The Skeletal System

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Chapter 6

• The Skeletal System

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Functions of Bone

• Support and Protection
• It provides a framework for the body and gives it
  shape.
• It supports and protects organs from injury.
• Leverage
• It provides a place for muscles, tendons,
  ligaments and tendons of the body to attach.
• It helps make movement possible.
• Storage
• It stores minerals (Calcium)
• Blood Cell Formation
• It provides a place for hemotpoeisis

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Bone Structure

• Two main types of bone
• Cancellous Bone
• Light and spongy inner layer of bone.
• Consists of tiny spicules with bone marrow
  between.
• Provides strength but prevents damage.
• Compact Bone
• Heavy and dense layer of outer bone
• Found in shafts of long bones
• Composed of haversian systems around a haversian
  canal

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Bone Structure continued

• Osteocytes- bone cells.
• Periosteum- covering of bone
• Inner layer contains osteoblasts.
• Endosteum- membrane that lines the hollow
  interior surface of bones.

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Haversian Systems

• Concentric layers of ossified bone matrix
  arranged around a central Haversian canal
• Layers of ossified bone matrix

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Bone Cells

• Osteoblasts
• Cells that form bone.
• Osteocytes
• Once osteoblasts are trapped inside matrix of
  osseous material.
• Can revert back to osteoblasts (remember this
  from last chapter?)
• Osteoclasts
• Eat bone away.
• Are the remodelers of bone
• Withdraw calcium when needed from bone
• Why is this important?

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Blood Supply to Bone

• Tiny vessels penetrate the periosteum.
• Volkmanns canals- tiny channels in the bone
  matrix that vessels pass through.
• Are at right angles to Haversian canals that run
  lengthwise in the bone.
• Nutrient Foramina- Where large vessels enter the
  bone.
• Carry blood into and out of bone marrow.
• Can be mistaken for fracture on radiographs.

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Bone Formation

• Bone is formed in 2 ways
• Endochondral Bone formation
• Intramembranous Bone formation
• Bone formation and growth is stimulated by Growth
  Hormone (GH) from the anterior pituitary
  (adenohypophysis) gland in the brain.

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Endochondral Bone Formation

• Endochondral Bone formation
• Cartilage bone formation
• Cartilage first, then bone
• How most bones develop
• Start as cartilage rods in long bones in
  diaphysis (shaft).
• Contains primary growth center
• Cartilage is removed gradually as bone is created
  and growth center expands.
• Secondary growth center
• Develop in epiphysis (ends) of bones

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Growth plates

• Located between diaphysis and epiphysis.
• May be called epiphyseal growth plates.
• Sites of creation of new bone that allows bone to
  lengthen as animal grows.
• Cartilage is created on epiphyseal side while
  bone is created on diaphyseal side.
• When bone reaches full length, all cartilage is
  replaced by bone and plates Close.
• Remodeling may take place but bone will not get
  any longer.
• Young animals may have epiphyseal fractures
  because this area is weaker than rest of bone.

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Panosteitis

• Growing Pains in dogs.
• Inflammation of various bone layers
• Seen prevalently in young giant breed dogs.
• Basically, body can not keep up as bones are
  growing very quickly.
• Can be confirmed through Radiographs.
• May cause Shifting Leg Lameness
• Treated with rest and anti-inflammatories
• Usually occurs in the center of the bone.

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Intramembranous Bone Formation

• Occurs only in certain skull bones
• Bone forms in the fibrous membranes that cover
  the brain in fetus.
• Bone forms directly from osteoblasts with no
  cartilage intermediary.

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Bone Shapes

• Long Bones
• Short Bones
• Flat Bones
• Irregular Bones

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Long Bones

• Longer than they are wide.
• Has a proximal and distal epiphysis consisting of
  cancellous bone.
• Main part of bone is diaphysis which composed of
  compact bone.
• Found in digits and limbs.

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Short Bones

• Shaped like cubes.
• Have core of cancellous bone covered by compact
  bone.
• Carpal and tarsal bones.

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Flat Bones

• Thin and flat bones
• Consists of two layers of compact bone separated
  by cancellous bone.
• Bones in skull, pelvis, and scapula are examples.

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Irregular Bones

• Miscellaneous bones that do not fit into another
  category.
• May have characteristics of more than one
  category.
• Include vertebrae and sesamoid bones.
• Patella is largest sesamoid bone in body.

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Bone Marrow

• Fills the spaces within bones
• Has two types
• Red bone marrow
• Hematopoietic tissue forms new blood cells.
• Majority of bone marrow in young animals but less
  of older animals
• Yellow bone marrow
• Consists primarily of adipose connective tissue.
• Common type of marrow in adult animals
• Does not produce blood cells but can revert to
  red marrow if needed.

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Common Bone Features

• Articular Surfaces
• Joint surfaces where bones come in contact with
  each other to form joints.
• Consists of
• Condyles
• Head
• Facet
• Covered by articular cartilage
• Composed of what type of cartilage?

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Condyle

• Large, round articular surface.
• Major condyle is located on end of humerus and
  femur.
• Also located in skull.

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Head

• Somewhat spherical articular surface on the
  proximal end of a long bone.
• Found on humerus, femur and rib.
• Head is usually joined with rest of bone by a
  neck.

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Femoral Head Osteotomy (FHO)

• Head of femur is removed in cases of trauma or
  severe arthritis.
• A false joint forms which gives more comfort to
  the patient.

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Facet

• A flat articular surface.
• Found in carpal and tarsal bones as well as in
  vertebrae, radius and ulna.

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Processes

• All projections of a bone.
• Heads and condyles are considered to be
  processes.
• Tendons may attach to processes

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Holes and Depressed Areas

• Foramen A hole in bone.
• Usually allow the passage of nerve or blood
  vessel.
• May exist simply to lighten structure
  (pelvis-obturator foramen)
• Fossa A depressed of sunken area on the surface
  of a bone.
• Usually occupied by muscles or tendons.

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Types of Skeletons

• Bones of head and trunk are Axial Skeleton
• Bones of limbs and appendages are Appendicular
  Skeleton.
• Some animals may have Visceral Skeleton- bones
  formed in the viscera or soft organs.

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Axial Skeleton bones of
head trunk

• Skull
• Hyoid bone
• Spinal column
• Ribs
• Sternum

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Skull

• Usually consists of 37 or 38 separate bones
• Most skull bones joined by sutures (fibrous
  joint)
• Mandible is connected to skull by a synovial
  joint (TMJ)

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Skull

• External bones
• Frontal bones (2)
• Occipital bones (1)
• Parietal bones (2)
• Temporal bones (2)
• Incisive (2 )
• Nasal (2)
• Maxillary (2)
• Zygomatic (2)
• Mandible (2)
• Palatine (2)
• Turbinates (2)

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Skull Bones Continued

• Categorized by
• Bones of Cranium
• Bones of the ear
• Bones of the face

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Bones of the Cranium

• Cranium-portion of skull that surrounds the
  brain.
• External Bones of Cranium
• Frontal Bones (2)
• Interparietal Bones (2)
• Occipital Bone (1)
• Parietal Bones (2)
• Temporal Bones (2)
• Internal Bones of Cranium
• Ethmoid Bone (1)
• Sphenoid Bone (1)

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External Bones of Cranium
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Occipital Bone

• Forms caudoventral portion or base of skull, most
  caudal skull bone.
• Important because
• Where spinal cord exits skull
• Skull bone that articulates with first cervical
  (neck) vertebrae.
• Foramen Magnum is in center of occipital bone.
• Occipital Condyles are on either side of foramen
  magnum

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Interparietal Bones

• Small bones located on dorsal midline between
  occipital and parietal bones
• Clearly visible in young animals, may fuse
  together in older animals.

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Parietal Bones

• Form the lateral walls of the cranium
• Well developed in dogs, cats and humans, but
  relatively small in horses and cattle.

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Temporal Bones

• Located ventral to the Parietal bones
• Form walls of the cranium
• Contain middle and inner ear structures
• Form Temporamandibular Joints (TMJs) with the
  mandible (Lower jaw)

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Frontal Bones

• Form forehead region of skull.
• Located rostral to parietal bone.
• Frontal sinus is contained within frontal bone.
• Horns are extension of frontal bone.

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Internal Bones of the Cranium

• Sphenoid Bone
• Forms ventral portion of the cranium and contains
  the pituitary fossa.
• This contains the pituitary gland.
• Contains the sphenoidal sinus in most animals.
• Ethmoid Bone
• Located rostral to sphenoid bone.
• Contains cribriform plate which has branches of
  olfactory nerve passing through.
• In horses and humans also have ethmoidal sinus in
  the ethmoid bone.

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Bones of the Ear

• Hidden in the middle ear from the outside in
• The Malleus- hammer
• The incus- anvil
• The stapes- stirrup
• Function is to transmit vibrations from the
  tympanic membrane (eardrum) to cochlea where
  vibrations are changed into nerve impulses.

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External Bones of the Face

• Incisive Bones
• Also called premaxillary bones
• In common domestic animals house upper incisor
  teeth. (Ruminants have a dental pad instead)
• Nasal Bones
• Form the bridge of the nose.
• Size depends on length of nose of animal.

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External Bones of the Face continued

• Maxillary Bones
• Make up upper jaw.
• House upper canine teeth, premolars and molars.
• Houses maxillary sinuses.
• Forms hard palate with palatine bones.
• Lacrimal Bones
• Form medial portion of the orbit of the eye.
• Contain lacrimal sac which is part of tear
  system.

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External Bones of the Face continued

• Zygomatic Bones
• Also known as the malar bones.
• Join process from temporal bones to form
  zygomatic arch.
• Mandible
• The lower jaw.
• Houses all lower teeth and is the only moveable
  part of the skull.
• Forms TMJ with the temporal bone on each side.
• Unite at mandibular symphysis at rostral end.
• Composed of shaft (horizontal portion) and ramus
  (vertical portion).

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Internal Bones of the Face

• Palatine Bones
• Make up part of hard palate
• Pterygoid Bones
• Support part of the lateral walls of the pharynx
  (throat).
• Vomer Bone
• Forms part of nasal septum (wall between left and
  right nasal passages).
• Turbinates
• Also called nasal conchae.
• Thin and scroll-like that fill most of nasal
  cavity space.

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Hyoid Bone

• Also called hyoid apparatus.
• Supports base of tongue, the pharynx, and the
  larynx and assists in swallowing.
• Composed of several parts that are united by
  cartilage.

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Spinal Column

• Also called the vertebral column
• Made up of vertebrae that extends from skull to
  tip of tail.
• Vertebrae are divided into 5 portions
• Cervical (neck)
• Abbreviated C
• Contain 7 vertebrae in dogs and cats
• Thoracic (chest)
• Abbreviated T
• Contain 13 vertebrae in dogs and cats
• Lumbar (abdomen)
• Abbreviated L.
• Contain 7 vertebrae in dogs and cats.
• Sacral (pelvis)
• Abbreviated S.
• Contain 3 vertebrae in dogs and cats.
• Coccygeal (tail)
• Abbreviated Cy.
• Number is variable.

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Vertebrae Characteristics

• Consists of
• Body
• The main, ventral portion of the bone.
• Separated from other vertebrae by intervertebral
  disks of cartilage.
• What type of cartilage makes this up?
• Arch (neural arch)
• Arch helps to make up vertebral foramen which
  allows passage of the spinal cord.
• Processes
• Transverse Processes
• Laterally projecting
• Site of muscle attachement.
• Articular Processes
• Found on cranial and caudal ends of vertebrae,
  help to join adjacent vertebrae.
• Spinous Processes
• Single and projects dorsally

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Intervetebral Disk Disease (IVDD)

• When disks slip
• More predominant in long-backed breeds
  (dachsunds, Bassets).
• Can confirm with radiographs.
• Usually once it occurs, animal is predisposed.
• Curative procedure is surgery to fuse spinal
  column together.
• Animals may become paralyzed (dragging rear
  legs).

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Cervical Vertebrae

• Atlas is C1
• Holds up head.
• Has wings of the atlas
• Has no vertebral body, just is a ring which
  spinal cord passes through.
• Axis is C2
• Has large spinous processes.

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Thoracic Vertebrae

• Have tall spinous processes.
• Have articular facets which communicate with the
  ribs.

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Lumbar Vertebrae

• Dorsal to abdominal region.
• Most massive-looking bones in spinal column.

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Sacral Vertebrae

• Fuse together to form one single, solid
  structure.
• Called the sacrum.
• Joins pelvis via sacroiliac joint.

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Coccygeal Vertebrae

• Bones of the tail.
• Appearance changes as bones progress down the
  tail.
• In humans fuse into coccyx.

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Ribs

• Flat bones that form the lateral walls of the
  thorax.
• Usually rib number is equal to the number of
  thoracic vertebrae.
• Have more of a moveable joints which is allows
  lungs to expand.
• Term for rib is costal.
• Sternal ribs attach to sternum
• Asternal ribs make up caudal part of thorax.
• Unattached ribs are called floating ribs.

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Sternum

• Breastbone
• Made up of sternebrae.
• Most cranial sternebrae is manubrium
• Most caudal sternebrae is called xiphoid process.
• May be broken during CPR.

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Appendicular Skeleton

• Made up of bones of limbs.
• Thoracic limbs (front leg)
• Scapula
• Humerus
• Radius
• Ulna
• Carpal bones
• Metacarpal bones
• Phalanges
• Pelvic Limbs (back leg)
• Pelvis
• Ilium
• Ischium
• pubis
• Femur
• Tibia
• Fibula
• Tarsal bones
• Metatarsal bones

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Appendicular Skeleton (limb bones)

• Thoracic Limb
• Scapula
• Humerus
• Radius
• Ulna
• Carpal bones (carpus)
• Metacarpal bones
• Phalanges

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Scapula

• Most proximal bone of the thoracic limb.
• Flat and triangular.
• Has prominent ridge on lateral surface referred
  to as spine of the scapula
• Glenoid cavity at distal end forms portion of
  ball and socket shoulder joint.

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Humerus

• Long bone of upper arm (brachium)
• Has head at proximal end with tubercles
  (processes) where muscles attach.
• Condyle at distal end is composed of trochlea,
  capitulum, medial and lateral epicondyles, and
  olecranon fossa.
• Is not funny bone (actually is a nerve not a
  bone).

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Ulna

• One of two bones that form antebrachium
  (forearm).
• Forms major portion of elbow joint with distal
  end of the humerus.
• Has large olecranon process at proximal end which
  forms the point of the elbow.
• Trochlear notch makes elbow tight.
• End of trochlear notch forms anconeal process.
• Where triceps brachii muscle attaches.
• Coronoid process articulates with the radius.

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Radius

• Main weight bearing bone of the antebrachium.
• Articulates with humerus and ulna.
• Styloid process articulates with carpus.

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Carpal Bones

• Carpus has two rows of bones.
• Is the wrist of the animals
• Proximal row bones have names (radial carpal
  bone, ulnar carpal bone, accessory carpal bone)
• Distal row bones numbered
• medial to lateral (1st carpal, 2nd carpal, etc)

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Metacarpal Bones

• Extend distally from distal row of carpal bones
  to proximal phalanges of the digits.
• Numbered from medial to lateral (dewclaw being
  number 1 in dogs and cats).
• Horses have one large metacarpal bone (cannon
  bone) and two non-weight bearing splint bones.
• Cattle have fused metacarpal bones with sesamoid
  bones (proximal and distal).

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Phalanges

• Each digit is made up of two or three phalanges
  (single bones are phalanx)
• In horses phalanges are the long and short
  pastern bone and coffin bone.
• Also have 2 proximal and 1 distal (navicular
  bone) sesamoid bones.
• Dogs and cats have a ungual process that
  surrounds the claw.

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Appendicular Skeleton- Hind Limb

• Connected to the axial skeleton at sacroiliac
  joint

• Pelvic Limb
• Pelvis
• Ilium
• Pubis
• Ischium
• Femur
• Tibia
• Fibula
• Tarsal bones (tarsus)
• Metatarsal bones
• Phalanges

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Pelvis

• May be referred to as the os coxae
• Develops as three separate bones on each side
  that fuse into a solid structure.
• Joined at pelvic symphysis.
• Bones are
• Ilium
• Cranial most bone of pelvis
• Wings of the ilium or hips.
• Ischium
• Most caudal pelvic bone
• Tailbone is ischial tuberosity.
• Pubis
• Smallest of the pelvic bones

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Pelvic landmarks

• Acetabulum- portion of hip bone that forms ball
  and socket joint with femur.
• We look at this area in cases of hip dysplasia.
• Obturator foramen- large holes in pelvis that
  help to lighten weight of pelvis.
• We look at this as landmark if radiograph is
  straight or not.

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Femur

• Long Bone of thigh
• Proximal end is the ball portion is called head.
• Proximal end also contains the trochanters where
  muscles attach.
• Distal end forms stifle joint (knee) with
  condyles.

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Patella

• Kneecap is largest sesamoid bone in the body.
• Formed in the distal tendon of the quadriceps
  femoris muscle on the cranial aspect of the
  stifle joint.
• Helps to protect tendon as it passes down over
  the trochlea of the femur.

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Patellar Luxation

• When trochlear groove is not deep enough to
  contain patella so it slips in and out of place.
• Usually will cause brief episodes of lameness
  which can correct.
• May hear or feel popping in and out of place.
• Found in small, toy breeds most prevalently.
• Can be surgically corrected.

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Fabellae

• Two small sesamoid bones located in the proximal
  gastrocnemius or calf muscles.
• Not present in cattle or horses.

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Tibia

• Main weight bearing bone of the lower leg
  (shinbone).
• Forms stifle joint with femur.
• Forms hock (ankle) with tarsus.
• Medial malleolus- medial to distal articular
  surface, knob of ankle.
• Tibial tuberosity of proximal end forms the
  tibial crest.
• Where patellar tendon attaches.

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Fibula

• Thin but complete bone in dogs and cats.
• Consists of proximal extremity, shaft, and distal
  extremity.
• Serves as a muscle attachment site.
• Horses and cattle do not have shaft of fibula.
• Forms lateral malleolus at distal end.

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Tarsal Bones

• Ankle but in four legged animals is termed the
  hock
• Consists of two rows of tarsal bones (similar to
  carpal bones in wrist).
• Proximal row is named and distal row is numbered.
• Contains calacaneal tuberosity which forms the
  point of the hock. Site of attachment for tendon
  of the gastrocnemius muscle.

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Metatarsal Bones

• Dogs and cats four metatarsal bones (II to V)
• Horses 1 large metatarsal bone (cannon bone) and
  2 small metatarsal bones (splint bones)

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Pelvic Limb Phalanges

• Similar to thoracic limb phalanges
• Exceptions dogs and cats
• Usually only 4 digits (II to V)

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Visceral Skeleton

• Bones that form in organs
• Examples
• os cordis in heart of cattle and sheep
• os penis in penis of dogs, beaver, raccoons, and
  walruses
• os rostri in nose of swine

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Joints

• Junctions between bones
• Can be moveable or immovable.
• Arthro and articular refer to joints.
• 3 Types of joints
• Fibrous Joints (synarthroses)
• Immovable
• Found in sutures of skull bones
• Cartilaginous Joints (amphiarthroses)
• Slightly moveable
• Vertebral disks between vertebrae
• Pelvic and mandibular symphisis
• Synovial Joints (diarthroses)
• Freely moveable

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Characteristics of Synovial Joints

• Have Articular surfaces on bones
• Articular cartilag covering articular surfaces
• Fluid-filled joint cavity
• Enclosed by a joint capsule
• Synovial membrane- outer membrane
• Synovial fluid- lubricates joint surfaces
• Ligaments - fibrous Connective tissue that join
  bones to other bones.

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Synovial Joint Movements

• Flexion and Extension
• Opposite movements
• Increase (extension) or decrease (flexion) angle
  between two bones
• Adduction and Abduction
• Opposite movements
• Move an extremity toward (Adduction) or away from
  (Abduction) medial plane
• Rotation
• Twisting movement of a part on its own axis
• Circumduction
• Movement of an extremity so that the distal end
  moves in a circle

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Types of Synovial Joints

• Hinge Joints
• One joint surface swivels around another
• Only capable of flexion and extension
• Gliding Joints
• Rocking motion of one joint surface on another
• Primarily capable of flexion and extension
• Abduction and adduction possible in humans not
  dogs/cats
• Pivot Joints
• One bone pivots (rotates) on another
• Only capable of rotation
• Ball-and-socket joints
• Allow for all joint movements