Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Musculoskeletal system
Structure of the skeleton
General overview of the skeleton

Throughout phylogeny, the Skeleton passes through three Selection/3.html">Stages of development: Connective Tissue (membranous), cartilaginous, and bony. In primitive Chordates (such as the lancelet), the skeleton is represented by the notochord, around which a membranous skeleton is formed. In adult cartilaginous fish, the skeleton remains cartilaginous, whereas in higher vertebrates, it is replaced by bone. Human Embryogenesis repeats the succession of these three skeletal development stages.

Chemical Composition and Structure of bones. Bone tissue consists of organic (ossein) — 1/3 and inorganic (2/3) (mainly calcium salts, 95%) substances. If a bone is treated with a Hydrochloric acid solution, the calcium salts dissolve, while the organic matter remains, preserving the shape of the bone. Such decalcified bone becomes exceptionally elastic and easily deforms. If, however, the bone is calcined (burned), the organic matter Burns away, and the inorganic matter remains. This bone retains its original shape but becomes extremely brittle. It can shatter at the slightest Touch. With age, the quantitative ratio of ossein to mineral salts changes. Children's bones contain more ossein and are therefore more elastic. In old age, bones contain more mineral salts, with their content reaching up to 80%. Consequently, the bones of elderly people are more brittle, and they often suffer fractures from falls.

Bones buried in the ground lose organic matter due to bacterial activity and become brittle. In dry soil, bones are preserved better because Bacteria require moisture to multiply. Such bones gradually mummify. In calcareous soil, bones become impregnated with calcium, or "fossilized."

The strongest bone in our skeleton is the Tibia; it bears the greatest weight when maintaining an upright body posture. This bone can withstand a load of up to 1650 kg, which is approximately 25 times its normal load. Such is the safety margin of this natural engineering design.

Bone is unique not only for its combination of hardness and elasticity, determined by its chemical composition, but also for its exceptional lightness. This is due to the peculiarities of its microscopic structure. The surface of the bone is covered by the periosteum (Fig. 1.1). It consists of two layers: an outer (connective tissue) layer and an inner osteogenic layer containing osteogenic stem Cells and osteoblasts. In bone fractures, osteoblasts "heal" the gap with woven bone tissue, forming a "bone callus." The periosteum is rich in nerves and Blood Vessels, through which the bone is nourished and innervated. A cross-section of the bone reveals its heterogeneous structure. Located On the surface is the so-called dense, or compact, bone (substantia compacta), while deep inside lies the spongy, or cancellous, bone (substantia spongiosa) (Fig. 1.2). The thickness of the compact bone layer varies depending on the load experienced by the bone and is most significant in the diaphysis region. Spongy bone is formed by very thin bone trabeculae, which are arranged not randomly, but in accordance with the distribution of functional loads on the entire bone or its parts. The epiphyses of long bones, all short bones, and some mixed and flat bones—that is, the light and strong PARTS OF THE skeleton that experience stress in various directions—consist predominantly of spongy bone. The diaphyses and some thin flat bones are almost completely devoid of spongy bone. They perform the Functions of support and movement.

Class="center">

Fig. 1.1. Tibia (part of the periosteum is cut and peeled back):

1 — outer and 2 — inner layers of the periosteum; 3 — anterior surface of the bone, stripped of the periosteum

Fig. 1.2. Proximal end of the Femur: A — frontal section: 1 — medullary cavity; 2 — spongy bone; 3 — compact bone; B — diagram of trabecular alignment in spongy bone

The structural unit of bone tissue is the osteon, or Haversian system (Fig. 1.3). An osteon is a system of bone lamellae in the form of concentric cylinders nested inside one another, between which lie bone cells called osteocytes. The Haversian canal, located in the center of the osteon, contains blood vessels that provide METABOLISM for the bone cells. Interstitial lamellae are located between the osteons. Compact bone and the trabeculae of spongy bone are composed of osteons. The distribution of compact and spongy bone depends on the functional conditions of the bone.

Fig. 1.3. Diagram of The structure of a long (tubular) bone:

A — periosteum; B — compact bone; C — endosteum; D — medullary cavity. 1 — interstitial lamellae; 2 — outer circumferential lamellae; 3 — blood vessels; 4 — osteocytes; 5 — osteon canal; 6 — perforating (Volkmann's) canal; 7 — fibrous layer of the periosteum; 8 — bone trabecula of spongy bone; 9 — inner circumferential lamellae; 10 — osteon

The spaces of spongy bone are filled with Cytology/practical/86.html">Red Bone Marrow. Yellow bone marrow is located in the central canal of long bones—the medullary cavity.

In adults, the entire cavity is filled with yellow bone marrow, but during the GROWTH AND DEVELOPMENT of a child, when intensive hematopoietic function is required, red bone marrow predominates. With age, it is gradually replaced by yellow marrow.

Bone shape. The bones that make up the skeleton account for approximately 10% of total body weight.

The size and shape of bones depend on their function. Bones are classified as long, short, flat (broad), and mixed.

Long bones are found, for example, in both limbs; their length significantly exceeds their other dimensions. The value of a bone as a lever to which Muscles attach increases with its length.

The relatively thinner middle part of long bones is called the body or diaphysis, and the thickened ends are called epiphyses.

The diaphysis of long bones contains an internal cavity, which is why these bones are also referred to as tubular. The thickening of the ends of long bones is functionally justified. The epiphyses serve as articulation sites for bones with one another, and this is where muscles attach. The wider the contact surface between the bones, the stronger and more stable their connection. At the same time, the thickened epiphysis distances the Muscle from the long axis of the bone, causing the muscle to approach its attachment site at a larger angle, thereby increasing its effective force in accordance with the parallelogram rule.

Short bones are located in the wrist (carpus) and ankle (tarsus), i.e., where both high strength and mobility of the skeleton are required simultaneously. These bones have a cubic or irregular shape and are of almost equal dimensions in all directions.

Flat (broad) bones form the walls of cavities containing Internal Organs. Such bones are concave on one side and convex on the other; their width and length significantly exceed their thickness. Examples include the hip bone, scapula, and cranial bones.

Mixed bones, such as the Sphenoid bone in the Skull, have various shapes, The complexity of which corresponds to the variety of functions they perform.

Among the flat and mixed BONES OF THE skull, there are pneumatic bones containing a cavity lined with mucous membrane and filled with air, which reduces the weight of the bone without compromising its strength (for example, the Maxilla and sphenoid bones).

Bones are directly involved in the metabolism of the entire body and undergo constant remodeling in response to changes in dynamic load. For example, when A change in occupation alters the load on a bone, old osteons are resorbed and new ones are formed to adapt to the new loading conditions.

Bone Development and growth. During ontogeny, as in phylogeny, the skeleton passes through three stages: connective tissue (mesenchymal), cartilaginous, and osseous. Bones arising from the mesenchyme are called primary or membrane bones (bones of the cranial vault). Most bones develop through a cartilaginous stage; these are called secondary or replacement bones.

In the second month of intrauterine life, the human skeleton is a cartilaginous structure. The cartilaginous Skeleton of the human embryo, in its rough outlines, resembles the fossil skeletons of ancient amphibians. Not all bones are represented yet; for example, the cranial vault is not yet formed, and many facial bones are absent—membrane bones will develop here later. Crucially, embryonic Cartilage does not turn into bone but is replaced by it.

A distinction is made between intracartilaginous, or endochondral, ossification and perichondral ossification, in which bone tissue first appears on the surface of the cartilage, beneath the perichondrium covering it (Fig. 1.4).

Fig. 1.4. Successive stages of tibia development:

Dots indicate parts ossifying endochondrally; cross-hatching indicates perichondrally; white indicates cartilage, and black indicates the medullary cavity of the diaphysis

Endochondral ossification occurs within the cartilaginous precursors. Bones consisting predominantly of cancellous bone (vertebrae, Sternum, etc.) develop endochondrally, while those consisting of both cancellous and compact bone develop both endo- and perichondrally.

Subsequently, new layers of bone tissue are continually deposited on the bone surface beneath the periosteum, allowing the bone to grow in thickness. Simultaneously, the medullary cavity is formed inside the bone through the resorption of its cancellous bone. The longitudinal growth of the diaphyses of long bones occurs due to the epiphyseal growth plate, a layer of cartilage between the epiphysis and diaphysis that persists throughout childhood and adolescence. The epiphyses of long bones remain cartilaginous for a long time, and endochondral ossification centers appear in them sequentially only During the first decade of life.

The onset of physical maturity in humans is marked by the completion of skeletal growth and the fusion (synostosis) of individual ossification centers in each bone. Skeletal development in men is completed by age 20—24, and in women 2—3 years earlier. Consequently, linear growth in women ceases at age 18—21. Throughout the entire growth period, the mass of the bony skeleton increases nearly 24-fold.

Even after growth ceases and full physical maturity is reached, the human skeleton does not stop changing. Thus, by old age, the balance between the rates of Bone Formation and resorption is disrupted; bones become thinner, their articular ends deform, and the medullary spaces and Paranasal Sinuses enlarge. Sex differences also affect The rate of skeletal Aging, the first signs of which appear earlier in women (starting at age 45—50).

Bone formation is influenced by environmental factors. The level of skeletal mineralization is largely related to diet. Indeed, it has been shown that children on diets deficient in Proteins, fats, and minerals have lower skeletal mineralization than their well-nourished peers. Mineral deficiency and the resulting Osteoporosis, or thinning of bone tissue, are particularly pronounced in areas where desalinated Water is used for drinking. Severe illnesses, vitamin and mineral deficiencies, and endocrine gland dysfunction also delay skeletal growth and development. In high-altitude conditions, the medullary cavity enlarges while the area of compact bone decreases. This feature is due to increased hematopoietic activity of the bone marrow under conditions of high-altitude Hypoxia.

Parts of the skeleton. The human skeleton is divided into three main parts: the Skeleton of the Trunk, the Skeleton of the limbs, and the Skeleton of the HEAD, or skull (Atl., Fig. 2).

The skeleton of the trunk, or Axial Skeleton, consists of THE Vertebral Column and the rib cage. It clearly exhibits a characteristic feature of vertebrate structure: segmentation, or metamerism. In primitive vertebrates, each segment of the trunk skeleton is formed by a vertebra and a pair of Ribs. In mammals and humans, complete segments are preserved only in the thoracic region of the trunk.

The skeleton of the limbs—upper and lower—is typically divided into the skeleton of the free limb (arms and legs)

and the skeleton of the girdle (pectoral and pelvic), which anchors it to the trunk. The skeleton of the arm consists of three regions: the upper arm, the forearm (formed by the radius and ulna), and the hand, while the Pectoral Girdle consists of two paired bones: the scapula and clavicle. The skeleton of the leg is also divided into three regions: the thigh, the lower leg (comprising the tibia and Fibula), and the FOOT, while the Pelvic Girdle consists of the paired hip bones. The skeletons of the hand and foot each consist of three parts: the hand comprises the carpus, metacarpus, and Phalanges of the fingers, while the foot includes the tarsus, metatarsus, and phalanges of the toes.

Compared to other parts of the skeleton, the skull is particularly complex in structure, which is explained by the diverse origins of its individual parts. The skull houses the Brain and forms the bony framework for the initial segments of the digestive and respiratory tracts. Accordingly, it is divided into two parts: the cranial and facial divisions. The bones of the skull are firmly joined together, and many of them fuse completely in adults.