The bone system performs a number of functions having either predominantly mechanical or predominantly biological significance. Let’s consider functions that have a predominantly mechanical meaning. All vertebrates are characterized by an internal skeleton, although among them there are species that, along with the internal skeleton, also have a more or less developed external skeleton that occurs in the skin (bony scales in the skin of fish). At the beginning of its appearance, the solid skeleton served to protect the body from harmful external influences (the external skeleton of invertebrates). With the development of the internal skeleton in vertebrates, it first became a support and support (framework) for soft tissues. Individual parts of the skeleton turned into levers driven by muscles, as a result of which the skeleton acquired a locomotor function. As a result, the mechanical functions of the skeleton are manifested in its ability to provide protection, support and movement. Support is achieved by attaching soft tissues and organs to various parts of the skeleton. Movement is possible due to the structure of bones in the form of long and short levers connected by movable joints and driven by muscles controlled by the nervous system. Finally, protection is carried out by forming from individual bones a bone canal – the vertebral canal, which protects the spinal cord; a bone box – the skull, which protects the brain; a bone cell – the thoracic, which protects the vital organs of the thoracic cavity (heart, lungs); a bone receptacle – the pelvis, which protects the reproductive organs important for the continuation of the species. The biological function of the bone system is associated with the participation of the skeleton in metabolism, especially in mineral metabolism (the skeleton is a depot of mineral salts – phosphorus, calcium, iron, etc.). It is important to take this into account for understanding metabolic diseases (rickets, etc.) and for diagnosis using radiant energy (X-rays, radioactive isotopes). In addition, the skeleton also performs a hematopoietic function. At the same time, the bone is not just a protective case for the bone marrow, but the latter is an organic part of it. A certain development and activity of the bone marrow affect the structure of the bone substance, and, conversely, mechanical factors affect the function of hematopoiesis: increased movement promotes hematopoiesis; therefore, when developing physical exercises, it is necessary to take into account the unity of all skeletal functions.

The following parts are distinguished in the skeleton: the skeleton of the trunk (vertebrae, ribs, sternum), the skeleton of the head (bones of the skull and face), the bones of the girdles of the limbs – upper (scapula, clavicle) and lower (pelvic) and the bones of the free limbs – upper (shoulder, forearm and hand bones) and lower (hip, bones lower legs and feet). The number of individual bones that make up the skeleton of an adult is more than 200, of which 36-40 are located along the midline of the body and are unpaired, the rest are paired bones. According to the external shape, long, short, flat and mixed bones are distinguished. However, this division, established in Galen’s time by only one feature (external shape), turns out to be one-sided and serves as an example of the formalism of the old descriptive anatomy, as a result of which bones that are completely heterogeneous in their structure, function, and origin fall into one group. Thus, the group of flat bones includes the parietal bone, which is a typical integumentary bone that ossifies endosmally, and the scapula, which serves for support and movement, ossifies on the basis of cartilage and is built of ordinary spongy substance. Pathological processes also occur quite differently in the phalanges and bones of the wrist, although both belong to short bones, or in the hip and rib, enrolled in the same group of long bones. Therefore, it is more correct to distinguish bones based on 3 principles on which any anatomical classification should be based: shape (structure), function and development.

At the lowest stages of organization, as well as in the embryonic period, in all vertebrates, the first rudiment of the internal skeleton is the dorsal string, the chorda dorsalis, originating from the mesoderm. The chord is a characteristic feature of the lowest representative of the chordate type, the lanceolate (Amphioxus lanceolatus), in which the skeleton consists of a dorsal dorsal string stretched along the body and surrounding connective tissue. In lower vertebrate species [round-mouthed, selachia (sharks), and cartilaginous ganoids], the connective tissue skeleton around the chord and throughout the rest is replaced by a cartilaginous skeleton, which in turn becomes bony in more highly organized vertebrates, starting with bony fish and ending with mammals. With the development of the latter, the chord disappears, with the exception of insignificant remnants (the gelatinous nucleus of the intervertebral disc). Aquatic forms could manage with a cartilaginous skeleton, since the mechanical load in the aquatic environment is incomparably less than in the air. But only the bony skeleton allowed the animals to come out of the water onto land, lift their bodies off the ground and stand firmly on their feet. Thus, in the process of phylogeny, as a phenomenon of adaptation to the environment, there is a sequential change of 3 types of skeleton. This change is repeated in the process of human ontogenesis, during which 3 stages of skeletal development are observed.: 1) connective tissue (membranous), 2) cartilaginous and 3) bony. Almost all bones go through these 3 stages of development, with the exception of the bones of the cranial vault, most of the bones of the face, and parts of the collarbone, which arise directly from the connective tissue, bypassing the cartilage stage. These integumentary bones, as they are called, can be considered as derivatives of the once-former outer skeleton, which shifted deep into the mesoderm and joined the inner skeleton as its complement in further evolution. Source: Meduniverse