in breathing.

2.1.4 The pectoral girdle and the upper limb

The pectoral girdle has two equal halves; each half has:

• the scapula (shoulder blade) — a triangular dorsal bone, broad at its inner end and pointed at its outer end where it carries a process joined to the clavicle. At the outer end there is a cup, the glenoid cavity, into which the head of the humerus fits, forming the shoulder joint.
• the clavicle (collar bone) — a thin ventral bone.

Each upper limb has:

1. Upper arm: one bone, the humerus.
2. Forearm: two bones, the radius and ulna. The upper end of the ulna has a depression into which the lower projection of the humerus fits (the elbow joint). The radius is smaller and can rotate around the fixed ulna, which allows the palm to turn up or down.
3. Wrist: 8 small bones, the carpals, in two rows. Their upper ends articulate with the radius; their lower ends with the bones of the palm.
4. Palm: 5 long thin bones, the metacarpals.
5. Digits (fingers): five, each of 3 bones called phalanges, except the thumb which has only 2.

Count: 1 + 2 + 8 + 5 + 14 = 30 bones in each arm and hand (plus the girdle).

2.1.5 The pelvic girdle and the lower limb

The pelvic girdle consists of two equal halves joined in front (ventrally) at the pubic symphysis. Each half has:

• the ilium (dorsal), attached in front and ventrally to the pubis, and behind and ventrally to the ischium.
• where the three bones meet there is a deep cup, the acetabulum, into which the head of the thigh bone (femur) fits to form the hip joint.

Each lower limb has:

1. Thigh: one bone, the femur (the longest, strongest bone in the body).
2. Leg (shank): two bones, the inner tibia and the outer fibula. The lower end of the femur has two projections that articulate with the tibia at the knee joint. In front of the knee is a small round bone, the patella (knee-cap).
3. Ankle: 7 irregular bones, the tarsals; the largest is at the back and forms the heel.
4. Instep: 5 long thin bones, the metatarsals.
5. Toes: five, each with 3 phalanges except the big toe, which has 2.

Why do limbs have girdles?

• The girdles connect the limbs to the axial skeleton, give a firm surface for the large muscles of the limbs, form the sockets (glenoid and acetabulum) of the ball-and-socket joints, and transfer the body weight (the pelvic girdle transfers weight from the vertebral column to the legs).
• They also protect organs (the pelvis protects the bladder and, in a woman, the uterus and ovaries).

Worked example 3 (counting). How many bones form the skull? 8 cranial + 14 facial = 22. How many thoracic cage bones? 12 pairs of ribs = 24 + 1 sternum = 25 (the book describes the structure; the number is not required but shows the logic). How many phalanges in one hand? Four fingers × 3 + thumb 2 = 14.