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Bone and Skeleton Structure

Q: What three bones fuse to form the hip bone (os coxae)?

In the adult, the hip bone (os coxae) is formed by the fusion of three bones: the ilium (os ilium), pubis (os pubis), and ischium (os ischii). They converge at the acetabulum, which serves as the socket for the femoral head.

Q: What is the function of apophyses?

Apophyses (apophysis) are pronounced bony projections that serve as strong attachment sites for muscles and ligaments.

Q: What is inside the medullary cavity?

The medullary cavity (cavitas medullaris), located inside the diaphysis of a tubular bone, contains yellow or red bone marrow.

Skeleton

For medical students2 min readUpdated 2026-10-10

The human skeleton (skeleton) is a complex framework of bones that provides structural support and protection for the body. In anatomy, each bone is considered an independent organ with a unique internal and external structure, and the entire skeletal system is traditionally divided into two main functional groups: axial and appendicular.

Skeletal DivisionsAxial skeleton (skull, vertebral column, thoracic cage) and appendicular skeleton (limbs).

Bone Tissue TypesComposed of compact bone externally and spongy (cancellous) bone internally.

MetaphysisA specialized growth zone located strictly between the diaphysis and epiphysis.

Blood SupplySupplied through a specialized nutrient foramen (*foramen nutricium*).

Classification of the Skeleton: Axial Division

The human skeleton (skeleton) is anatomically divided into two major parts. The first is the axial skeleton (skeleton axiale), which forms the central axis of the body and protects vital organs.

The axial skeleton includes the following structures:

Skull (cranium) — the bony framework of the head protecting the brain.
Vertebral column (columna vertebralis) — the main axis of the trunk.
Thoracic cage (skeleton thoracis) — the bony enclosure for the thoracic cavity organs.

Appendicular Skeleton

The second major part of the skeletal system is the appendicular skeleton (skeleton appendiculare). It includes the bones of the upper and lower limbs. For ease of study, each limb is divided into a girdle (connecting segment to the trunk) and the free part.

Bones of the Upper Limb (Ossa membri superioris)

The upper limb is designed for complex movements. Its skeleton is divided into:

Pectoral girdle (cingulum membri superioris): includes the scapula and clavicle.
Free part (pars libera): consists of the arm (humerus), forearm bones (radius — radius, ulna — ulna), and hand bones (ossa manus).

Bones of the Lower Limb (Ossa membri inferioris)

The lower limb bears the entire weight of the body and provides locomotion. Its components include:

Pelvic girdle (cingulum membri inferioris): represented by the massive hip bone (os coxae).
Free part: includes the thigh (femur), patella (patella), leg bones (tibia — tibia, fibula — fibula), and foot bones (ossa pedis).

Bone as an Organ: External Structure

When studying the skeleton, it is important to understand that each bone is a living organ. A classic example for study is a long tubular bone. Its macroscopic structure includes several distinct regions:

Diaphysis (diaphysis) — the central shaft or body of the bone.
Epiphysis (epiphysis) — the widened articular ends. These include the proximal epiphysis (epiphysis proximalis), located closer to the trunk, and the distal epiphysis (epiphysis distalis), located farther from the body center.
Metaphysis (metaphysis) — the transitional zone between the shaft and the end of the bone. This is the location of the epiphyseal plate (growth plate) responsible for longitudinal bone growth.
Apophysis (apophysis) — specialized bony projections. Their main function is to provide secure attachment sites for skeletal muscles and ligaments.

Internal Bone Architecture (Cross-Section)

A cross-section of a long tubular bone reveals its internal architecture. Bone tissue is distributed unevenly to form structures of varying density:

Compact bone (substantia compacta) — the dense outer layer providing mechanical strength to the diaphysis.
Spongy bone (substantia spongiosa) — the porous internal structure that predominates in the epiphyses.
Medullary cavity (cavitas medullaris) — the space inside the bone shaft containing bone marrow (yellow or red depending on age and location).

Any living organ requires a blood supply. Bone tissue is nourished via a specialized opening known as the nutrient foramen (foramen nutricium), through which blood vessels and nerves enter the bone.

Frequently asked questions

What types of bone marrow exist, and where are they specifically located in an adult human?

There are two types of bone marrow: red and yellow. In adults, they have distinct distributions:

Red bone marrow — located between the trabeculae of spongy bone in flat bones (sternum, hip bones, skull bones), vertebral bodies, ribs, and in the epiphyses and metaphyses of long bones (predominantly proximally).
Yellow bone marrow — fills the medullary cavities of the diaphyses of long bones.

Which structure enables a tubular bone to grow in thickness?

Tubular bones grow in thickness via the inner cambial layer of the periosteum (periosteum). The periosteum covers the outer surface of the bone and consists of two layers:

Cambial layer — the inner layer containing osteogenic cells responsible for appositional growth (thickness).
Fibrous layer — the outer protective layer that continues into ligaments and joint capsules.

How are skeletal bones classified by shape (osteological classification)?

Bones are classified by shape into long, short, and flat bones.

Long bones — include tubular bones of the free limbs and long spongy bones (ribs, sternum).
Short bones — divided into short tubular bones of the distal limbs and sesamoid bones developing within tendons (e.g., patella).
Flat bones — form limb girdles (scapula, hip bone) and provide large surface areas for muscle attachment.

What three bones fuse to form the hip bone (os coxae)?

In the adult, the hip bone (os coxae) is formed by the fusion of three bones: the ilium (os ilium), pubis (os pubis), and ischium (os ischii). They converge at the acetabulum, which serves as the socket for the femoral head.

What is the structural and functional unit of compact bone?

The structural and functional unit of compact bone is the osteon.

Osteon — consists of concentric lamellae of bone matrix arranged around a central canal (Haversian canal).

Central canals contain nerves, blood vessels, and lymphatic vessels. Osteons are separated from each other and the outer bone surface by a cementing substance.

What is the difference between the axial and appendicular skeleton?

The axial skeleton includes the structures of the central body axis (skull, vertebral column, thoracic cage), while the appendicular skeleton consists of the bones of the upper and lower limbs, including their girdles.

What is the metaphysis and what is its role?

The metaphysis is the region of a long bone located between the shaft (diaphysis) and the end (epiphysis). It functions as the growth zone for longitudinal bone elongation.

What is the function of apophyses?

Apophyses (apophysis) are pronounced bony projections that serve as strong attachment sites for muscles and ligaments.

What is inside the medullary cavity?

The medullary cavity (cavitas medullaris), located inside the diaphysis of a tubular bone, contains yellow or red bone marrow.

Go deeper

Anatomy of the skull (Cranium) and vertebral column (Columna vertebralis)
Structure of the thoracic cage (Skeleton thoracis)
Detailed analysis of hand bones (ossa manus)
Topography of foot bones (ossa pedis)
Characteristics of red and yellow bone marrow