Every day we use our muscles and joints to carry out voluntary movements. Many of these actions, such as walking, demand very little concentration, but complex tasks, such as playing the piano, require more conscious effort, supported by a subconscious system of coordination learned when the skill was first mastered. All movements are based on mechanical changes in the muscles, which contract or relax, making specific bones pivot, hinge, rotate, or glide at the joints.
In this section
Skeletal muscles move the body at its joints by contracting. In addition, they maintain a steady tension, or tone, that gives the body the support it needs to maintain its posture, such as keeping the head upright on the neck. This postural tone is automatic but does require alertness. Unlike horses, people cannot sleep standing up.
There are two other types of muscle in the body: cardiac muscle, found only in the heart; and smooth muscle, occurring in hollow organs such as the intestines. Most muscle of these types is not under conscious control.
Types of bone
Bones come in many shapes and sizes, ranging from the flat bones found in the skull to the long bones of the limbs. The outer layer of each bone is made of dense, heavy, compact bone. The inner layer consists of spongy bone that is made up of numerous trabeculae (struts) arranged in such a way that they provide maximum support without excessive weight.
Bone gives the body shape and supports the body’s structure. It is a living tissue, which is constantly being renewed. Bone also serves as a reservoir for various minerals, such as calcium and phosphorus. Bone marrow, the soft, fatty substance that fills the cavities in bones, produces most of the body’s blood cells (see Formation of blood cells).
Joints, which are formed where bones meet, are covered with lubricated cartilage that allows smooth movement. The range of movement of the joints is determined both by their structure and by the ligaments that stabilize and support them; the hip joint, for example, moves less freely than the shoulder. In contrast, the joints in the wrist, foot, and spine sacrifice mobility for stability; their bones are joined by strong, inflexible ligaments that allow little movement. Most of the skull bones fuse together and become immobile once growth has ceased.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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