Lecture Notes in Human Anatomy - Hryhorieva O.A., Svitlytskyi A.O. 2020

Anatomy of the Musculoskeletal System
Anatomy of Bones and Their Joints
Osteology

Lecture Plan:

1. Concept of The Musculoskeletal System.

2. Chemical composition of bones.

3. Structure and shape of bones.

4. Classification of Bones.

5. Skeleton of the Skull

6. Skeleton of the Trunk (Axial Skeleton)

7. Skeleton of the limbs

Osteology (from Greek osteon — bone, and logos — study, science) is a branch of anatomy dedicated to The Study of the skeleton as a whole, individual bones, and Bone tissue.

The following subdivisions are distinguished:

General osteology (studies the bone as an organ in inseparable connection with its function, as well as The chemical composition of bones and their physical properties, structure, development, and growth, provides a classification of bones, and takes into account the Influence of External factors on Bone Structure and development)

Special osteology (the study of The structure of individual BONES OF THE skeleton)

Comparative osteology (comparative Study of the bone structure of humans and various species of vertebrate animals)

Age-related osteology (examination of bone structure during their development and changes across various age periods; radiosteology is dedicated to the study of the bone system in a living Organism).

Osteology is studied in conjunction with syndesmology.

The skeleton (from Greek skeletos — dried up, desiccated) is represented by a set of bones that form a rigid framework in The Human Body, providing the following Functions:

- support;

- acting as a depot for macro- and microelements;

- locomotion;

- METABOLISM;

- protection;

- hemopoiesis.

The musculoskeletal system (apparatus of movement and support) comprises the bones, their joints, and skeletal Muscles. Through Cartilage, joint capsules, and fascia, bones form a unified functional system — the passive part of the musculoskeletal system, which is set in motion by muscles (the active part of the musculoskeletal system) receiving impulses from the Central Nervous System.

Chemical composition of bones. The bone of a living human contains 50% Water, 28% organic substances (12% ossein and 16% Lipids), and 22% inorganic substances (compounds of calcium, phosphorus, magnesium, etc.). Macerated (degreased, bleached, dried) bone consists of 1/3 organic matter and 2/3 inorganic matter. The specific physicochemical combination of organic and inorganic substances in bones determines their fundamental properties — resilience, elasticity, strength, and hardness. When organic substances predominate in the bone (in children), the bone is more elastic; when inorganic substances predominate (in the elderly), the bone is more brittle and fragile. The chemical composition of bones changes with age and depends on functional loads, Nutrition, and other factors.

Bone structure. A bone is a living, plastic organ consisting of several Tissues and possessing a specific morphological structure. The primary tissue of the bone is bone tissue; in addition, it contains Cytology/practical/45.html">Dense Connective Tissue, reticular connective tissue, and cartilage tissue. Within bone tissue, Three types of cellular elements are distinguished that participate in The formation of new bone tissue: osteoblasts — young Cells involved in the formation of new bone tissue; osteocytes

- mature cells incapable of division, and osteoclasts, which are bone-resorbing cells. Both processes occur continuously within bone tissue. The structural unit of bone is the osteon, a system of osteocytes and bone lamellae arranged concentrically around a central canal (haversian canal).

An osteon is a tubular system of thin bone lamellae surrounding a central canal filled with loose collagenous connective tissue, through which nourishable Blood Vessels and nerves pass. It also contains osteoblastocytes, bone cells that subsequently differentiate into osteocytes. Their role is to form new bone matrix. The function of another type of bone Cell—osteoclastocytes—is to break down old bone tissue.

The outer surface of a bone is covered by a thin connective tissue membrane known as the periosteum, which consists of Fibrous connective tissue. The periosteum contains Blood and Lymphatic vessels as well as nerves that penetrate the bone and connect it to surrounding tissues. It comprises two layers: an outer and an inner layer. The outer layer consists of Dense Fibrous Connective tissue, while the inner layer is dense and contains osteoblasts. Through the inner, osteogenic layer of the periosteum, young bone cells—osteoblasts—are formed, which facilitate bone thickening by depositing on the bone surface, playing a bone-forming role during bone fractures. This inner layer drives bone growth in thickness and its repair following structural damage. The periosteum performs protective, trophic, and osteogenic functions.

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On cross-sections, ground sections, or radiographs, two structural Types of bone tissue are distinguished: compact bone (substantia compacta), located superficially, and spongy (trabecular) bone (substantia spongiosa), located internally (see figure), which consists of numerous bone lamellae and plates (trabeculae).

All bones, both externally and internally (within the compact substance of diaphysis and the cavities of spongy bone), are covered by two highly vascularized connective tissue membranes. Both membranes—the outer periosteum and the inner endosteum—participate in the nourishment and Development of the bone.

Beneath the periosteum lies the compact substance, constructed of lamellar bone tissue penetrated by a system of fine bone canaliculi. These canaliculi are continuations of larger nutrient canals that open onto the bone surface as foramina, through which Arteries and nerves enter the bone and Veins exit. The layer of compact substance in the shaft of long bones is thicker than in the epiphyses, and thinner in short and flat bones. Bones that functionally endure greater mechanical loads possess a thicker compact layer. Beneath the compact substance lies the porous spongy substance, which has a trabecular architecture and contains Red Bone Marrow, performing hematopoietic (hemopoiesis) and protective functions. The trabeculae of the spongy substance are oriented along specific trajectories corresponding to the biomechanical forces acting on the bone, thereby ensuring bone strength. The entire internal cavity of the bone is filled with bone marrow, which is classified as yellow or red. Red marrow is located within the cavities of the spongy bone. The medullary cavities of the diaphyses of long bones contain yellow bone marrow, which is rich in adipose cells.

With age, the layer of compact substance thickens, and the trabeculae of the spongy bone become coarser; by the age of 18–20, bones acquire adult structural characteristics, though their internal remodeling continues throughout life.

Bone classification is based on shape, function, and structural features. Bones are categorized into the following groups:

- long bone (os longum), possessing a tubular structure;

- short bone (os breve), possessing a spongy structure;

- flat bone (os planum), performing a protective function;

- pneumatic bone (os pneumaticum), containing air-filled cells;

- irregular (mixed) bone (os irregulare), composed of parts belonging to the aforementioned groups or differing in origin;

- sesamoid bone (os sesamoideum), belonging to the Accessory apparatus of muscles.

Tubular bones are constructed of spongy and compact substance, forming a tube with a medullary cavity, and perform supportive, protective, and locomotor functions. Tubular bones are subdivided into long and short ones. In long tubular bones (humerus, bones of the forearm, Femur, bones of the leg), the longitudinal dimension predominates, and they act as levers of movement. Long tubular bones feature a middle section—the shaft or diaphysis—and two extremities—the proximal and distal epiphyses. In young individuals, a growth cartilage plate is located in this region, which drives longitudinal bone growth. The interior of the diaphysis contains the medullary cavity, which in adults is filled with yellow bone marrow. The epiphyses of long tubular bones are constructed primarily of spongy substance covered externally by a layer of compact substance.

Short tubular bones (Phalanges of the digits, etc.) primarily perform a supportive function, with dimensions approximately equal in all three planes.

Spongy bones are composed of spongy substance covered by a thin layer of compact substance. They are subdivided into long (Ribs, Sternum) and short (vertebrae, carpal and Tarsal Bones) spongy bones. Sesamoid bones (Patella, pisiform bone, sesamoid bones of the fingers and toes) are also classified as spongy bones; they are embedded within Muscle tendons, thereby increasing the mechanical moment of force of the muscle and its functional strength. Sesamoid bones are located adjacent to joints, facilitating movement within them while lacking direct skeletal articulations.

Flat bones are constructed of two thin plates of compact substance enclosing a thin layer of spongy substance that contains vascular channels; they perform protective functions (bones of the cranium), support and protection (bones of the limb girdles), and enclose cavities for organ protection (pelvic bones).

Mixed bones (vertebrae, Temporal bone, etc.) are bones whose various parts are heterogeneous in shape and structure, incorporating elements of both flat and spongy bones.

Pneumatic bones (sphenoid, ethmoid, frontal, Maxilla) contain internal cavities (sinuses) lined with mucous membrane and filled with air, which reduces their weight without compromising structural strength.

During ontogenesis, the majority of human skeletal bones pass through membranous, cartilaginous, and osseous developmental stages, largely reflecting the phylogenetic stages of chordate internal skeleton evolution. Based on their ossification pattern, these bones are termed secondary. Certain bones (most cranial bones, the body of the clavicle) develop directly from mesenchyme, bypassing the cartilaginous stage. They are designated as primary. Most bones develop from cartilaginous precursors (models). Ossification is the gradual process of transforming a cartilaginous precursor into bone tissue via the deposition of mineral salts, predominantly calcium.

Ossification of each bone begins with the appearance of one, two, or multiple centers (points) of ossification.

Ossification of primary bones typically begins with the appearance of one or several ossification centers within the mesenchymal tissue.

This type of ossification is termed desmal (intramembranous) ossification.

The process of secondary bone ossification occurs along the entire circumference of the diaphysis from the outside inward within the deep layers of the periosteum (perichondral ossification) and from the inside outward (enchondral ossification) within the cartilage tissue, resulting in the formation of spongy bone tissue.

A distinction is made between primary ossification centers, which emerge in cartilaginous and connective tissue bone precursors, and secondary ones, which appear at later Selection/3.html">Stages of development. Both Primary and secondary centers are referred to as main centers.

✵ At the end of the 2nd to the beginning of the 3rd month of embryonic development, primary ossification centers appear in the diaphyses of long bones.

✵ Secondary centers primarily ossify the epiphyses as well as the apophyses (bone protrusions) near the epiphyses. The ossification centers of the apophyses are called accessory centers.

The human skeleton is divided into the axial skeleton (HEAD and trunk) and the Appendicular Skeleton (upper and lower extremities). The adult human skeleton contains more than 200 bones, of which 23 form the skull, 26 THE Vertebral Column, 25 The thoracic cage, 64 the upper extremities, and 62 the lower extremities.

The human Skeletal System is divided into the axial skeleton (skull, vertebral column, thoracic cage) and the appendicular skeleton, which includes the bones of the upper and lower limbs. The skeleton performs supportive, locomotive, protective, and biological functions. Its supportive function lies in providing a framework for soft tissues attached to it, forming the walls of cavities that house Internal Organs, and giving the body its specific shape and spatial orientation. The locomotive function is due to skeletal bones acting as levers set into motion by the contraction of attached muscles. The protective function is carried out by forming bony cavities (such as the cranial, thoracic, and pelvic cavities) that shield internal organs from injury and other environmental impacts. The biological function is associated with the skeleton's participation in Mineral Metabolism and the hemopoietic function of the bone marrow.

The Skeleton of the head is the skull (cranium); it houses and protects the Brain and Sensory Organs associated with it, and serves as the attachment site for the initial sections of the respiratory and digestive systems. Accordingly, the skull is divided into the neurocranium (braincase) and the facial skeleton. The boundary between these divisions runs along the supraorbital margin, the Zygomatic bone, and a line extending to the external acoustic meatus.

The neurocranium comprises the vault (calvaria) and the base. It consists of eight bones—paired and unpaired. The paired bones include the temporal and parietal bones, while the unpaired ones are the frontal, occipital, ethmoid, and sphenoid bones.

The Frontal bone forms the anterior part of the cranial vault and consists of the frontal squama, the nasal part, and two orbital parts. The frontal squama is positioned almost vertically; its upper margin articulates with the parietal bones, and its lower margin with the Sphenoid bone. Within the thickness of the frontal squama lies the air-filled frontal sinus. Between the two orbital parts lies a notch filled by the Ethmoid bone; the orbital parts are horizontal, with their lower concave surfaces facing the orbits and their upper surfaces facing the cranial cavity, while their posterior margins articulate with the sphenoid bone.

The Parietal bone forms the superior-lateral part of the cranial vault and is situated between the frontal and occipital bones. Its anterior margin articulates with the frontal bone, the posterior with the Occipital bone, the medial with the contralateral parietal bone, and the lateral with the squamous part of the temporal bone.

The temporal bone forms the base and lateral part of the cranial vault, located between the parietal, sphenoid, and occipital bones; it bounds the external acoustic opening and forms a joint with the Mandible (temporomandibular joint). It consists of squamous, tympanic, and petrous (pyramidal) parts. The squamous part forms the lateral section of the cranial vault and gives rise to the zygomatic process, which articulates with the zygomatic bone. The tympanic part forms the anterior, inferior, and posterior margins of the external acoustic opening, which continues into the external acoustic meatus leading to the tympanic cavity. The petrous part (pyramid) is pyramid-shaped, contributes to the cranial base, and serves as the osseous housing for the organs of Hearing and Equilibrium (vestibular apparatus).

The sphenoid bone forms the central part of the cranial base, articulating anteriorly with the frontal and ethmoid bones, and posteriorly with the occipital bone. It consists of a body, lesser wings, greater wings, and pterygoid processes. The body contains an air-filled cavity known as the sphenoidal sinus, which is divided by a septum into two halves that communicate with the Nasal cavity via apertures. On the superior (cerebral) surface of the body, along the midline, lies a depression called the sella turcica, at the bottom of which is the hypophyseal fossa housing the Pituitary Gland, an endocrine organ. The anterior surface of the body, which contributes to the posterior wall of the nasal cavity, features a crest that articulates anteriorly with the perpendicular plate of the ethmoid bone. The wings and processes contain canals, foramina, and fissures transmitting Blood Vessels and nerves; situated between the lesser and greater wings is the superior orbital fissure, which connects the cranial cavity to the orbits.

The occipital bone forms the posterior and inferior walls of the cranium, contributing to both the cranial vault and the cranial base. It consists of four parts surrounding the foramen magnum: the body (basilar part), the squamous part (occipital squama), and two lateral parts. The foramen magnum connects the cranial cavity with the vertebral canal, transmitting the Spinal Cord, blood vessels, and nerves. The body is located anterior to the foramen magnum and articulates with the sphenoid bone. The occipital squama lies superior to the foramen magnum; its superior border articulates with the parietal bones, and its inferior border with the temporal bones. The inferior surfaces of the lateral parts bear the occipital condyles for articulation with the first cervical vertebra, the atlas.

The ethmoid bone participates in forming the Base of the cranium, the orbital walls, and the nasal cavity, and is composed of the cribriform plate, the perpendicular plate, and the ethmoidal labyrinths. The cribriform plate bridges the notch between the orbital PARTS OF THE frontal bone and is perforated by numerous (30–40) small foramina that transmit Olfactory nerve fibers and blood vessels. The perpendicular plate extends inferiorly from the cribriform plate, forming the anterosuperior section of the bony nasal septum; the Vomer attaches to its inferior margin, while the ethmoidal labyrinths flank it laterally. The ethmoidal labyrinths consist of numerous air cells enclosed laterally (towards the Orbit) by the thin orbital lamina, while their medial surfaces facing the perpendicular plate form the superior and middle nasal conchae.

The bones of the facial skeleton provide the bony framework for the Sense Organs and the initial segments of the digestive and respiratory systems, which determines their structural Organization. The facial bones include the paired maxilla, nasal, lacrimal, zygomatic, palatine, and Inferior nasal concha bones, as well as the unpaired mandible, vomer, and Hyoid bone.

The maxilla contributes to the formation of cavities for the sense organs (the orbits and the nasal cavity), the septum separating the nasal and oral cavities, and the masticatory apparatus. Occupying the central part of the face, it consists of a body and four processes: frontal, zygomatic, alveolar, and palatine. The body contains the large maxillary sinus, which opens widely into the nasal cavity via the middle meatus. The maxillary, sphenoid, and frontal sinuses, along with the ethmoidal air cells, comprise the Paranasal Sinuses lined with mucous membrane. The frontal process articulates with the nasal part of the frontal bone, and the zygomatic process with the zygomatic bone. The alveolar process extends along the inferior margin of the maxilla and terminates in the alveolar arch, which contains eight dental alveoli for the upper Teeth. The palatine processes, together with the palatine bones, close off the nasal cavity inferiorly and separate it from the Oral Cavity.

The Nasal bone, together with its contralateral counterpart, forms the bony bridge of the Nose.

The Lacrimal bone articulates with the frontal process of the maxilla and forms the anterior part of the medial wall of the orbit. The lateral surface of the lacrimal bone features the lacrimal groove, which, together with the corresponding groove on the maxilla, forms the fossa for the lacrimal sac.

The zygomatic bone is situated between the maxilla, frontal, and temporal bones, articulating with the zygomatic processes of the temporal and frontal bones, the maxilla, and the greater wing of the sphenoid bone.

The Palatine bone lies posterior to the maxilla and consists of two plates joined at a right angle that Complement the maxilla. The horizontal plate contributes to the Formation of the hard palate, while the perpendicular plate forms the lateral wall of the nasal cavity.

The inferior nasal concha is an independent bone, unlike the superior and middle nasal conchae, which are parts of the ethmoid bone. Below the inferior nasal concha lies the inferior meatus; below the superior concha is the superior meatus, and below the middle concha is the middle meatus. The inferior meatus communicates with the orbital cavity, whereas the middle and superior meatuses communicate with the paranasal sinuses of the sphenoid, maxilla, ethmoid, and frontal bones.

The mandible is the only movable bone of the skull, features a horseshoe shape, and consists of a body and two rami. The body is positioned horizontally; its superior border bears the alveolar arch, which contains 16 dental alveoli. Projecting vertically upward from the body are the right and left rami, each terminating in two processes: the coronoid process, for the attachment of the temporalis muscle, and the condylar process, which ends in the head of the mandible (mandibular condyle) participating in the temporomandibular joint. Blood vessels and nerves enter the body of the mandible through the mental foramen on its external surface.

The vomer is an irregularly shaped quadrilateral plate that forms part of the bony nasal septum; its posterior border divides the posterior apertures of the nasal cavity, known as the choanae, which connect the nasal cavity to the nasopharynx.

The hyoid bone is located between the mandible and the Larynx, consisting of a body and two pairs of horns (greater and lesser horns). It is suspended from the cranial base by two long fibrous ligaments extending from the lesser horns to the styloid processes of the temporal bones.

The human trunk skeleton consists of the vertebral column (spine) and the thoracic cage.

The vertebral column (columna vertebralis) is located on the posterior surface of the trunk; its position and shape are shaped by human bipedalism. It serves as the primary rigid axis that Supports body weight, thus performing a support function; it protects the Organs of the thoracic, abdominal, and pelvic cavities, as well as the spinal cord located within the vertebral canal, performing a protective function; it participates in the Movements of the body and head, performing a locomotor function; and it cushions shocks during movement, performing a Shock-absorbing function. The vertebral column consists of 33–34 individual bones called vertebrae, which are stacked sequentially upon one another. It is divided into cervical (7 vertebrae), thoracic (12 vertebrae), lumbar (5 vertebrae), sacral (5 vertebrae), and coccygeal (4–5 vertebrae) regions. In the sagittal plane, the vertebral column exhibits physiological curves directed forward (lordoses) and backward (kyphoses).

These include cervical and lumbar lordoses, as well as thoracic and sacral kyphoses. The vertebral column is flexible, allowing movement around three axes of rotation: flexion (bending the trunk forward) and extension (returning to the starting position and bending backward) occur around the frontal (transverse) axis; lateral bending occurs around the sagittal axis; and rotation occurs around the vertical axis. With age, these physiological curves tend to flatten. Due to the reduction in height of the intervertebral discs and vertebrae, alongside a loss of tissue elasticity, the vertebral column bends forward, forming a thoracic curve (hump), and the overall length of the spine decreases.

Regardless of their region, all vertebrae share a uniform structural plan consisting of a body facing forward, an arch directed backward, and seven processes. Between the vertebral body and the arch lies the vertebral foramen; when the vertebrae are aligned, these foramina form the vertebral canal, which houses the spinal cord. At the junction of the vertebral body and arch are notches that, when vertebrae articulate, form the intervertebral foramina for the passage of Spinal Nerves. Seven processes extend from the vertebral arch to serve as muscle attachment sites. A single, unpaired spinous process projects backward along the midline; paired transverse processes extend laterally; and paired superior and inferior articular processes project upward and downward. The articular processes form the intervertebral joints that enable spinal movement, while the transverse and spinous processes serve for the attachment of muscles and ligaments.

The vertebrae are interconnected via intervertebral discs, intervertebral symphyses, and joints. The vertebral bodies are joined together by symphyses. Disc height varies, being greater in the sacral and cervical regions and smaller in the thoracic region; a greater disc height relative to the vertebral body allows for increased mobility. Each disc is a fibrocartilaginous plate whose peripheral region consists of concentric layers of connective tissue fibers. These fibers form the anulus fibrosus, enclosing The Nucleus pulposus in the center, which is composed of soft fibrocartilage (the remnants of the notochord). The nucleus is compressed by the vertebral bodies, ensuring the resilience of the joint. The vertebral arches are connected by joints and ligaments situated both between the arches themselves and between their articular processes.

Vertebrae vary in size and shape across different Regions of the vertebral column. The Morphology of the 1st and 2nd cervical vertebrae is adapted to their Participation in the mobile articulation with the skull: the first vertebra (atlas) lacks a body, most of which has fused onto the second vertebra to form the dens (odontoid process), which articulates with the anterior arch of the atlas. In most cervical vertebrae (except the 6th and 7th), the spinous processes are bifid, with that of the 7th vertebra being particularly prominent. In general, the bodies (weight-bearing parts) of the cervical vertebrae are poorly developed, increasing in size downward to accommodate greater loads, and reaching their maximum dimensions in the lumbar region. A distinctive feature of the thoracic vertebrae is the presence of costal facets on the lateral surfaces of their bodies for articulation with the ribs. The sacral vertebrae, which must support the weight of the head, trunk, and upper limbs and connect this part of the skeleton to the Lower limb girdle, fuse between the ages of 16 and 18 into a single bone—the sacrum. The coccygeal vertebrae, representing the remnants of a vanished tail, are vestigial and fuse into a small, wedge-shaped bone.

The thoracic cage (compages toracis) is located in the upper part of the trunk, forming the thoracic cavity which houses internal organs; it serves as a support for the upper limbs, an attachment site for muscles, and participates in respiratory movements. The thoracic cage is formed by the ribs, sternum, thoracic vertebrae, and their articulations.

The sternum is situated along the anterior midline and serves as the attachment site for the clavicles and ribs. It consists of the manubrium, the body, and the xiphoid process. The manubrium lies superior to the body; its upper margin features the jugular notch, flanked on either side by clavicular notches for articulation with the clavicles. The lateral margins of both the manubrium and the sternal body bear costal notches for rib attachment. The xiphoid process is located inferior to the body and can vary significantly in shape and size.

There are 12 pairs of ribs, corresponding to the number of thoracic vertebrae, with their posterior ends articulating with the bodies of the thoracic vertebrae. The first 7 pairs attach anteriorly to the sternum and are known as true ribs. False ribs—pairs VIII, IX, and X—connect to the cartilage of the 7th rib to form the costal margin. The anterior ends of the XI and XII pairs lie free within the musculature and are more mobile, earning them the name floating ribs. Each rib is a narrow, curved plate consisting of two parts: a longer posterior bony part and an anterior cartilaginous part. The bony portion, or rib bone, is classified as a flat bone and features posterior and anterior ends with a shaft in between; the posterior end bears a head, followed by a constricted region known as the neck of the rib.

The thoracic cage has two apertures—superior and inferior. The superior aperture is bounded by the first thoracic vertebra, the first ribs, and the manubrium of the sternum, and transmits the Esophagus, Trachea, major blood vessels, and nerves. The larger inferior aperture is bounded by the twelfth thoracic vertebra, the XI and XII ribs, the costal margins, and the xiphoid process, and is sealed by the Diaphragm. The intercostal spaces are filled with muscles and ligaments. The thoracic cage houses vital organs such as The Heart, Lungs, and esophagus. The shape and dimensions of the thoracic cage are individual, determined by the degree of development of the muscles and lungs. Three main chest shapes are distinguished: flat, cylindrical, and conical. Individuals with well-developed muscles and lungs have a broad, short, conical chest, where the lower part is wider than the upper, the ribs have a slight downward slope, and the inferior aperture is significantly larger than the superior. Conversely, individuals with poorly developed muscles and lungs present a narrow, long, Flat chest, characterized by a reduced anteroposterior diameter, a nearly vertical anterior wall, and sharply inclined ribs. The cylindrical shape occupies an intermediate position between the conical and flat types. The female thoracic cage is shorter and narrower in its lower part compared to that of males, and is more rounded.

Age-related Changes in the thoracic cage are associated with the GROWTH AND DEVELOPMENT of its bony components—the sternum and ribs—as well as alterations in its overall shape. Growth of the thoracic cage is uneven, with its middle section expanding most intensively. Maximum enlargement of chest dimensions occurs in boys at ages 12–13, and in girls 1–2 years earlier. Sexual Dimorphism in the shape of the thoracic cage becomes apparent around the age of fifteen. Chest mobility increases up to the age of 17; in children, chest mobility is more pronounced during inspiration than expiration. In older age, with declining muscle tone, the dimensions of the thoracic cage decrease, the inclination of the ribs increases, and their mobility becomes restricted due to the Ossification of the costal cartilages.

The appendicular skeleton is represented by the bones of the upper and lower limbs. Locomotion in most terrestrial vertebrates is limb-dependent. During evolutionary history, the limb skeleton underwent significant transformations: in humans, the upper limbs evolved into organs of labor, while the lower limbs maintain an upright posture, bearing the functions of support and locomotion. The limb skeleton consists of two parts: the girdle skeleton and the free limb skeleton.

The bones of the upper limb are divided into the bones of the pectoral (upper limb) girdle and the free upper limb.

The Pectoral Girdle is located in the upper part of the thoracic cage and anchors the bones of the free upper limb; it helps maintain an upright body posture and increases the range of motion of the entire upper limb and its segments across multiple planes. The bones of the pectoral girdle include the scapulae and clavicles.

The clavicle connects the upper limb to the axial skeleton and holds the shoulder joint at a proper distance from the thoracic cage, thereby ensuring greater freedom of movement for the free limb. It is a paired, long tubular bone with an S-shaped curvature, consisting of a shaft, a medial end, and a lateral end. Situated along the frontal axis on the anterior surface of the thoracic cage, the clavicle slightly overlies the first rib. Its medial (sternal) end articulates with the sternum, while its lateral (acromial) end articulates with the scapula.

The scapula is positioned on the posterior surface of the thoracic cage near the vertebral column, spanning the 2nd to 7th ribs. Connected to the spine via muscles, it shifts easily when these muscles contract. The scapula presents costal and dorsal surfaces, lateral, medial, and superior borders, and lateral, inferior, and superior angles. The costal surface is concave, forming the subscapular fossa, which houses the subscapular muscle. The dorsal surface features a prominent ridge—the spine of the scapula—which divides the bone into the supraspinous and infraspinous fossae, lodging the supraspinatus and infraspinatus muscles, respectively. The spine of the scapula terminates in the acromion, which articulates with the clavicle. The lateral angle is thickened and ends in the glenoid cavity, which articulates with the head of the humerus. The medial border faces the vertebral column between the superior and inferior angles, whereas the lateral border lies between the inferior and lateral angles. The superior border, which is the shortest, connects the superior and lateral angles and features a notch for the passage of vessels and nerves; a well-defined coracoid process projects from this border.

The skeleton of the free upper limb is subdivided into the arm (brachium), forearm (antebrachium), and hand (manus).

The arm bone is the humerus, a long tubular bone comprising a shaft and two extremities (epiphyses). Its upper end articulates with the scapula, while its lower end articulates with the forearm bones. The upper extremity terminates in the humeral head, which fits into the shoulder joint. Just below the head is a constriction known as the anatomical neck; inferior to this, between the head and the shaft, lies the surgical neck—a site prone to approximately 53% of all humeral fractures. The lower, distal end of the humerus is widened and slightly curved forward, terminating in the humeral condyle. The medial part of the condyle forms a pulley-like structure (trochlea) that articulates with the ulna, while the adjacent sphere-shaped capitulum articulates with the radius.

The ulna is located on the medial side of the forearm (the little-finger side). Its upper end is thickened and features a trochlear notch, through which it articulates with the trochlea of the humerus. The lower end of the ulna bears a head that articulates with the ulnar notch of the radius, with a styloid process projecting from the medial side of the head.

The radius is situated on the lateral side of the forearm (the thumb side). Its proximal end features a cylindrical head with an articular fovea for articulation with the capitulum of the humerus. Below the head lie the neck and the radial tuberosity, to which the biceps brachii muscle attaches. The distal end of the radius is enlarged, bearing a styloid process laterally and an ulnar notch medially for articulation with the ulnar head. The carpal articular surface of the radius connects with the proximal row of Carpal Bones.

The hand consists of the carpal bones, Metacarpal bones, and the bones of the digits (phalanges). The carpus comprises 8 small bones arranged in two rows of four. The proximal row articulates with the forearm, and the distal row with the metacarpals. On the dorsal side, the carpal bones form a convexity, whereas on the palmar side they form a concavity that serves as a passageway for the flexor tendons of the digits. The metacarpus contains 5 tubular bones articulating proximally with the carpus and distally with the phalanges. There are 14 phalanges in total: 2 in the first digit (thumb) and 3 in each of the other four.

The bones of the Pelvic Girdle, together with the sacrum and coccyx, form a closed bony ring known as the pelvis, through which body weight is transmitted to the lower limbs.

The pelvis is divided into the greater (false) pelvis and the lesser (true) pelvis. The greater pelvis is bounded laterally by the iliac wings and posteriorly by the lower lumbar vertebrae and the base of the sacrum. It is separated from the lesser pelvis by a plane passing posteriorly along the upper margin of the Pubic Symphysis to the sacral base. Open anteriorly, the greater pelvis communicates inferiorly with the cavity of the lesser pelvis and continues superiorly into the Abdominal cavity, housing the organs of the lower abdomen. The lesser pelvis acts as a container for several urogenital and digestive organs (in females: the Uterus, Ovaries, rectum, Urinary Bladder, and Vagina; in males: the Prostate Gland, Seminal Vesicles, portions of the Ductus Deferentes, rectum, and urinary bladder), as well as blood vessels and nerves. Abdominal and thigh muscles attach to the pelvic bones. In the habitual upright posture, the pelvis is tilted, which helps maintain balance while standing without requiring extra muscular effort. The pelvic girdle is formed by the hip (coxal) bone, which consists of three separate components during childhood—the ilium, pubis, and ischium—separated by layers of cartilage; by the ages of 14–16, these layers ossify, fusing them into a single coxal bone. On the external surface of the coxal bone is a deep depression, the acetabulum, which contains an articular surface for articulation with the head of the femur and an acetabular fossa for the attachment of the ligament of the head of the femur.

The ilium extends upward from the acetabulum. It consists of a thickened portion called the body, which contributes to the formation of the acetabulum, and an expanded, thin portion known as the ala (wing). The internal surface of the ala is concave, forming the iliac fossa, which supports internal organs. Posterior to the iliac fossa lies the auricular surface for articulation with the sacrum. The gluteal (external) surface features gluteal lines that serve as attachment sites for the corresponding muscles. The upper free margin of the iliac ala is thickened and forms the iliac crest, where various muscles attach.

The pubis is located anterior to the acetabulum. It comprises a body, which forms part of the acetabulum, and two rami: the superior ramus, which runs almost horizontally forward, and the inferior ramus, descending from the superior ramus at an almost right angle. At the junction of the superior and inferior rami, There is a rough surface for articulation with the pubic bone of the opposite side.

The ischium is situated inferior to the acetabulum and consists of a body—which participates in forming the lower part of the acetabulum—and a ramus that projects upward from the body to fuse with the inferior ramus of the pubic bone, together enclosing the obturator foramen.

At the junction of the body and the ramus, the ischial tuberosity is formed, which supports the human body weight when sitting.

The free lower limb begins at the hip joint and comprises three main segments: the thigh (proximal segment), the leg, and the FOOT (distal segment).

The femur serves as the skeletal framework of the thigh and is the longest bone in the skeleton. Classified as a long tubular bone, its proximal epiphysis terminates in a head that transitions into the neck. Near the junction of the head and the shaft, There are two prominences: the greater and lesser trochanters. The distal end of the bone is thickened and features two large prominences with articular surfaces—the medial and lateral condyles—between which lies a deep intercondylar fossa.

The patella is a flat sesamoid bone located anterior to the knee joint within the tendon of the quadriceps femoris muscle. Its broader base points upward, while its narrower apex points downward. The patella protects the knee joint and enhances the leverage and strength of the quadriceps femoris muscle.

The bones of the leg include the Tibia and the Fibula.

The tibia is the larger and thicker of the two, located medially on the side of the great toe. The shaft of the tibia is triangular in cross-section; its sharp anterior border lies directly beneath the Skin, making it prone to frequent injury. The proximal end of the tibia is enlarged and features medial and lateral condyles, along with an articular surface for articulation with the femur. Two fibrocartilaginous discs, known as menisci, lie between the articular surfaces of the tibial and femoral condyles and are frequently subject to injury. The distal epiphysis of the tibia articulates with the talus of the tarsus and terminates inferiorly in a process called the medial malleolus, which is clearly visible and palpable beneath the skin.

The fibula is situated on the lateral side of the leg (on the side of the little toe). Its proximal end is enlarged to form the head, which bears an articular surface for articulation with the lateral condyle of the tibia. The distal epiphysis is elongated, forming the lateral malleolus, which extends further distally than the medial malleolus of the tibia.

The skeleton of the foot is divided into three regions: the tarsus, metatarsus, and phalanges, and consists of 26 bones: 7 spongy tarsal bones (talus, calcaneus, navicular, cuboid, and 3 cuneiforms), 5 short tubular Metatarsal Bones, and 14 phalanges. All toes have three phalanges except the big toe, which has two. The Bones of the foot are connected by ligaments to form the arches of the foot, which facilitate even load distribution and provide springiness and elasticity during walking.