Human Anatomy - Kotsan I. Ya. 2009

Bones and Their Joints

One of the most essential properties of a living Organism is movement and spatial displacement. In mammals (including humans), this function is performed by The Musculoskeletal System, which consists of two parts: passive and active. The former includes Bones and Their Joints, while the latter comprises Muscles.

The Skeleton (from the Greek skeleton, meaning dried up or desiccated) is a complex of Bones and joints that form the rigid framework of The Human Body (Fig. 15). The skeleton serves multiple Functions. It defines the external shape of body segments and the organism as a whole. Skeletal bones form cavities that house vital Organs, protecting them from external influences: the cranial cavity contains the Brain, the spinal canal protects the Spinal Cord, the thoracic cavity encloses The Heart, major Blood Vessels, Lungs, and Esophagus, and the pelvic cavity contains the urogenital organs. Furthermore, skeletal bones act as levers set into motion by muscles, allowing body parts to change their position relative to one another and propelling the body through space. Ligaments, muscles, tendons, and fascia attach to the bones. Bones also serve as the body's primary mineral reservoir, storing excess minerals and releasing them as needed, thereby actively participating in Mineral METABOLISM. Additionally, bones contain Cytology/practical/86.html">Red Bone Marrow, which is responsible for hematopoiesis (blood Cell Formation) and immune functions.

The human skeleton consists of 206 bones—85 paired (170 in total) and 36 unpaired. In men, they account for 18% of total body mass, in women 16%, and in newborns 14%.

A bone (os) is the fundamental Structural and functional unit of the skeleton. Each bone comprising the skeleton functions as an organ formed by all tissue types, with Bone tissue playing the primary role. Bone tissue belongs to the group of Connective Tissues. It consists of Cells and a dense Extracellular matrix rich in Collagen and minerals. Together, these components determine the physical and Chemical properties of bones, namely their strength and elasticity.

Each bone contains approximately 50% Water, 22% other inorganic substances, and 28% organic matter (Proteins, fats, CARBOHYDRATES). Among the proteins, ossein predominates, whereas calcium phosphate constitutes the highest percentage of Inorganic Compounds (excluding water). Living bone also contains Vitamins A, D, C, etc. Bones used in anatomy classes have a different composition than those of a living human or cadaver because they undergo preliminary Processing, which includes maceration (soaking), degreasing, drying, and bleaching. These prepared bones consist of 1/3 organic matter and 2/3 inorganic compounds.

One of the CORE PROPERTIES OF bone—its mechanical strength—depends directly on The ratio of organic to inorganic substances. If a bone is immersed in an acid solution (such as hydrochloric or nitric acid), the mineral salts dissolve while the organic components remain. The shape of the bone is preserved, but it becomes soft and pliable. Conversely, if a bone is carefully calcined, the organic matter Burns away, leaving the inorganic compounds intact. While the bone retains its shape, it becomes extremely brittle. Thus, bone elasticity is determined by its organic content, whereas its strength relies on the quantity of mineral salts. The combination of organic and inorganic substances in living bone grants it both extraordinary strength and resilience. It is often said that bone possesses the tensile strength of copper and the elasticity of oak.

The ratio of bone tissue components, determined through chemical analysis, varies among individuals and can even change within the same person depending on age, diet, physical activity, and other factors. In childhood, the relative content of organic substances is higher, which is why children's bones are more flexible and elastic, fracturing less frequently. In elderly individuals, the organic content decreases, making bones more brittle and prone to fractures. There are two primary Types of bone tissue: coarse-fibrous (reticulofibrous) and lamellar. The former develops directly from mesenchyme and is characteristic mainly of embryonic and young organisms. It is distinguished by collagen bundles grouped into thick, coarse fibers that are arranged haphazardly in an amorphous matrix, intersecting in various directions with osteocytes scattered randomly among them. The latter, lamellar bone tissue, forms during the remodeling of coarse-fibrous tissue and the ingrowth of blood vessels. The bones of an adult human are built predominantly of lamellar bone tissue. Its framework consists of bone lamellae formed by osteocytes and fine-fibrous extracellular matrix. Within each lamella, Connective Tissue fibers run parallel to one another in a precise, orderly arrangement.

Depending on the arrangement of these bone lamellae, lamellar bone tissue is subdivided into two types: spongy (cancellous) and dense (compact).

In spongy bone tissue, the bone lamellae form trabeculae (bony struts) of varying thickness that intersect in multiple directions. The layout of these trabeculae is dictated by mechanical forces: the thicker trabeculae, composed of a greater number of lamellae, are oriented along lines of maximal stress. This architectural design achieves maximum structural strength with minimal weight. The spaces between the trabeculae are filled with red bone marrow.

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Fig. 15. The human skeleton

1 — Skull, 2 — Vertebral Column, 3 — thoracic cage, 4 — BONES OF THE upper free limb, 5 — coxal (hip) bone, 6 — bones of the lower free limb

In dense (compact) bone tissue, bone lamellae are arranged in an orderly fashion, forming sophisticated systems known as osteons. An osteon is the structural unit of a bone (Fig. 16). It consists of 5–20 bone lamellae arranged concentrically around a central (Haversian) canal that houses blood Vessels and nerves.

The spaces between osteons are filled with interstitial lamellae. The outer surfaces of tubular bones are covered by a system of outer circumferential lamellae, whereas the inner surfaces of bone cavities are lined with inner circumferential lamellae.

Unlike osteons, the arrangement of circumferential lamellae is not directly associated with blood vessels.

Osteons are not distributed randomly; their orientation corresponds to the physical loads applied to the bone: in tubular bones, they run parallel to the long axis of the bone, whereas in spongy bones, they lie perpendicular to the forces of compression and tension.

Dense (compact) bone tissue forms the outer layer of bones, beneath which lies the spongy substance. The distribution of compact and spongy bone varies across different skeletal elements, determined by the functional role of the bone and the specific mechanical forces of compression and tension it experiences. Compact bone predominates in the walls of the diaphyses of tubular bones, whereas spongy bone is dominant in the epiphyses of tubular bones, as well as in flat and short bones.

Externally, bones are covered by the periosteum, except at articular surfaces where they are covered by Cartilage.

The periosteum is a thin, durable connective tissue membrane that envelops the bone externally and attaches to it via bundles of connective tissue fibers (Sharpey's fibers) that penetrate the bone matrix through specialized canals. The periosteum performs protective, trophic (nutritive), neuroregulatory, and osteogenic (bone-forming) functions. It consists of two layers: an outer fibrous layer and an inner osteogenic (cellular) layer.

Fig. 16. Microscopic Structure of osteonic bone

1 — outer circumferential lamella, 2 — osteogenic layer of the periosteum, 3 — fibrous layer of the periosteum, 4 — lacunae containing osteocytes, 5 — canaliculi, 6 — cement line, 7 — interstitial lamella, 8 — osteon, 9 — inner circumferential lamella, 10 — blood vessel, 11 — endosteal lining, 12 — central canal, 13 — endosteum, 14 — Volkmann's canal

The outer layer of the periosteum is denser and composed of thick bundles of collagen fibers. It contains nerves and blood vessels that branch through apertures into the bone matrix, extending branches into the osteonic canals to nourish the bone.

The inner layer of the periosteum contains collagen and elastic fibers, along with a significant population of osteoblasts. Through proliferation, these osteoblasts differentiate into osteocytes, thereby driving bone growth in thickness and facilitating bone regeneration following injury.

Depending on their shape, structure, function, and development, bones are classified into tubular, spongy, flat, irregular (mixed), and pneumatic types (Table 4).

Table 4

Long bones are characterized by a predominance of compact bone tissue. The shape of long bones is approximately cylindrical.

Long bones can be divided into long (bones of the arm, thigh, forearm, and leg) and short (metacarpal and Metatarsal Bones, and Phalanges of the fingers and toes) (Fig. 17, 1).

A distinctive feature of long bones is that each has an elongated middle section, known as the shaft or diaphysis (diaphysis), and thickened ends called epiphyses (epiphysis). The epiphysis located closer to the trunk is termed the proximal epiphysis, while the one farther from the trunk is the distal epiphysis. The epiphyses feature articular surfaces (facies articulares) covered with hyaline cartilage, which serve to connect the bone with adjacent bones. In childhood and adolescence, a layer of cartilaginous tissue lies between the diaphysis and the epiphysis; this is known as the epiphyseal cartilage (cartilago epiphysialis) or the metaepiphyseal growth plate. Through the proliferation of cells in this plate, which deposit intermediate cartilage matrix, the bone grows in length. Long tubular bones grow in length from both ends (di-epiphyseal), whereas short tubular bones grow through the action of the epiphyseal cartilage on only one side (mono-epiphyseal). The articular cartilage also serves as a longitudinal growth zone, although of lesser significance. The ends of the diaphysis adjacent to the epiphyses (or epiphyseal cartilages in children) are called metaphyses (metaphysis). Bone prominences on the epiphyses where Muscle tendons and ligaments attach are called apophyses (apophysis). The interior of a long bone contains the medullary cavity (cavitas medullaris), which is lined with a very thin and delicate connective tissue membrane called the endosteum (endosteum). The medullary cavity is filled with bone marrow.

Fig. 17. Types of bones

1 — long bone, 2 — flat bone, 3 — spongy (short) bones, 4 — irregular bone

Bone marrow (medulla ossium) is an organ of hematopoiesis and biological defense of the organism. It also participates in the Nutrition, development, and growth of bone. The mass of bone marrow constitutes about 5% of body weight. Approximately half of the bone marrow is red bone marrow (medulla ossium rubra), and the other half is yellow bone marrow (medulla ossium flava). The structural framework of bone marrow consists of reticular connective tissue and cellular elements. Among the cellular elements of red bone marrow, blood stem cells predominate, giving rise to all other Blood Cells. The cellular elements of yellow bone marrow contain A large number of fat cells (lipocytes), which impart its characteristic yellow color. The bone marrow also contains osteoblasts (cells that produce bone tissue) and osteoclasts (cells that resorb bone). Bone marrow is permeated with nerves and blood vessels that nourish not only the bone marrow itself but also the inner layers of the bone. During the prenatal period and in newborns, all bone cavities contain red bone marrow, which is gradually replaced by yellow bone marrow over time. In adults, red bone marrow remains only in the trabecular spaces of spongy bone tissue within flat bones (Sternum, iliac wings), short bones, and the epiphyses of long bones. In the diaphyses of long bones, the medullary cavities of adults contain yellow bone marrow. During significant blood loss, when it is necessary to accelerate hematopoiesis in the body, a reverse process of partial replacement of yellow bone marrow with red bone marrow can occur.

Spongy bones are bones composed primarily of spongy (trabecular) bone tissue covered by a thin layer of compact bone (Fig. 17, 3). Spongy bones are categorized into long (Ribs), short (carpals, tarsals), and sesamoid bones (Patella, pisiform bone, sesamoid bones of the fingers and toes).

Flat bones are bones in which length and width exceed thickness (Fig. 17, 2). Flat bones are constructed of two thin plates of compact bone enclosing a layer of spongy bone. In the flat bones of the skull, this spongy layer is called the diploë (diploё) or middle layer. Flat bones participate in The formation of Body Cavities and limb girdles. They perform protective and supportive functions, and their surfaces serve as attachment sites for muscles. A typical example is the Parietal bone.

Irregular bones are bones formed by the fusion of several parts that differ in function, structure, and origin. Examples include the bones of the skull base and the vertebrae (the vertebral bodies are spongy bones, while their arches and processes are flat bones) (Fig. 17, 4).

Among flat and irregular bones, there are those that contain cavities lined with a mucous membrane and filled with air on the inside; these are termed pneumatic bones. Such a structure significantly reduces bone mass without compromising strength. Pneumatic bones include the Maxilla, frontal, sphenoid, and ethmoid bones.