Chapter 16: The musculoskeletal system Flashcards
Functions of the bones
The functions of bones include:
- providing the body framework
- giving attachment to muscles and tendons
- allowing movement of the body as a whole and of parts of the body, by forming joints that are moved by muscles
- forming the boundaries of the cranial, thoracic and pelvic cavities, and protecting the organs they contain
- hemopoiesis, the production of blood cells in red bone marrow
- mineral storage, especially calcium phosphate – the mineral reservoir within bone is essential for maintenance of blood calcium levels, which must be tightly controlled.
Types of bones
Bones are classified as long, short, irregular, flat, and sesamoid.
Long bones
- These consist of a shaft and two extremities. As the name suggests, these bones are longer than they are wide. Most long bones are found in the limbs; examples include the femur, tibia, and fibula.
- These have a diaphysis (shaft) and two epiphyses (extremities). The diaphysis is composed mainly of compact bone with a central medullary canal, containing fatty yellow bone marrow. The epiphyses consist of an outer covering of compact bone with the spongy (cancellous) bone inside. The diaphysis and epiphyses are separated by epiphyseal cartilages, which ossify when growth is complete.
- Long bones are almost completely covered by a vascular membrane, the periosteum, which has two layers. The outer layer is tough and fibrous and protects the bone underneath. The inner layer contains osteoblasts and osteoclasts, the cells responsible for bone production and the breakdown, and is important in the repair and remodeling of the bone. The periosteum covers the whole bone except within joint cavities, allows attachments of tendons, and is continuous with the joint capsule. Hyaline cartilage replaces the periosteum on bone surfaces that form joints. Thickening of a bone occurs by the deposition of new bone tissue under the periosteum.
Short, Irregular, flat, and sesamoid bones
•These have no shafts or extremities and are diverse in shape and size. Examples include:
-short bones – carpals (wrist)
-irregular bones – vertebrae and some skull bones
-flat bones – sternum, ribs, and most skull bones
-sesamoid bones – patella (kneecap).
•These have a relatively thin outer layer of compact bone, with the spongy bone inside containing red bone marrow. They are enclosed by the periosteum except for the inner layer of the cranial bones where it is replaced by the dura mater.
Blood and nerve supply
One or more nutrient arteries supply the bone shaft; the epiphyses have their own blood supply, although in the mature bone the capillary networks arising from the two are heavily interconnected. The sensory nerve supply usually enters the bone at the same site as the nutrient artery, and branches extensively throughout the bone. Bone injury is, therefore, usually very painful.
The microscopic structure of bone
Bone is a strong and durable type of connective tissue. Its major constituent (65%) is a mixture of calcium salts, mainly calcium phosphate. This inorganic matrix gives bone great hardness, but on its own would be brittle and prone to shattering. The remaining third is an organic material, called osteoid, which is composed mainly of collagen. Collagen is very strong and gives bone slight flexibility. The cellular component of bone contributes less than 2% of bone mass.
Bone cells
• There are three types of bone cell:
- osteoblast
- osteocyte
- osteoclast
Osteoblasts
•These bone-forming cells are responsible for the deposition of both inorganic salts and osteoid in bone tissue. They are therefore present at sites where the bone is growing, repairing, or remodeling, e.g.:
-in the deeper layers of periosteum
-in the centers of ossification of immature bone
-at the ends of the diaphysis adjacent to the epiphyseal cartilages of long bones
-at the site of a fracture.
•As they deposit new bone tissue around themselves, they eventually become trapped in tiny pockets (lacunae) in the growing bone and differentiate into osteocytes.
Osteocytes
These are mature bone cells that monitor and maintain bone tissue and are nourished by tissue fluid in the canaliculi that radiate from the central canals.
Osteoclasts
These cells break down bone, releasing calcium and phosphate. They are very large cells with up to 50 nuclei, which have formed from the fusion of many monocytes. The continuous remodeling of healthy bone tissue is the result of the balanced activity of the bone’s osteoblast and osteoclast populations. Osteoclasts are found in areas of the bone where there is active growth, repair, or remodeling, e.g:
- under the periosteum, maintaining bone shape during growth and removing excess callus formed during the healing of fractures
- round the walls of the medullary canal during growth and canalize callus during healing.
Compact (cortical) bone
Cortical bone is the dense outer surface of bone that forms a protective layer around the internal cavity. This type of bone also known as compact bone makes up nearly 80% of skeletal mass and is imperative to body structure and weight-bearing because of its high resistance to bending and torsion.
Spongy (cancellous trabecular) bone
To the naked eye, spongy bone looks like a honeycomb. Microscopic examination reveals a framework formed from trabeculae (meaning ‘little beams’), which consist of a few lamellae and osteocytes interconnected by canaliculi. Osteocytes are nourished by interstitial fluid diffusing into the bone through the tiny canaliculi. The spaces between the trabeculae contain red bone marrow. In addition, spongy bone is lighter than compact bone, reducing the weight of the skeleton.
Development of bone tissue
- Also called osteogenesis or ossification, this begins before birth and is not complete until about the 21st year of life. Long, short and irregular bones develop in the fetus from rods of cartilage, cartilage models. Flat bones develop from membrane models and sesamoid bones from tendon models.
- During ossification, osteoblasts secrete osteoid, which gradually replaces the initial model; then this osteoid is progressively calcified, also by osteoblast action. As the bone grows, the osteoblasts become trapped in the matrix of their own making and become osteocytes.
- In mature bone, a fine balance of osteoblast and osteoclast activity maintains normal bone structure. If osteoclast activity exceeds osteoblast activity, the bone becomes weaker. On the other hand, if osteoblast activity outstrips osteoclast activity, the bone becomes stronger and heavier.
Development of long bones
- In long bones the focal points from which ossification begins are small areas of osteogenic cells or centers of ossification in the cartilage model. This is accompanied by the development of a bone collar at about 8 weeks of gestation. Later the blood supply develops, and bone tissue replaces cartilage as osteoblasts secrete osteoid in the shaft. The bone lengthens as ossification continues and spreads to the epiphyses. Around birth, secondary centers of ossification develop in the epiphyses, and the medullary canal forms when osteoclasts break down the central bone tissue in the middle of the shaft. During childhood, long bones continue to lengthen because the epiphyseal plate at each end of the bone, which is made of cartilage, continues to produce new cartilage on its diaphyseal surface (the surface facing the shaft of the bone).
- This cartilage is then turned to bone. If cartilage production matches the rate of ossification, the bone continues to lengthen. At puberty, under the influence of sex hormones, the epiphyseal plate growth slows down and is overtaken by bone deposition. Once the whole epiphyseal plate is turned to bone, no further lengthening of the bone is possible.
Hormonal regulation of bone growth
Growth hormone and the thyroid hormones, thyroxine, and triiodothyronine are especially important during infancy and childhood, deficient or excessive secretion of these results in abnormal development of the skeleton.
Exercise and bone
- Although bone growth lengthways permanently ceases once the epiphyseal plates have ossified, thickening of bone is possible throughout life. This involves the laying down of new osteons at the periphery of the bone through the action of osteoblasts in the inner layer of the periosteum. Weight-bearing exercise stimulates the thickening of bone, strengthening it and making it less liable to fracture. Lack of exercise reverses these changes, leading to lighter, weaker bones.
- Testosterone and estrogens influence the physical changes that occur at puberty and help maintain bone structure throughout life. Rising levels of these hormones are responsible for the growth spurt of puberty, but later stimulate closure of the epiphyseal plates, so that bone growth lengthways stops (although bones can grow in thickness throughout life). Average adult male height is usually greater than female because male puberty tends to occur at a later age than female puberty, giving a male child’s bones longer to keep growing.
- Calcitonin and parathyroid hormone control blood levels of calcium by regulating its uptake into and release from bone. Calcitonin increases calcium uptake into bone (reducing blood calcium), and parathormone decreases it (increasing blood calcium).
Diet and bone
Healthy bone tissue requires adequate dietary calcium and vitamins A, C, and D. Calcium, and smaller amounts of other minerals such as phosphate, iron, and manganese, are essential for adequate mineralization of bone. Vitamin A is needed for osteoblast activity. Vitamin C is used in collagen synthesis, and vitamin D is required for calcium and phosphate absorption from the intestinal tract.
Bone markings
Most bones have rough surfaces, raised protuberances, and ridges that give attachment to muscle tendons and ligaments. These are not included in the following descriptions of individual bones unless they are of note, but many are marked on illustrations.
Healing of bone
•There are several terms used to classify bone fractures, including:
-simple: the bone ends do not protrude through the skin
-compound: the bone ends protrude through the skin
-pathological: fracture of a bone weakened by disease.
•1: A hematoma (collection of clotted blood) forms between the ends of the bone and in surrounding soft tissues.
•2: There follows the development of acute inflammation and accumulation of inflammatory exudate, containing macrophages that phagocytose the hematoma and small dead fragments of bone (this takes about 5 days). Fibroblasts migrate to the site; granulation tissue and new capillaries develop.
•3: New bone forms as large numbers of osteoblasts secrete spongy bone, which unites the broken ends, and is protected by an outer layer of bone and cartilage; the new deposits of bone and cartilage is called a callus. Over the next few weeks, the callus matures, and the cartilage is gradually replaced with new bone.
•4: Reshaping of the bone continues and gradually the medullary canal is reopened through the callus (in weeks or months). In time the bone heals completely with the callus tissue completely replaced with mature compact bone. Often the bone is thicker and stronger at the repair site than originally, and a second fracture is more likely to occur at a different site.
Tissue fragments between cone ends
Splinters of dead bone (sequestrate) and soft tissue fragments not removed by phagocytosis delay healing.
Deficient blood supply
This delays growth of granulation tissue and new blood vessels. Hypoxia also reduces the number of osteoblasts and increases the number of chondrocytes that develop from their common parent cells. This may lead to the cartilaginous union of the fracture, which results in a weaker repair. The most vulnerable sites, because of their normally poor blood supply, are the neck of the femur, the scaphoid, and the shaft of the tibia.
Poor alignment of bone ends
This may result in the formation of a large and irregular callus that heals slowly and often results in permanent disability.
Continued mobility f bone ends
Continuous movement results in fibrosis of the granulation tissue followed by the fibrous union of the fracture.
Miscellaneous
These include infection, systemic illness, malnutrition, drugs, e.g., corticosteroids and aging.
Infections
Pathogens enter through broken skin, although they may occasionally be blood-borne. Healing will not occur until the infection resolves.
Fat embolism
Emboli consisting of fat from the bone marrow in the medullary canal may enter the circulation through torn veins. They are most likely to lodge in the lungs and block blood flow through the pulmonary capillaries.
axial skeleton
- The bones of the skeleton are divided into two groups: the axial skeleton and the appendicular skeleton.
- The axial skeleton consists of the skull, vertebral column, ribs, and sternum. Together the bones forming these structures constitute the central bony core of the body, the axis. The appendicular skeleton consists of the shoulder and pelvic girdles and the limb bones.
Skull
The skull rests on the upper end of the vertebral column and its bony structure is divided into two parts: the cranium and the face.
Sinuses
Sinuses containing air are present in the sphenoid, ethmoid, maxillary, and frontal bones. They all communicate with the nasal cavity and are lined with ciliated mucous membrane. They give resonance to the voice and reduce the weight of the skull, making it easier to carry.
Cranium
•The cranium is formed by several flat and irregular bones that protect the brain. It has a base upon which the brain rests and a vault that surrounds and covers it. The periosteum lining the inner surface of the skull bones forms the outer layer of the dura mater. In the mature skull, the joints (sutures) between the bones are immovable. The bones have numerous perforations (e.g., foramina, fissures) through which nerves, blood, and lymph vessels pass. The bones of the cranium are:
- 1 frontal bone
- 2 parietal bones
- 2 temporal bones
- 1 occipital bone
- 1 sphenoid bone
- 1 ethmoid bone
Frontal bone
- This is the bone of the forehead. It forms part of the orbital cavities (eye sockets) and the prominent ridges above the eyes, the supraorbital margins. Just above the supraorbital margins, within the bone, are two air-filled cavities or sinuses lined with ciliated mucous membrane, which open into the nasal cavity.
- The coronal suture joins the frontal and parietal bones and other sutures are formed with the sphenoid, zygomatic, lacrimal, nasal, and ethmoid bones. The frontal bone originates in two parts joined in the midline by the frontal suture
Parietal bones
•These bones form the sides and roof of the skull. They articulate with each other at the sagittal suture, with the frontal bone at the coronal suture, with the occipital bone at the lambdoidal suture, and with the temporal bones at the squamous sutures. The inner surface is concave and is grooved to accommodate the brain and blood vessels.
Temporal bones
- These bones lie one on each side of the head and form sutures with the parietal, occipital, sphenoid, and zygomatic bones. The squamous part is the thin fan-shaped area that articulates with the parietal bone. The zygomatic process articulates with the zygomatic bone to form the zygomatic arch (cheekbone).
- The mastoid part contains the mastoid process, a thickened region easily felt behind the ear. It contains many very small air sinuses that communicate with the middle ear and are lined with squamous epithelium.
- The petrous portion forms part of the base of the skull and contains the organs of hearing (the spiral organ) and balance.
- The temporal bone articulates with the mandible at the temporomandibular joint, the only movable joint of the skull. Immediately behind this articulating surface is the external acoustic meatus (auditory canal), which passes inwards towards the petrous portion of the bone.
- The styloid process projects from the lower process of the temporal bone and supports the hyoid bone and muscles associated with the tongue and pharynx.
Occipital bone
This bone forms the back of the head and part of the base of the skull. It forms sutures with the parietal, temporal, and sphenoid bones. Its inner surface is deeply concave, and the concavity is occupied by the occipital lobes of the cerebrum and by the cerebellum. The occiput has two articular condyles that form condyloid joints with the first bone of the vertebral column, the atlas. This joint permits nodding movements of the head. Between the condyles is the foramen magnum (meaning ‘large hole’) through which the spinal cord passes into the cranial cavity.
Sphenoid bone
This bone occupies the middle portion of the base of the skull, and it articulates with the occipital, temporal, parietal, and frontal bones. It links the cranial and facial bones and cross-braces the skull. On the superior surface in the middle of the bone is a little saddle-shaped depression, the hypophyseal fossa (Sella turcica) in which the pituitary gland rests. The body of the bone contains some large air sinuses lined with ciliated mucous membrane with openings into the nasal cavity. The optic nerves pass through the optic foramina on their way to the brain.
Ethmoid bone
The ethmoid bone occupies the anterior part of the base of the skull and helps to form the orbital cavity, the nasal septum, and the lateral walls of the nasal cavity. On each side are two projections into the nasal cavity, the superior and middle conchae or turbinated processes. It is a very delicate bone containing many air sinuses lined with ciliated epithelium and with openings into the nasal cavity. The horizontal flattened part, the cribriform plate, forms the roof of the nasal cavity and has numerous small foramina through which nerve fibers of the olfactory nerve (sense of smell) pass upwards from the nasal cavity to the brain.
Face
The skeleton of the face is formed by 13 bones in addition to the frontal bone already described. -2 zygomatic (cheek) bones -1 maxilla -2 nasal bones -2 lacrimal bones -1 vomer -2 palatine bones -2 inferior conchae 1 mandible
Zygomatic (cheek) bone
The zygomatic bone originates as two bones that fuse before birth. They form the prominences of the cheeks and part of the floor and lateral walls of the orbital cavities.
Maxilla (upper jawbone)
This originates as two bones that fuse before birth. The maxilla forms the upper jaw, the anterior part of the roof of the mouth, the lateral walls of the nasal cavity, and part of the floor of the orbital cavities. The alveolar ridge, or process, projects downwards and carries the upper teeth. On each side is a large air sinus, the maxillary sinus, lined with a ciliated mucous membrane and with openings into the nasal cavity.
Nasal bone
These are two small flat bones that form the greater part of the lateral and superior surfaces of the bridge of the nose.
Lacrimal bones
These two small bones are posterior and lateral to the nasal bones and form part of the medial walls of the orbital cavities. Each is pierced by a foramen for the passage of the nasolacrimal duct that carries the tears from the medial canthus of the eye to the nasal cavity.
Vomer
The vomer is a thin flat bone that extends upwards from the middle of the hard palate to form most of the inferior part of the nasal septum. Superiorly it articulates with the perpendicular plate of the ethmoid bone.
Palatine bones
These are two small L-shaped bones. The horizontal parts unite to form the posterior part of the hard palate and the perpendicular parts project upwards to form part of the lateral walls of the nasal cavity. At their upper extremities, they form part of the orbital cavities.
Inferior conchae
Each concha is a scroll-shaped bone, which forms part of the lateral wall of the nasal cavity and projects into it below the middle concha. The superior and middle conchae are parts of the ethmoid bone. The conchae collectively increase the surface area in the nasal cavity, allowing inspired air to be warmed and humidified more effectively.
Mandible (lower jawbone)
This is the lower jaw, the only movable bone of the skull. It originates as two parts that unite at the midline. Each half consists of two main parts: a curved body with the alveolar ridge containing the lower teeth and a ramus, which projects upwards almost at right angles to the posterior end of the body.
•At the upper end, the ramus divides into the condylar process which articulates with the temporal bone to form the temporomandibular joint, and the coronoid process, which gives attachment to muscles and ligaments that close the jaw. The point where the ramus joins the body is the angle of the jaw.
hyoid bone
This is an isolated horseshoe-shaped bone lying in the soft tissues of the neck just above the larynx and below the mandible. It does not articulate with any other bone but is attached to the styloid process of the temporal bone by ligaments. It supports the larynx and gives attachment to the base of the tongue.
Fontanelles of the skull
At birth, ossification of the cranial sutures is incomplete. The skull bones do not fuse earlier to allow for molding of the baby’s head during childbirth. Where three or more bones meet there are distinct membranous areas or fontanelles. The two largest are the anterior fontanelle, not fully ossified until the child is between 12 and 18 months old, and the posterior fontanelle, usually ossified 2–3 months after birth.
Functions of the skull
The various parts of the skull have specific and different functions:
- the cranium protects the brain
- the bony eye sockets protect the eyes and give attachment to the muscles that move them
- the temporal bone protects the delicate structures of the inner ear
- the sinuses in some face and skull bones give resonance to the voice
- the bones of the face form the walls of the posterior part of the nasal cavities and form the upper part of the air passages
- the maxilla and the mandible provide alveolar ridges in which the teeth are embedded
- the mandible, controlled by muscles of the lower face, allows chewing.
Vertebral column
- There are 26 bones in the vertebral column. Twenty-four separate vertebrae extend downwards from the occipital bone of the skull; then there is the sacrum, formed from five fused vertebrae, and lastly, the coccyx, or tail, which is formed from between three and five small, fused vertebrae. The vertebral column is divided into different regions. The first seven vertebrae, in the neck, form the cervical spine; the next 12 vertebrae are the thoracic spine, and the next five are the lumbar spine, the lowest vertebra of which articulates with the sacrum. Each vertebra is identified by the first letter of its region in the spine, followed by a number indicating its position. For example, the topmost vertebra is C1, and the third lumbar vertebra is L3.
- The movable vertebrae have many characteristics in common, but some groups have distinguishing features
The body
This is the broad, flattened, largest part of the vertebra. When the vertebrae are stacked together in the vertebral column, it is the flattened surfaces of the body of each vertebra that articulate with the corresponding surfaces of adjacent vertebrae. However, there is no direct bone-to-bone contact since between each pair of bones is a tough pad of fibrocartilage called the intervertebral disc. The bodies of the vertebrae lie to the front of the vertebral column, increasing greatly in size towards the base of the spine, as the lower spine must support much more weight than the upper regions.
The vertebral (neural) arch
This encloses a large vertebral foramen. It lies behind the body and forms the posterior and lateral walls of the vertebral foramen. The lateral walls are formed from plates of bone called pedicles, and the posterior walls are formed from laminae. Projecting from the regions where the pedicle meets the lamina is a lateral prominence, the transverse process, and where the two laminae meet at the back is a process called the spinous process. These bony prominences can be felt through the skin along the length of the spine. The vertebral arch has four articular surfaces: two articulate with the vertebra above and two with the one below. The vertebral foramina form the vertebral (neural) canal that contains the spinal cord.
Cervical vertebrae
- These are the smallest vertebrae. The transverse processes have a foramen through which a vertebral artery passes upwards to the brain. The first two cervical vertebrae, the atlas, and the axis are atypical.
- The first cervical vertebra (C1), the atlas, is the bone on which the skull rests. Below the atlas is the axis, the second cervical vertebra (C2).
- The atlas is essentially a ring of bone, with no distinct body or spinous process, although it has two short transverse processes. It possesses two flattened facets that articulate with the occipital bone; these are condyloid joints, and they permit nodding of the head.
- The axis sits below the atlas and has a small body with a small superior projection called the odontoid process (also called the dens, meaning tooth). This occupies part of the posterior foramen of the atlas above and is held securely within it by the transverse ligament. The head pivots (i.e., turns from side to side) on this joint.
- The 7th cervical vertebra, C7, is also known as the vertebra prominins. It possesses a long spinous prominence terminating in a swollen tubercle, which is easily felt at the base of the neck.
Thoracic vertebrae
The 12 thoracic vertebrae are larger than the cervical vertebrae because this section of the vertebral column must support more bodyweight. The bodies and transverse processes have facets for articulation with the ribs.
Lumbar vertebrae
These are the largest of the vertebrae because they must support the weight of the upper body. They have substantial spinous processes for attachment of the muscles of the lower back.
Sacrum
This consists of five rudimentary vertebrae fused to form a triangular or wedge-shaped bone with a concave anterior surface. The upper part, or base, articulates with the 5th lumbar vertebra. On each side, it articulates with the ilium to form a sacroiliac joint, and at its inferior tip, it articulates with the coccyx. The anterior edge of the base, the promontory, protrudes into the pelvic cavity. The vertebral foramina are present, and on each side of the bone, there is a series of foramina for the passage of nerves.
Coccyx
This consists of the four-terminal vertebrae fused to form a very small triangular bone, the broad base of which articulates with the tip of the sacrum.
Intervertebral discs
The bodies of adjacent vertebrae are separated by intervertebral discs, consisting of an outer rim of fibrocartilage (annulus fibrosus) and a central core of soft gelatinous material (nucleus pulposus). They are thinnest in the cervical region and become progressively thicker towards the lumbar region, as spinal loading increases. The posterior longitudinal ligament in the vertebral canal helps to keep them in place. They have a shock-absorbing function and the cartilaginous joints they form contribute to the flexibility of the vertebral column.
Intervertebral discs
- When two adjacent vertebrae are viewed from the side, a foramen formed by a gap between adjacent vertebral pedicles can be seen.
- Throughout the length of the column there is an intervertebral foramen on each side between every pair of vertebrae, through which the spinal nerves, blood vessels, and lymph vessels pass
Intervertebral foramina
- When two adjacent vertebrae are viewed from the side, a foramen formed by a gap between adjacent vertebral pedicles can be seen.
- Throughout the length of the column there is an intervertebral foramen on each side between every pair of vertebrae, through which the spinal nerves, blood vessels, and lymph vessels pass
Ligaments of the vertebral column
- These ligaments hold the vertebrae together and keep the intervertebral discs in position.
- The transverse ligament holds the odontoid process of the axis in the correct position in relation to the atlas.
- The anterior longitudinal ligament extends the whole length of the column and lies in front of the vertebral bodies.
- The posterior longitudinal ligament lies inside the vertebral canal and extends the whole length of the vertebral
- Column in close contact with the posterior surface of the bodies of the bones.
- The ligament Flava connects the laminae of adjacent vertebrae.
- The ligament nuchae and the supraspinous ligament connect the spinous processes, extending from the occiput to the sacrum.
Curves of the vertebral column
- When viewed from the side, the vertebral column presents four curves: two primary and two secondaries.
- The fetus in the uterus lies curled up so that the head and the knees are touching. This position shows the primary curvature. The secondary cervical curve develops when the child can hold up their head (after about 3 months) and the secondary lumbar curve develops when able to stand (after 12–15 months). The thoracic and sacral primary curves are retained.
movement of the vertebral column
Movement between the individual bones of the vertebral column is very limited. However, the movements of the column are quite extensive and include flexion (bending forward), extension (bending backward), lateral flexion (bending to the side), and rotation. There is more movement in the cervical and lumbar regions than elsewhere.
Functions of the vertebral column
These include:
- collectively the vertebral foramina form the vertebral canal, which provides strong bony protection for the delicate spinal cord lying within it
- the pedicles of adjacent vertebrae form intervertebral foramina, one on each side, providing access to the spinal cord for spinal nerves, blood vessels, and lymph vessels
- the numerous individual bones with their intervertebral discs allow movement of the whole column
- support of the skull
- the intervertebral discs act as shock absorbers, protecting the brain
- formation of the axis of the trunk, giving attachment to the ribs, shoulder girdle, and upper limbs, and the pelvic girdle and lower limbs.
Thoracic cage
The thorax (thoracic cage) is formed by the sternum anteriorly, twelve pairs of ribs forming the lateral bony cages, and the twelve thoracic vertebrae.
Sternum (breastbone)
- This flat bone can be felt just under the skin in the middle of the front of the chest.
- The manubrium is the uppermost section and articulates with the clavicles at the sternoclavicular joints and with the first two pairs of ribs.
- The body or middle portion gives attachment to the ribs.
- The xiphoid process is the inferior tip of the bone. It gives attachment to the diaphragm, muscles of the anterior abdominal wall, and the Linea alba (literally ‘white line’)
Ribs
- The 12 pairs of ribs form the lateral walls of the thoracic cage. They are elongated curved bones that articulate posteriorly with the vertebral column. Anteriorly, the first seven pairs of ribs articulate directly with the sternum and are known as the true ribs. The next three pairs articulate only indirectly. In both cases, costal cartilages attach the ribs to the sternum. The lowest two pairs of ribs, referred to as floating ribs, do not join the sternum at all, their anterior tips being free.
- Each rib forms up to three joints with the vertebral column. Two of these joints are formed between facets on the head of the rib and facets on the bodies of two vertebrae, the one above the rib and the one below. Ten of the ribs also form joints between the tubercle of the rib and the transverse process of (usually) the lower vertebra.
- The inferior surface of the rib is deeply grooved, providing a channel along which intercostal nerves and blood vessels run. Between each rib and the one below are the intercostal muscles, which move the rib cage during breathing.
- Because of the arrangement of the ribs, and the quantity of cartilage present in the ribcage, it is a flexible structure that can change its shape and size during breathing. The first rib is firmly fixed to the sternum and to the 1st thoracic vertebra and does not move during inspiration. Because it is a fixed point, when the intercostal muscles contract, they pull the entire ribcage upwards towards the first rib.
Shoulder girdle
The shoulder girdle consists of two clavicles and two scapulae.
Clavicle (collar bone)
The clavicle is an S-shaped long bone. It articulates with the manubrium of the sternum at the sternoclavicular joint and forms the acromioclavicular joint with the acromion process of the scapula. The clavicle provides the only bony link between the upper limb and the axial skeleton.
Scapula (shoulder blade)
- The scapula is a flat triangular-shaped bone, lying on the posterior chest wall superficial to the ribs and separated from them by muscles.
- At the lateral angle is a shallow articular surface, the glenoid cavity, which, with the head of the humerus, forms the shoulder joint.
- On the posterior surface runs a rough ridge called the spine, which extends beyond the lateral border of the scapula and overhangs the glenoid cavity. The prominent overhang, which can be felt through the skin as the highest point of the shoulder, is called the acromion process and forms a joint with the clavicle, the acromioclavicular joint, a slightly movable synovial joint that contributes to the mobility of the shoulder girdle. The coracoid process, a projection from the upper border of the bone, gives attachment to muscles that move the shoulder joint.
Homarus
- This is the bone of the upper arm. The head sits within the glenoid cavity of the scapula, forming the shoulder joint. Distal to the head are two roughened projections of bone, the greater and lesser tubercles, and between them there is a deep groove, the bicipital groove or intertubercular sulcus, occupied by one of the tendons of the biceps muscle.
- The distal end of the bone presents two surfaces that articulate with the radius and ulna to form the elbow joint.
Ulna and radius
•These are the two bones of the forearm. The ulna is longer than and medial to the radius and when the arm is in the anatomical position, i.e., with the palm of the hand facing forward, the two bones are parallel. They articulate with the humerus at the elbow joint, the carpal bones at the wrist joint, and with each other at the proximal and distal radioulnar joints. In addition, an interosseous membrane, a fibrous joint, connects the bones along their shafts, stabilizing their association and maintaining their relative positions despite forces applied from the elbow or wrist.
Carpal (wrist bones)
•There are eight carpal bones arranged in two rows of four. From outside inwards they are:
-proximal row: scaphoid, lunate, triquetrum, pisiform
-distal row: trapezium, trapezoid, capitate, hamate.
•These bones are closely fitted together and held in position by ligaments that allow a limited amount of movement between them. The bones of the proximal row are associated with the wrist joint and those of the distal row form joints with the metacarpal bones. Tendons of muscles lying in the forearm cross the wrist and are held close to the bones by strong fibrous bands called retinacula
