Adaptation to the environment by movement is the principal property distinguishing an animal from a plant. In his book Twenty Years' Experience in Objective Study of Nervous Activity published in 1951, Pavlov wrote that the most important manifestation of the higher activity of an animal, i.e. its visible response to the external world, is movement, the result of the activity of its skeleto-muscular system. Three types of movement are encountered in the animal world: (1) amoeboid movement by means of pseudopodia, protrusions of the protoplasm, e.g. in amoeba; (2) ciliary movement by means of cilia, e.g. in infuso­ria; (3) muscular movement by means of special contractile muscle elements in most animals. In reflection of the process of phylogenesis, man has pre­served all three types of movement in his body: the amoeboid movement of the leucocytes, the oscillation of the cilia of the ciliated epithelium, and the contraction of special cell elements, muscle fibres, which compose com­plexes called muscles. Contraction of the muscles is responsible for all move­ments of the body and its organs. All the musculature in the organism can be classified as visceral or somatic. The visceral musculature is a component of the viscera located in the body; it consists for the most part of unstriated muscle cells and only partly of striated fibres (the cranial end of the alimentary tract, the muscles of the larynx and heart). The movements effected by it are mainly restricted to the viscera and do not change the position of the body itself in space. The somatic musculature, composed entirely of striated muscles, is located in the walls of the body (soma) cavities enclosing the viscera and also forms the main mass of the limbs. The movements performed by the somatic mus­culature are manifested by a change in the position of the whole body and its parts in their surroundings. The totality of the somatic musculature along with a small part of the skeletal muscles of the head (see section "Muscles of the Head") make up the locomotor system of the body to which the skeleton and its articulations also belong. Since in addition to the function of move­ment, this system also performs the function of bearing the weight of the body on the ground, it is also called the weight-bearing and locomotor sys­tem. It should be kept in mind that the human organism is born, develops, and exists under the earth's gravitational pull (L gravis heavy). Each move­ment of the body is concerned with overcoming this gravitation and the weight-bearing and locomotor system therefore performs an antigravitation function too. That is why it can also be called the antigravitation system. It is thus quite natural to distinguish the passive (the skeleton and its articulations) and active (muscles) parts of the locomotor system. Both parts are closely related functionally and originate from one and the same germ, the mesoderm. As a result, the weight-bearing and locomotor system is made up of three systems of organs: (1) bones, (2) their articulations or joints, and (3) muscles and their auxiliary devices. The weight-bearing and loco­motor system constitutes the greater part of the total body mass. Its organs account for 72.45 per cent of the body weight of an adult. The musculature makes up about one-fifth of the total body weight, the skeleton between one-seventh and one-fifth. This explains the fact that the weight-bearing and locomotor system is the main determinant of the shape of the body and its separation into the trunk, head, and limbs.