Chapter 6

The Skeletal System

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

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

Bone Structure continued

• Osteocytes- bone cells.• Periosteum- covering of bone

• Inner layer contains osteoblasts.• Endosteum- membrane that lines the

hollow interior surface of bones.

Haversian Systems• Concentric layers of ossified bone matrix arranged around a

central Haversian canal • Layers of ossified bone matrix

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?

Blood Supply to Bone

• Tiny vessels penetrate the periosteum.• Volkmann’s 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.

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.

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

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.

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.

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.

Bone Shapes

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

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.

Short Bones

• Shaped like cubes.• Have core of cancellous bone covered by

compact bone.• Carpal and tarsal bones.

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.

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.

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.

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?

Condyle

• Large, round articular surface.• Major condyle is located on end of

humerus and femur.• Also located in skull.

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.

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.

Facet

• A flat articular surface.• Found in carpal and tarsal bones as well

as in vertebrae, radius and ulna.

Processes

• All projections of a bone.• Heads and condyles are considered to be

processes.• Tendons may attach to processes

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.

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.

Axial Skeleton bones of head & trunk

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

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)

SkullExternal 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)

Skull Bones Continued

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

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)

External Bones of Cranium

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

Interparietal Bones

• Small bones located on dorsal midline between occipital and parietal bones

• Clearly visible in young animals, may fuse together in older animals.

Parietal Bones

• Form the lateral walls of the cranium• Well developed in dogs, cats and humans,

but relatively small in horses and cattle.

Temporal Bones• Located ventral to the Parietal bones• Form walls of the cranium• Contain middle and inner ear structures• Form Temporamandibular Joints (TMJ’s)

with the mandible (Lower jaw)

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.

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.

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.

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.

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.

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).

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.

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.

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.

• Refer to vertebrae by letter designation and number designation.

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

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).

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.

Thoracic Vertebrae

• Have tall spinous processes.• Have articular facets which communicate

with the ribs.

Lumbar Vertebrae

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

column.

Sacral Vertebrae

• Fuse together to form one single, solid structure.

• Called the sacrum.• Joins pelvis via sacroiliac joint.

Coccygeal Vertebrae

• Bones of the tail.• Appearance changes as bones progress

down the tail.• In humans fuse into coccyx.

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.

Sternum

• Breastbone• Made up of sternebrae.• Most cranial sternebrae is manubrium• Most caudal sternebrae is called xiphoid

process.• May be broken during CPR.

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• Phalanges

Appendicular Skeleton Appendicular Skeleton (limb bones)(limb bones)

Thoracic Limb• ScapulaScapula• HumerusHumerus• RadiusRadius• UlnaUlna• Carpal bones (carpus)Carpal bones (carpus)• Metacarpal bonesMetacarpal bones

• PhalangesPhalanges

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.

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).

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.

Radius

• Main weight bearing bone of the antebrachium.

• Articulates with humerus and ulna.• Styloid process articulates with carpus.

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)

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).

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.

Appendicular Skeleton- Hind Limb• Connected to the axial skeleton at sacroiliac jointConnected to the axial skeleton at sacroiliac joint

Pelvic Limb::• PelvisPelvis

• IliumIlium

• PubisPubis

• IschiumIschium

• FemurFemur• TibiaTibia• FibulaFibula• Tarsal bones (tarsus)Tarsal bones (tarsus)• Metatarsal bonesMetatarsal bones• PhalangesPhalanges

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

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.

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.

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.

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.

Fabellae

• Two small sesamoid bones located in the proximal gastrocnemius or calf muscles.

• Not present in cattle or horses.

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.

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.

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.

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)

Pelvic Limb Phalanges• Similar to thoracic limb phalanges• Exceptions: dogs and cats

• Usually only 4 digits (II to V)

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

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

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.

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

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