Axial Skeleton

Role of the axial skeleton in the musculoskeletal system

The human musculoskeletal system comprises more than 200 bones in total. This entire complex system is anatomically and functionally divided into two large groups: the axial skeleton (Skeleton axiale) and the appendicular skeleton (Skeleton appendiculare).

The axial skeleton forms the central support of our body. It consists of three main divisions:

• Skull (Cranium) — includes 29 bones.
• Vertebral column (Columna vertebralis) — consists of 26 bones.
• Thoracic cage (Skeleton thoracis) — formed by 25 bones.

By comparison, the appendicular skeleton includes 126 bones and consists of the girdles and free parts of the upper and lower extremities (shoulder, forearm, hand, thigh, leg, foot).

Vertebral column: structure and biomechanics

The vertebral column (Columna vertebralis) acts as the main axis of the body. It has a pronounced metameric structure, meaning it consists of sequentially arranged segments — vertebrae. From the perspective of osteology, vertebrae belong to short spongy bones.

This structure performs three crucial functions in the body:

1. Supportive — serves as a reliable axial framework for the entire torso.
2. Protective — forms a bony canal that protects the spinal cord from damage.
3. Locomotor — ensures diverse movements of the torso and skull.

For adequate load distribution, the human spine has physiological curvatures in the sagittal plane. Curvatures convex anteriorly are called lordoses (they form in the cervical and lumbar regions). Curvatures convex posteriorly are called kyphoses (present in the thoracic and sacral regions).

Regions of the spine

The vertebral column is subdivided into five regions, each having its own Latin designation and a strictly defined number of vertebrae:

Phylogeny and embryonic development

The development of the musculoskeletal system in chordate animals is inextricably linked to the formation of the axial organ complex. During neurulation, embryos develop the neural tube, gut, and notochord — the primary axial skeletal element. The fact that these organs have the same origin and mutual arrangement in all chordates proves their complete homology and evolutionary continuity. That is why a notochord is initially laid down in the human embryo, which is completely replaced by the spine during development.

Evolutionarily, the axial skeleton has traveled a long path. In lower acrania, the notochord persists throughout life. In primary aquatic vertebrates (fish and amphibians), it regresses, giving way to a cartilaginous and then bony spine. The skull skeleton also successively passes through membranous, cartilaginous, and bony stages. The visceral skeleton, which in fish is represented by gill arches, transforms in higher vertebrates into the jaw apparatus and auditory ossicles of the middle ear. Understanding this process is critical for explaining ontophylogenetic anomalies of the maxillofacial region in humans.

Furthermore, the axial complex obeys the laws of topographic coordinations — stable connections between spatially close organs. In chordates, the development of the musculoskeletal axis is historically associated with the nervous system and gill slits. In mammals, this coordinated complex is complemented by the appearance of the diaphragm, the left aortic arch, hair, and mammary glands.

Histological structure of bones

The bones forming the axial skeleton have a complex tissue structure. Externally, they are covered by the periosteum — a connective tissue membrane containing an osteogenic layer responsible for bone growth and repair.

The internal structure is represented by two types of tissue:

• Compact bone — is built of osteons. Each osteon is a system of 5–20 lamellae arranged concentrically around a central canal.
• Spongy bone — consists of bone trabeculae. They are not arranged randomly, but strictly along lines of compression and tension, which grants the bone maximum strength.

Tubular bones of the appendicular skeleton are also characterized by macroscopic division into diaphysis, epiphyses, and metaphyses.

Class and exam questions

What else to study on this topic

• Appendicular skeleton: structure of girdles and free limb parts
• Anatomy of tubular bones: diaphysis, metaphyses, and epiphyses
• Ontophylogenetic developmental anomalies of the maxillofacial region
• Topographically coordinated traits of the class Mammalia

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Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.