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UNIT- 2
SKELETON SYSTEM
keleton System deals with the study of bones and joints. Apart from these, the skeleton system also deals with the study of joints, their types and various movements of joints. For the study purposes the whole skeletal system is divided into: 1.Axial Skeleton 2.Appendicular Skeleton
- Axial skeleton consists of study of Skull,Ribs,Sternum and Vertebral Column.
- Appendicular Skeleton consists of study the bones of upper limb (Hand) and lower limb(Leg). Bone Bone tissue makes up about 17-18% of the weight of the human body. Composition of bones: Because the bone is a types of connective tissue so it contains extracellular matrix and 23 - 25% water, 22-26% collagen fibers, and 49- 51% crystallized mineral salts i.e- calcium phosphate [Ca 3 (PO^4 ) 2 ] and calcium hydroxide [Ca(OH) 2 ].After combination they form crystals of hydroxyapatite [Ca 10 (PO^4 )^6 (OH) 2 ]. Other mineral salts, are calcium carbonate (CaCO3), and ions are magnesium, fluoride, potassium, and sulfate. Functions of Skeleton System. 1. Body framework and Support. The skeleton serves as the structural framework for the body by supporting soft tissues and. 2. Internal organ Protection. The skeleton protects the most important internal organs from injury. For example:
- Cranial bones protect the brain
- Vertebrae (backbones) protect the spinal cord
- Rib cage protects the heart and lungs 3. Body Movement. Most skeletal muscles attach to bones to produce movement.
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4. Mineral storage and release. It stores calcium and phosphorus, which contribute to the strength of bone. 5. Blood cell production. A process called hemopoiesis in red bone marrow which is presnt in long bones. 6. Triglyceride storage. Adipose cells present in Yellow bone marrow stores triglycerides. Types of Bones: S.No. Types of Bones Example 1. Long bones Femur,Humerus 2. Short bones Carpals and Tarsals 3. Flat bones Skull bones,Sternum,Ribs 4. Irregular bones Bones of Vertebrae 5. Sesamoid bones Patella
7. Endosteum is a thin membrane that lines the internal bone surface facing the medullary cavity. It contains a single layer of cells and a small amount of connective tissue. Various Types of Cells in bone Four types of cells are present in bone tissue: 1.Osteogenic cells Function: unspecialized stem cells derived from mesenchymal origin.They are the only bone cells to undergo cell division and develop into osteoblasts. Location: Inner portion of the periosteum, in the endosteum. 2.Osteoblasts Function: bone-building cells. They synthesize and secrete collagen fibers and other organic components which forms extracellular matrix of bone tissue and initiate calsification. Note: The ending blast in the name of a bone cell or any other connective tissue cell means that the cell secretes extracellular matrix. 3.Osteocytes Function: As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes. These are the main cells in bone tissue.Like osteoblasts, osteocytes do not undergo cell division. Note: The ending - cyte in the name of a bone cell or any other tissue cell means that the cell maintains the tissue. 4.Osteoclasts Function: Giant cell formed by fusion of approximately 50 monocytes. These cells have lysosomal enzymes and acids that digest the protein and mineral components of the underlying bone matrix. Note: The breakdown of bone extracellular matrix, is called as resorption which is essential for normal development, maintenance, and repair of bone. Bone formation:
The process by which bone forms is called ossification or osteogenesis. Bone formation occurs in four principal situations:
1. During embrogenesis. 2. Growth of bone during normal development from infant to adult. 3. During replacement of bone through out life from old to newer bone tissue. 4. During fracture healing. Bones are formed mainly by two methods: 1.Intramembranous ossification. 2.Endochondral ossification.
- First type of ossification, called intramembranous ossification , bone forms directly within mesenchyme arranged in sheet like layers that resemble membranes.
- Second type, endochondral ossification , bone forms within hyaline cartilage that develops from mesenchyme. Intramembranous Ossification Intramembranous ossification is the simpler of the two methods of bone formation. The flat bones of the skull and mandible (lower jawbone) are formed in this way. **Steps are as follows:
- Development of the ossification center.** At the place of bone development specific chemical messages cause the mesenchymal cells to cluster together and differentiate, first into osteogenic cells and then into osteoblasts. The site of such a cluster is called an ossification center. During this phase extracellular matrix secretion occurs. 2.Calcification. During calcification the deposition of calcium and other minerals ocuurs over extracellular matrix and causes hardening of the developing bone causes calcification. 3.Formation of trabeculae. As soon as the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone. Angiogenesis also occurs during this phase in between the spaces between the trabeculae along with bone marrow formation.
By the action of osteoclast the newly formed spongy bone breakes down in the center leaves a hollow space called medullary cavity. Gradually, most of the wall of the diaphysis is replaced by compact bone. 5.Development of the secondary ossification centers. When branches of the epiphyseal artery enter the epiphyses, secondary ossification centers develop, usually around the time of birth. 6.Formation of articular cartilage and the epiphyseal plate. The hyaline cartilage that covers the epiphyses becomes the articular cartilage and helps in joint formation. Difference between Compact and Spongy Bone Tissue Compact bone tissue Location: It contains few spaces and is the strongest form of bone tissue. It is found beneath the periosteum of all bones and makes up the bulk of the diaphyses of long bones. Functions: Compact bone tissue provides protection and support and resists the stresses produced by weight and movement. Characteristics of Compact Bones:
- Compact bone has "Osteons" the basic structural unit.
- It contains Volkmann’s canals from where the nerve,blood vessels passes through one osteon to another.
- It contains central or haversian canals (longitudinal hollow canal within the bone.
- Around the central canals are concentric lamellae rings of calcified extracellular matrix. Between the lamellae are small spaces called lacunae , which contain osteocytes.
- From the lacunae,in all directions there are tiny canaliculi , filled with extracellular fluid which contain fingerlike processes of osteocytes.
- The components of compact bone tissue are arranged into repeating structural units called osteons or haversian systems.
- Each osteon consists of a central (haversian) canal with its concentrically arranged lamellae, lacunae, osteocytes, and canaliculi.
- Osteons in compact bone tissue are arranged longitudinally. Spongy Bone vs.Compact Bone Spongy Bone Tissue
- Does not contain osteons.
- Spongy bone consists of lamellae arranged in an irregular lattice of thin columns called trabeculae.
- The spaces between the trabeculae help make bones lighter and can sometimes be filled with red bone marrow, which contains numerous small blood vessels.
- Within each trabecula are lacunae that contain osteocytes. Canaliculi radiate outward from the lacunae.
- Spongy bone is always covered by a layer of compact bone for protection. The trabeculae of spongy bone tissue appear to be randomly arranged.
- Spongy bone tissue tends to be located where bones are not heavily stressed or where stresses are applied from many directions.
The Axial Skeleton System Skull:
- The skull (cranium), with its 22 bones, rests on the superior end of the vertebral column (backbone).
- The bones of the skull are grouped into two categories: cranial bones and facial bones.
- The cranial bones form the cranial cavity, which encloses and protects the brain. The eight cranial bones are as follows: Depressions for passing Nerves, Blood Vessels, ligaments, tendons. S.No Marking Description 1. Fissure Narrow slit between adjacent parts of bones 2. Foramen Hole through which nerves or blood vessels pass. 3. Fossa Wide, shallow Depression 4. Sulcus Furrow like space along bones. 5. Meatus Tube like space inside bone. Elevated part of bones which forms junctions with tendons & ligaments 6. Condyle Rounded enlarge part over bone endings 7. Facet Smooth flat articular surface. 8. Head Rounded projection on neck of bone. 9. Crest Prominent ridge 10. Epichondyle Present above condyle 11. Line Long narrow ridge 12. Spinous process Sharp projection in bone 13. Trochanter Large projection 14. Tubercle Small ,rounded projection 15. Tuberosity Large, roughened projection
Fourteen facial bones are as follows:
- Frontal bone =
- Parietal bones =
- Temporal bones=
- Occipital bone=
- Sphenoid bone =
- Ethmoid bone=
- Nasal bones=
- Maxillae (or maxillas)=
- Zygomatic bones=
- Mandible =
- Lacrimal bones=
- Palatine bones=
- Inferior nasal conchae=
- Vomer=
Ethmoid Bone
- The ethmoid bone (like a sieve) is spongelike in appearance and is located on the midline in the anterior part of the cranial floor medial to the orbits. It is anterior to the sphenoid and posterior to the nasal bones.
- The ethmoid bone forms: 1. Part of the anterior portion of the cranial floor; 2. The medial wall of the orbits. 3. The superior portion of the nasal septum, a partition that divides the nasal cavity into right and left sides. 4. Most of the superior sidewalls of the nasal cavity. The ethmoid bone is a major superior supporting structure of the nasal cavity. FACIAL BONES
- The 14 facial bones includes: ✓ 2 nasal bones, ✓ 2 maxillae (or maxillas), ✓ 2 zygomatic bones, ✓ 1 mandible, ✓ 2 lacrimal bones, ✓ 2 palatine bones, ✓ 2 inferior nasal conchae, ✓ 1 vomer. Nasal Bones The paired nasal bones meet at the midline and form the bridge of the nose. Zygomatic bones The 2 zygomatic bones commonly called cheekbones, form the prominences of the cheeks and part of the lateral wall and floor of each orbit. Lacrimal Bones The 2 lacrimal bones resemble a fingernail in size and shape. These bones are the smallest bones of the face, they are posterior and lateral to the nasal bones and form a part of the medial wall of each orbit.
Palatine Bones The 2 (L-shaped) palatine bones form the posterior portion of the hard palate, part of the floor and lateral wall of the nasal cavity, and a small portion of the floors of the orbits. Inferior Nasal Conchae The 2 inferior nasal conchae, form a part of the inferior lateral wall of the nasal cavity and project into the nasal cavity. All three pairs of nasal conchae (superior, middle, and inferior) increase the surface area of the nasal cavity and help swirl and filter air before it passes into the lungs. Vomer The vomer is a triangular bone on the floor of the nasal cavity that articulates superiorly with the perpendicular plate of the ethmoid bone and inferiorly with both the maxillae and palatine bones along the midline. It forms the inferior portion of the nasal septum. Mandible The mandible or lower jawbone, is the largest, strongest facial bone. It is the only movable skull bone (other than the auditory ossicles). Features of The Skull The skull exhibits several unique features not seen in other bones of the body. These include sutures, paranasal sinuses, and fontanels. Sutures A suture (seam) is an immovable joint in most cases in an adult skull that holds most skull bones together. Types of sutures:
1. The coronal suture (crown) unites the frontal bone and both parietal bones. 2. The sagittal suture (arrow) unites the two parietal bones on the superior midline of the skull . 3. The lambdoid suture unites the two parietal bones to the occipital bone. 4. The squamous sutures unite the parietal and temporal bones on the lateral aspects of the skull. Paranasal Sinuses
- The vertebral column, also called the spine, backbone, or spinal column, makes up about two-fifths of your total height and is composed of a series of bones called vertebrae.
- At about 71 cm (28 in.) in an average adult male and about 61 cm (24 in.) in an average adult female.
- The total number of vertebrae during early development is 33. As a child grows, several vertebrae in the sacral and coccygeal regions fuse.
- As a result, the adult vertebral column typically contains 26 vertebrae.
- These are distributed as follows: ✓ 7 cervical vertebrae- ( cervic- _ neck) are in the neck region. ✓ 12 thoracic vertebrae- ( thorax _ chest) are posterior to the thoracic cavity. ✓ 5 lumbar vertebrae - support the lower back. ✓ 1 sacrum - consists of five fused sacral vertebrae. ✓ 1 coccyx - usually consists of four fused coccygeal vertebrae. Note: The cervical, thoracic, and lumbar vertebrae are movable. The sacrum and coccyx are not movable.
Normal Curves of the Vertebral Column ✓ When viewed from the side, it shows four slight bends called normal curves. ✓ Relative to the front of the body, the cervical and lumbar curves are convex (bulging out). ✓ The thoracic and sacral curves are concave (cupping in). ✓ The curves of the vertebral column increase its strength, help maintain balance in the upright position, absorb shocks during walking, and help protect the vertebrae from fracture. ✓ Due to abnormal day to day activities,the change in normal curves occurs as-kyphosis, lordosis, and scoliosis. Regions of the Vertebral Column ❖ The regions are the: ✓ Cervical ✓ Thoracic ✓ Lumbar ✓ Sacral ✓ Coccygeal
Difference between Cervical, Thoracic, and Lumbar Vetebrae
Characteristic Lumbar Thoracic Cervical Structure Body Largest Larger Small Foramina One vertebral One Vertebral Two transverse and one vertebral Spinous Process Short & Blunt Long & Fairly thick Slender & Bifid Transverse processes Large and Blunt Large Small Size of Intervertebral Discc Huge Relatively thin Thick Sacrum
- The sacrum is a triangular bone formed by the union of five sacral vertebrae (S1–S5).
- The sacral vertebrae begin to fuse in individuals between 16 and 18 years of age, a process usually completed by age 30. Coccyx
- The coccyx, like the sacrum, is triangular in shape.
- It is formed by the fusion of usually four coccygeal vertebrae(Co 1 – Co 4 ).
- The coccygeal vertebrae fuse somewhat later than the sacral vertebrae, between the ages of 20 and 30. The dorsal surface of the body of the coccyx contains two long coccygeal cornua that are connected by ligaments to the sacral cornua.
Thorax
- The term thorax refers to the entire chest.
- The skeletal part of the thorax, the thoracic cage, is a bony enclosure formed by the sternum, ribs and their costal cartilages, and the bodies of the thoracic vertebrae. Sternum
- The sternum, or breastbone, is a flat, narrow bone located in the center of the anterior thoracic wall that measures about 15 cm (6 in.) in length and consists of three parts.
- The superior part is the manubrium , the middle and largest part is the body; and the inferior, smallest part is the xiphoid process. Ribs
- Twelve pairs of ribs, numbered 1–12 from superior to inferior, give structural support to the sides of the thoracic cavity.
- The ribs that have costal cartilages and attach directly to the sternum are called true ribs.
- The remaining five pairs of ribs are termed false ribs because their costal cartilages either attach indirectly to the sternum or do not attach to the sternum at all.
- The eleventh and twelfth pairs of ribs are false ribs designated as floating (vertebral) ribs because the costal cartilage at their anterior ends does not attach to the sternum at all. These ribs attach only posteriorly to the thoracic vertebrae.
