The locomotor system

Abstract

The locomotor system comprises the skeleton, composed principally of bone and cartilage, the joints between them, and the muscles which move bones at joints. The skeleton forms a supporting framework for the body and provides the levers to which the muscles are attached to produce movement of parts of the body in relation to each other or movement of the body as a whole in relation to its environment. The skeleton also plays a crucial role in the protection of internal organs. The skeleton is shown in outline in Figure 2.1A. The skull, vertebral column, and ribs together constitute the axial skeleton. This forms, as its name implies, the axis of the body. The skull houses and protects the brain and the eyes and ears; the anatomy of the skull is absolutely fundamental to the understanding of the structure of the head and is covered in detail in Section 4. The vertebral column surrounds and protects the spinal cord which is enclosed in the spinal canal formed by a large central canal in each vertebra. The vertebral column is formed from 33 individual bones although some of these become fused together. The vertebral column and its component bones are shown from the side in Figure 2.1B. There are seven cervical vertebrae in the neck, twelve thoracic vertebrae in the posterior wall of the thorax, five lumbar vertebrae in the small of the back, five fused sacral vertebrae in the pelvis, and four coccygeal vertebrae—the vestigial remnants of a tail. Intervertebral discs separate individual vertebrae from each other and act as a cushion between the adjacent bones; the discs are absent from the fused sacral vertebrae. The cervical vertebrae are small and very mobile, allowing an extensive range of neck movements and hence changes in head position. The first two cervical vertebrae, the atlas and axis, have unusual shapes and specialized joints that allow nodding and shaking movements of the head on the neck. The thoracic vertebrae are relatively immobile. combination of thoracic vertebral column, ribs, and sternum form the thoracic cage that protects the thoracic organs, the heart, and lungs and is intimately involved in ventilation (breathing).