The human body has hundreds of muscles. The three types are skeletal(骨骼的) muscles, smooth muscles and cardiac, or heart muscles. Each kind of muscle has a special job to do. The skeletal muscles are attached to the bones; they actually hold the skeleton together. Strong cords, called tendons(肌腱), help connect these muscles to the bones. Skeletal muscles give us strength and allow us to move in many ways. If you look at skeletal muscles through a microscope, you will see that it has bands of fibers that look like stripes(条纹). These stripes are called striations. Skeletal muscles usually move because the brain has sent a conscious message to them telling them what to do. This is called voluntary movement. Another type of muscle is smooth muscle. Unlike skeletal muscle, smooth muscle does not have striations. This is found in organs such as the stomach, the bladder(膀胱), the lungs and the eyes. Smooth muscles are smaller than the skeletal muscles and they move involuntarily. The brain tells these muscles what to do, but we aren’t even aware of it. For example, we don’t have to think about breathing or digesting food. The smooth muscles just do their jobs automatically. For this reason smooth muscles are sometimes called involuntary muscles. Like smooth muscle, cardiac muscle also works involuntarily. It is a very thick muscle that is found only in the walls of the heart. Its job is to pump blood out of the heart and into blood vessels called arteries(动脉). The arteries help carry the blood to all parts of the body. When the cardiac muscles relax, they let blood back into the heart. Cardiac muscles have striations, just like skeletal muscles do. 小题1:When we _______, the job is done by smooth muscles.
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