Muscles are the active part of the motor apparatus; their contraction produces various movements.
The muscles may be divided from a physiological standpoint into two classes: the voluntary muscles, which are under the control of the will, and the involuntary muscles, which are not.
All muscular tissues are controlled by the nervous system. The involuntary muscles are controlled by a specialized part of the nervous system.
When muscular tissue is examined under the microscope, it is seen to be made up of small, elongated threadlike cells, which arc called muscle fibres, and which are bound into bundles by connective tissue.
There are three varieties of muscle fibres:
1) striated muscle fibres, which occur in voluntary muscles;
2) unstriated muscles which bring about movements in the internal organs;
3) cardiac or heart fibres, which are striated like (1), but are otherwise different. Both unstriated and cardiac muscles are involuntary. All living cells can move to some degree, but this ability is highly developed in muscles. Muscle tissue comprises about 40% of human weight. Muscle consists of threads, or muscle fibers, supported by connective tissue, which act by fiber contraction: the fibers can shorten to two – thirds of their resting length. There are two types of muscles smooth and striated. Smooth, or «involuntary» muscles are found in the walls of all the hollow organs and tubes of the body, such as blood vessels and intestines. These react slowly to stimuli from the autonomic nervous system. The striated, or «voluntary» muscles of the body mostly attach to the bones and move the skeleton. Under the microscope their fibres have a cross – striped appearance. Striated muscle is capable of fast contractions. The heart wall is made up of special type of striated muscle fibres called cardiac muscle. Muscles vary greatly in structure and function in different organs and animals: some invertebrates have only smooth muscles, while all the arthropods have only striated muscles. The body is composed of about 600 skeletal muscles. In the adult about 35-40% of the body weight is formed by the muscles. According to the basic part of the skeleton all the muscles are divided into the muscles of the trunk, head and extremities.
According to the form all the muscles are traditionally divided into three basic groups: long, short and wide muscles. Long muscles compose the free parts of the extremities. The wide muscles form the walls of the body cavities. Some short muscles, of which stapedus is the smallest muscle in the human body, form facial musculature.
Some muscles are called according to the structure of their fibres, for example radiated muscles; others according to their uses, for example extensors or according to their directions, for example, – oblique. The muscles are formed by a mass of muscle cells. The muscle fibres are connected together by connective tissue. There are many blood vessels and nerves in the muscles.
Great research work was carried out by many scientists to determine the functions of the muscles. Three basic methods of study were used: experimental work on animals, the study of the muscles on a living human body and on the cadavers. Their work helped to establish that the muscles were the active agents of motion and contraction.