Skeletal classification: axial division
The human skeleton (skeleton) is anatomically subdivided into two major parts. The first of these is the axial skeleton (skeleton axiale), which forms the central support of our body and protects vital organs.
The axial skeleton includes the following structures:
- Skull (cranium) — the bony framework of the head that protects the brain.
- Vertebral column (columna vertebralis) — the main axis of the trunk.
- Thoracic cage (skeleton thoracis) — the receptacle for the organs of the thoracic cavity.
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 convenience of study, each limb is divided into a girdle (connecting link to the trunk) and the free part.
Bones of the upper limb (Ossa membri superioris)
The upper limb is designed to perform 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), bones of the forearm (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 ensures locomotion. Its composition includes:
- 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 every bone is a living organ. A classic example for study is a long tubular bone. Its macroscopic structure includes several distinct parts:
- Diaphysis (diaphysis) — the central part, or shaft of the bone.
- Epiphyses (epiphysis) — the thickened articular ends. There is the proximal epiphysis (epiphysis proximalis), located closer to the trunk, and the distal epiphysis (epiphysis distalis), located farther from the center of the body.
- Metaphysis (metaphysis) — the transitional zone between the shaft and the end of the bone. This is precisely where the growth zone is located, through which the bone lengthens.
- Apophyses (apophysis) — specialized bony prominences. Their main function is to provide secure attachment for skeletal muscles and ligaments.
Internal structure of the bone (cross-section)
If a long tubular bone is sawn through, its internal architecture can be studied in detail. Bone tissue is distributed unevenly and forms structures of varying density:
- Compact bone (substantia compacta) — the dense outer layer providing the mechanical strength of 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 (which can be yellow or red depending on age and localization).
Any living organ needs blood supply. The nutrition of bone tissue is carried out through a special opening appropriately called the nutrient foramen (foramen nutricium). Blood vessels and nerves penetrate into the bone through it.
Briefly: answers to topical questions
What types of bone marrow exist and where exactly are they localized in an adult human?
There are two types of bone marrow: red and yellow. In an adult, they have different localizations:
- Red bone marrow — located between the trabeculae of spongy bone tissue in flat bones (sternum, pelvic bones, skull bones), in the vertebral bodies, ribs, and also in the epiphyses and metaphyses of long tubular bones (primarily in the proximal parts).
- Yellow bone marrow — fills the medullary cavities of the diaphyses of tubular bones.
Due to which structure does a tubular bone grow in thickness?
A tubular bone grows in thickness due to the inner cambial layer of the periosteum (periosteum). The periosteum covers the bone externally and consists of two layers:
- Cambial layer — the inner layer containing osteogenic cells, responsible for bone growth in thickness.
- Fibrous layer — the outer layer that performs a protective function and continues into ligaments and joint capsules.
How are skeletal bones classified by shape (osteological classification)?
By shape, skeletal bones are classified into long, short, and flat.
- Long bones — include the tubular bones of the free limbs and long spongy bones (ribs, sternum).
- Short bones — are divided into short tubular bones of the distal limb parts and sesamoid bones developing in muscle tendons (e.g., the patella).
- Flat bones — form the limb girdles (scapula, hip bone) and provide a large surface area for muscle attachment.
What three bones make up the hip bone (os coxae)?
In an 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 join in the region of the acetabulum (acetabulum), which serves as the socket for the head of the femur.
What is the structural and functional unit of compact bone?
The structural and functional unit of compact bone is the osteon.
- Osteon — consists of collagen bone lamellae arranged circularly around the central canal of the osteon (the Haversian canal system).
Nerves, blood vessels, and lymphatic vessels run inside the osteon canals. The osteons themselves are separated from each other and from the outer surface of the bone by a special cementing substance.
Class and exam questions
What is the difference between the axial and appendicular skeleton?
The axial skeleton includes the structures of the central body axis (skull, spine, rib 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 a section of a long tubular bone located between its shaft (diaphysis) and end (epiphysis). It functions as the growth zone of the bone in length.
What are apophyses used for?
Apophyses (apophysis) represent pronounced bony prominences. They serve as sites for the strong attachment of muscles and ligaments to the bone.
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.
What else to study on this topic
- Anatomy of the skull (Cranium) and vertebral column (Columna vertebralis)
- Structure of the thoracic cage (Skeleton thoracis)
- Detailed analysis of the bones of the hand (ossa manus)
- Topography of the bones of the foot (ossa pedis)
- Characteristics of red and yellow bone marrow
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