Muscle
Muscle tissue, which comprises about 40 percent of human body weight, consist of threads, or muscle fibers, supported by connective tissue. All living cells can move to some degree, but this ability is highly developed in muscles, which act by fiber contraction; the fibers can shorten to two thirds of their resting length. Muscle vary greatly in structure and function in different organs and animals. Based on structure there are two types of muscles; smooth and s treated, "Involuntary", or smooth, muscles are found in the walls of all the hollow organs and tubes of the body (for example, blood vessels and intestines). There react slowly to stimuli from the autonomic nervous system. The "voluntary," or striated, muscles of the body attach mostly to the bones to move the skeleton, and under the microscope their fibers have a cross striped appearance. Striated muscle is capable of fast contractions. The heart wall is made up of special muscle fibers (cardiac muscle),