[单项选择]Are you aware that you actually (47)______ six senses The sixth is a muscular sense responsible for directing your muscles intelligently to the exact (48)______ necessary for each action you perform. For example, when you reach for an object, the sensory nerves linking the muscles to the brain stop your hand at the (49)______ spot. This (50)______ perception of the position of your muscles in (51)______ to the object is your muscular sense in action. Muscles are stringy bundles of fibers (52)______ from one five thousandth of an inch to about three inches. They have three (53)______ characteristics: they can become shorter and thicker; they can stretch; and they can retract to their (54)______ position. Under a high-powered microscope, muscle tissue is seen as long, slender cells with a grainy texture like wood. More than half of a person’’s body is (55)______ of muscle fibers, most of which are involuntary, in other words, work without conscious direction. The voluntary mus