skeletal reading AS111

Introduction to Bones and the Skeleton

  • Imagining an animal's body without a skeleton:
    • Appears as a furry sac of semisoft, gelatin-like material.
    • Without bones, muscles cannot leverage movement, rendering the body immobilized.
  • Definition of Skeleton:
    • The framework of bones that supports and protects the soft tissues of the body.
    • Serves multiple functions beyond skeletal structure.

Bone Overview

Bone Terminology

  • Terms associated with bone include:
    • Os and Osteo-: Generally refer to bone.
    • Example: os penis (dog's bone) and osteocytes (bone cells).

Bone Characteristics

  • Bone is the second hardest natural substance in the body.
    • Harder than all but dental enamel.
  • Vital living tissue with excellent repair capacity:
    • Healing of broken bones requires alignment and immobilization for weeks or months.
    • Clinical application emphasizes fracture repair techniques.
  • Composition of bone:
    • Sparse cell population in a hard intercellular matrix.
    • Osteoblasts: Cells that produce bone by secreting a matrix initially soft with collagen fibers.
    • The matrix undergoes ossification when it hardens through infiltration of calcium and phosphate in hydroxyapatite crystals.
    • Osteoblasts become Osteocytes after being trapped within lacunae of the ossified matrix:
    • Connect with other cells and blood supply through canaliculi (tiny channels).

Functions of Bones

Support

  • Bones serve as scaffolding for the body, supporting softer tissues.
    • Other body tissues are weaker and rely on bones for structural integrity.

Protection

  • Bones protect vital organs by completely or partially enclosing them:
    • Example: Skull bones shield the brain, eyes, and ears.

Leverage

  • Bones act as levers for muscles to facilitate body movement.
    • Muscles are attached to bones through tendons, enabling joint movement.

Storage

  • Bones serve as storage for minerals, predominantly calcium:
    • Function as reservoirs for calcium to maintain controlled blood levels critical for various body functions.
    • Calcium’s roles include: muscle contraction, blood clotting, milk secretion, and skeleton maintenance.
    • Hormonal regulation:
    • Calcitonin: Prevents hypercalcemia (high calcium).
    • Parathyroid hormone: Prevents hypocalcemia (low calcium).

Blood Cell Formation

  • Some bones are sites for hematopoiesis (blood cell formation) within bone marrow.

Bone Structure

Types of Bone

  • Two main types of bone:
    • Cancellous Bone (Spongy Bone):
    • Light but strong, appearing sponge-like with spaces for bone marrow.
    • Arranged to resist forces of muscle and gravity while minimizing weight.
    • Compact Bone:
    • Dense, forming shafts of long bones and outer layers of all bones.
    • Comprised of Haversian systems (cylinders of bone matrix around central canals for blood vessels).

Bone Cells

Types

  • Osteoblasts:
    • Cells that secrete bone matrix and facilitate mineral deposition for hardening.
  • Osteocytes:
    • Mature osteoblasts trapped in matrix, capable of reverting to osteoblasts for bone formation.
  • Osteoclasts:
    • Cells that break down bone; important for remodeling and calcium release.

Blood Supply to Bone

  • Blood vessels penetrate from periosteum into the bone matrix via Volkmann's canals and Haversian canals.
    • Larger vessels enter through nutrient foramina, often mistaken for fractures in radiographs.

Bone Formation Mechanisms

  • Two mechanisms for bone formation:
    • Endochondral Bone Formation:
    • Bone replaces cartilage model.
    • Primary growth centers develop in diaphysis, then secondary centers in epiphyses.
    • Intramembranous Bone Formation:
    • Development from fibrous tissue membranes, primarily in skull bones.

Bone Shapes

  • Four basic shapes:
    • Long Bones:
    • Longer than wide; includes limb bones.
    • Short Bones:
    • Cube-shaped; e.g. carpal and tarsal bones.
    • Flat Bones:
    • Thin and flat; sandwich-like structure with compact and cancellous layers (e.g. scapula).
    • Irregular Bones:
    • Do not fit other categories; e.g. vertebrae.

Bone Marrow Types

Red Bone Marrow

  • Hematopoietic tissue, forming blood cells.
    • Most prevalent in young animals, only found in certain areas in older animals.

Yellow Bone Marrow

  • Primarily composed of fat tissue, predominant in mature animals.
    • Can convert back to red marrow under certain conditions like chronic blood loss.

Common Bone Features

Articular Surfaces

  • Smooth, compact bone areas where bones meet to form joints.
    • Covered with articular cartilage to reduce friction.

Processes

  • Projections on bones labeled differently based on location.
    • Include condyles, heads, and tubercles which indicate muscle attachment or joint formation.

Holes and Depressed Areas

  • Foramen: Holes for nerves/blood vessels, may serve structural purposes.
  • Fossa: Depressed areas, often housing muscles or tendons.

Axial vs. Appendicular Skeleton

Axial Skeleton

  • Comprises bones of head and trunk, includes:
    • Skull, hyoid bone, spinal column, ribs, and sternum.
    • Situated along the body’s median plane.

Skull Overview

  • The most complex part, made up of 37-38 separate bones united by sutures.
    • Grouped into cranium, ear bones, and facial bones.

External Bones of the Cranium

  • Occipital: Forms skull base, important for spinal cord passage; articulates with cervical vertebra.
  • Interparietal: Small bones between occipital and parietal bones.
  • Parietal: Forms the dorsolateral skull walls.
  • Temporal: Houses ear structures; forms temporomandibular joint with mandible.
  • Frontal: Forms the forehead; houses frontal sinus and cornual process in cattle.

Internal Bones of the Cranium

  • Sphenoid: Ventral cranium bone housing the pituitary gland.
  • Ethmoid: Contains cribriform plate facilitating olfactory nerve passage.

Bones of the Ear

  • Ossicles: Malleus (hammer), incus (anvil), stapes (stirrup) transmit vibrations from tympanic membrane to cochlea.

External Bones of the Face

  • Incisive: Houses upper incisor teeth, varies among species.
  • Nasal: Forms the bridge of the nose, shape varies by species.
  • Maxillary: Forms upper jaw; houses teeth and sinuses, contributes to hard palate.
  • Lacrimal: Contains structures for tear drainage.
  • Zygomatic: Forms part of eye orbit and zygomatic arches.
  • Mandible: Lower jaw, movable; critical for TMJ formation and susceptible to fractures.

Internal Bones of the Face

  • Palatine: Partly forms hard palate, separates mouth/nasal cavity.
  • Pterygoid: Supports pharynx wall.
  • Vomer: Forms nasal septum.
  • Turbinates (Nasal Conchae): Enhance inhalation by warming and humidifying air.

Hyoid Bone

  • Supports tongue, pharynx, and larynx, facilitates swallowing; resembles the letter H.

Spinal Column

  • Also known as vertebral column, consists of vertebrae divided into cervical, thoracic, lumbar, sacral, and coccygeal regions.
  • Numbering convention refers to each region (C, T, L, S, Cy) followed by vertebra number.

Vertebrae Characteristics

  • Composed of a body, arch, and processes.
    • Body: Strongest portion, separated by intervertebral discs.
    • Hollow arch protects the spinal cord.
    • Processes for muscle attachment and vertebrae articulation.

Cervical Vertebrae

  • 7 in all mammals, includes unique atlas (C1, supports head) and axis (C2, allows rotation).

Thoracic Vertebrae

  • Associated with ribs, number correlates with rib pairs.

Lumbar Vertebrae

  • Support abdominal weight; larger/stronger.

Sacral Vertebrae

  • Fused to form sacrum; articulates with pelvis.

Coccygeal Vertebrae

  • Tail bones; number varies greatly.

Ribs

  • Flat bones forming thorax, number typically equals thoracic vertebrae pairs.
  • Each rib has bony (dorsal) and cartilaginous (ventral) parts.
    • Sternal ribs connect with sternum; asternal ribs connect to adjacent cartilage; floating ribs do not connect.

Sternum

  • Breastbone made of sternebrae; named manubrium (first) and xiphoid (last).

Appendicular Skeleton

  • Comprised of limb bones; thoracic limb has no direct bony connection to axial skeleton.

Thoracic Limb

Scapula

  • Triangular bone forming shoulder joint's socket (glenoid cavity).

Humerus

  • Long bone of upper arm; articulates with scapula and forms elbow with radius and ulna.

Ulna and Radius

  • Ulna: Forms elbow with humerus, has distinct processes for muscle attachment.
  • Radius: Weight-bearing bone of forearm; articulates with humerus and carpal bones.

Carpal Bones

  • Two rows of short bones; function varies among species.

Metacarpal Bones

  • Extend from carpal bones to digits; vary according to species, with unique adaptations.

Phalanges

  • Digital bones; horses have three (proximal, middle, distal), while dogs and cats have 5.

Pelvic Limb

  • Connected to axial skeleton via sacroiliac joint.

Pelvis

  • Contains ilium, ischium, and pubis; joined at the acetabulum for hip joint connection.

Femur

  • Long bone of thigh; forms hip joint with pelvis and knee joint with tibia.

Patella

  • Largest sesamoid bone, protecting knee tendon.

Tibia and Fibula

  • Tibia: Main weight-bearing bone of lower leg.
  • Fibula: Supports muscle attachment without bearing weight.

Tarsal Bones

  • Form ankle/hock; named proximal row, numbered distal row.

Metatarsal Bones

  • Similar to metacarpal bones, relevant differences based on animal foot structure.

Phalanges of Pelvic Limb

  • Similar to thoracic limb with variations across species.

Visceral Skeleton

  • Composed of variable bones not found in all animals, forming in soft organs.
    • Examples: os cordis (heart), os penis (dogs), os rostri (pigs).

Joints

Joint Terminology

  • Terms arthro- and articular are related to joints; confusion exists with complex anatomical terms.

Types of Joints

  • Three main classifications
    • Fibrous Joints (Synarthroses): Immovable, as in skull sutures.
    • Cartilaginous Joints (Amphiarthroses): Slightly movable, as in intervertebral discs.
    • Synovial Joints (Diarthroses): Freely movable, including several common types.

Synovial Joint Characteristics

  • Include structures like articular surfaces, articular cartilage, joint cavity, and ligaments.

Types of Synovial Joints

  • Hinge Joints (Ginglymus): Allow flexion/extension (e.g., elbow).
  • Gliding Joints (Arthrodial): Rocking movements (e.g., carpus).
  • Pivot Joints (Trochoid): Rotation around an axis (e.g., atlantoaxial joint).
  • Ball-and-Socket Joints (Spheroidal): Allow extensive movement (e.g., shoulder, hip).

Conclusion

  • The complex nature of bones and joints illustrates their essential roles in animal anatomy and function covering support, movement, and protection. Understanding these concepts is crucial for comprehending the skeletal system's overall function and health.