Skeletal System – body’s framework of bones and their associated cartilage & ligaments Functions of skeletal system

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Comparative Anatomy and Physiology

Skeletal System
Skeletal System – body’s framework of bones and their associated cartilage & ligaments
Functions of skeletal system


Protection – surround vital organs (eg. brain, heart and lungs)

Levers for movement – muscles attached

Mineral homeostasis – stores and releases minerals as needed

(eg. calcium and phosphorus)

Blood cell production – in red marrow
Bone Histology – bone is a solid connective tissue

1/3 fibers (organic matrix)– collagen

2/3 ground substance (inorganic matrix)– mineral salts (hydroxyapatite)

mainly calcium salts

calcification – deposition of mineral salt crystals in bone

hardness depends on amount of mineral salts

resistance to deformation depends on collagen fibers
Fixed Cells

osteoblastsform new bone by secreting matrix around themselves
osteocytesmature bone cells trapped within lacunae maintain daily metabolic processes of bone cells
osteoclasts – huge cells from fusion of many phagocytic cells

contain enzymes and acids to digest bone matrix (resorption)

involved in bone repair and remodelling

remove Ca+2 from bone when it is needed in blood

2 Types of Bone – compact and spongy bone

1) Compact bone – bone arranged in cylindrical structures called osteons or

Haversian systems

very dense and strong
Structure of an Osteon

central(Haversian)canal – runs longitudinally through center contains blood vessels and nerves supplying the bone

lamellae – layers of calcified matrix

lacunae – holes between lamellae which contain osteocytes

canaliculi – tiny tunnels connecting lacunae

pathways for exchange of nutrients and wastes with blood vessels in the central canal

osteons align along lines of stress to resist bending of bone

compact bone forms the shafts of long bones and the outer shell

of all bones
2) Spongy(cancellous) bone – arranged in an irregular network of small spicules

with spaces between

arrangement gives strength but lightness

red marrow – fills spaces between spicules

site of blood cell formation (hematopoiesis)

spicules are composed of layers of matrix with lacunae containing osteocytes, connected by canaliculi

nutrients and wastes are exchanged with the red marrow

spongy bone is found in the ends of long bone and center of other bones
Periosteum – membrane covering outer surface of bone (except at joints)

outer fibrous layerdense irregular connective tissue

site of tendon, ligament, and joint capsule attachment

inner cellular layer – contains osteoblast cells for growth, remodeling and

fracture repair

some collagen fibers (Sharpey’s fibers) penetrate bone to anchor the periosteum

Endosteum – thin cellular layer lining inner surfaces of bone

contains osteoblasts for remodelling and repair

Blood and Nerve Supply of Bone – has a rich blood supply

bones are very metabolically active and usually heal rapidly

Periosteal arteries – many small vessels that penetrate the periosteum and bone

matrix through Volkman’s canals

supply vessels in central canals of compact bone
Nutrient arteries – large vessels that enter bones through large holes (nutrient

foramina) and supply mainly the marrow of spongy bone
veins and nerves accompany the arteries the periosteum is rich in sensory nerves which detect pain

Long bonesgreater length than width levers for body motion

(most bones of limbs and paws)

Structure of Long Bones
diaphysis – shaft; mainly compact bone

epiphyses (sing. –sis) – proximal and distal ends mainly spongy bone with an outer shell of compact bone

spaces filled with red marrow

metaphyses (sing. –sis) – narrow zone between diaphysis and epiphysis

epiphyseal plate (growth plate) – in immature bones plate of hyaline cartilage at metaphysis site where bones grow in length

epiphyseal line – in mature bones

line of bone where epiphyseal plate has ossified (closed)
articular cartilagehyaline cartilage covering joint surfaces of epiphyses

reduces friction and absorbs shock at joints

medullary cavity(marrow cavity) – space in central diaphysis which contains yellow (fatty) marrow
Short bonesnearly equal length and width spongy bone except at surface

(carpals and tarsals of paws)

Flat bones2 thin, flat layers of compact bone enclosing spongy bone

surround and protect organs provide large surface area for muscle attachment (most skull bones, rib cage, shoulder blades)

Irregular bonesdon’t fit other categories complex shapes and variable composition (vertebrae, some facial bones)

Sesamoid bones (shaped like a sesame seed) – develop in tendons

provide strength to areas of unusual mechanical stress

(patella is the largest)
Bone formation (ossification) embryonic“skeleton” is composed of loose fibrous connective tissue membranes and hyaline cartilage which serve as a template(model for the skeleton) ossification is replacement of these tissues with bone tissue

(remember all connective tissues come from embryonic mesoderm)

2 methods of ossification – intramembranous and endochondral
Intramembranous ossification – bone forms within loose fibrous connective

tissue membranes forms flat bones of skull
ossification center develops in fibrous membrane

embryonic mesenchyme cells differentiate into osteoblasts and secrete bone matrix around themselves

osteocytes become trapped in lacunae

matrix around adjacent osteocytes fuses to form spicules of spongy bone connective tissue between spicules differentiates into red marrow

osteoblast cells and connective tissue around spongy bone condense to form periosteum and endosteum osteoblasts of periosteum form a thin layer of compact bone on

surface of bone

Endochondral ossification – bone forms within a hyaline cartilage model most bones form this way
cartilage model develops

embryonic mesenchyme cells cluster in shape of future bone

differentiate into chondroblasts, which secrete hyaline cartilage matrix

outer layer of chondroblasts and connective tissue condense to

form perichondrium

cartilage model grows

interstitial growth – chondrocytes of model divide and secrete matrix between themselves model grows in length

appositional growth – chondroblasts in perchondrium divide and

secrete matrix beneath perichondrium model grows in thickness

primary ossification center develops

chondrocytes in center of diaphysis die and are replaced by


osteoblasts secrete bone matrix, forming spongy bone

ossification center grows outwards towards metaphyses of bone

osteoclasts hollow out medullary cavity

remodeling replaces spongy bone of diaphysis with compact bone

secondary ossification centers develop

begin in center of each epiphysis and spread outwards, filling

epiphyses with spongy bone

articular cartilage – hyaline cartilage remains on joint surface

epiphyseal plate (growth plate)– remaining hyaline cartilage between epiphysis and diaphysis

Bone Growth – regulated by hormones

growth hormone(GH) – from pituitary gland promotes growth of bone

sex steroids (estrogens and androgens)

anabolic steroids – stimulate tissue growth, including bone

high levels at maturity eventually close growth plates

Growth in Lengthinterstitial growth only at episphyseal plates

injury to the plate can cause early closure

Growth in Thicknessappositional growth

osteoblasts in periosteum divide and secrete matrix beneath periosteum

Bone remodeling – constant process in which osteoclasts remove bone tissue and osteoblasts replace it renews aging bone tissue, repairs damaged bone redistributes bone along lines of mechanical stress
Features of Bones – develop in response to mechanical forces on bone surfaces
articular surfaces – smooth, contoured ends of bones that form joints

with other bones

condyle – cylindrical to rounded end

facet – flat surface

head – spherical to rounded end

depressions and openings – result from compressive forces or allow passage of soft tissues

fissure – crack fossa – pocket or cup

foramen – hole sulcus – groove

meatus – tunnel fovea – small depression

processesprominences on bones resulting from tension (pulling)forces attachment points for tendons and ligaments

line tubercle epicondyle

crest tuberosity

spine trochanter


Remodeling Processes

bone resorption – breakdown of bone matrix by osteoclast secretions

enzymes - digest collagen (organic matrix)

acid – dissolves mineral salts (inorganic matrix)

ossification (osteogenesis) – formation of new bone by osteoblasts

calcification – deposition of mineral salts in bone by osteoblasts and


Bone Homeostasis resorption = ossification and calcification
exercise – weight bearing exercise places stress on bones which stimulates

osteogenesis and calcification exercise increases bone mass and strength

osteoporosis – weak, porous bone resorption is greater than ossification and calcification

may be due to lack of exercise, poor nutrition, hormonal imbalance

Calcium Homeostasis – bone stores 99% of body’s calcium calcium aids in nerve and muscle function, blood clotting, and enzyme function

blood calcium level is regulated within a narrow range

Hormonal regulation of blood calcium

Calcitonin negative feedback loop

high blood calcium

thyroid gland produces and secretes more calcitonin

calcitonin inhibits osteoclasts

less bone is resorbed

more calcium remains in bone

blood calcium decreases

Parathyroid Hormone(PTH) negative feedback loop

low blood calcium

parathyroid gland produces and secretes more PTH

PTH stimulates osteoclasts more bone is resorbed

calcium released from bone matrix enters bloodstream

blood calcium increases

(these hormones also help regulate how much calcium is absorbed from

the diet and excreted from the body in urine)

Fracture Repair
fracture hematoma – forms within hours

injured bone tissue forms a clot and becomes inflamed phagocytic cells and osteoclasts remove damaged tissue

cartilage callus – forms within weeks fibroblasts enter hematoma and differentiate into chondroblasts produce hyaline cartilage

bony callus – forms within months beginning near healthy bone ends, osteoblasts replace cartilage callus with spongy bone bridges broken ends and stabilizes fracture bone remodeling – may take months to years bony callus replaced with “normal” bone
Joint (arthrosis or articulation) – connection between 2 bones, bone and cartilage, or bone and teeth
Naming Joints – usually named after the articulating structures (eg. intervertebral joints, radiocarpal joint, sacroiliac joint)
some large, complex joints are given special names (eg. stifle, elbow,

hip, shoulder, hock, carpal joint)

skull joints can be named using directional terms (eg. sagittal suture)
Classification of Joints

Fibrous Joints – connected by fibrous connective tissue

no movement (synarthroses)

sutures – skull joints

gomphoses – tooth joints

Cartilaginous Joints – connected by cartilage

slightly moveable as cartilage compresses (amphiarthroses)

symphysis – connected by fibrocartilage
Synovial Joints – freely moveable (diarthroses)

2 bone ends covered by articular cartilage

joint cavity – small space between bone ends

joint capsule – connective tissue enclosing joint cavity

fibrous outer layerdense irregular connective tissue attaches to periosteum; flexible yet strong

synovial membrane – inner loose connective tissue layer

synovial fluid – secreted by synovial membrane viscous fluid that lubricates the joint
Structures Associated with Synovial Joints

ligaments – dense fibrous connective tissue that attaches bone

to bone

extracapsular – outside joint capsule (eg. collateral ligaments)

intracapsular – inside joint capsule (eg. cruciate ligaments of stifle)

menisci (sing. meniscus) – extra pads of fibrocartilage between articulating bones

stabilize joint and provide extra cushioning

bursae – sac-like synovial structures located at points of friction associated with a joint

filled with viscous fluid to provide cushioning

bursitis – inflammation of a bursa

tendon sheath – tube-like bursa surrounding tendons at sites of friction
Types of Synovial Joints – based on shape of articulating surfaces determines type of movement at the joint
gliding(planar)joint – surfaces relatively flat glide over each other (eg. intercarpal joints)

hinge joint – one rod-like and one groove-like surface

allows monaxial flexion or extension (eg. elbow)

pivot joint – one pointed and one ring-like surface

one bone rotates around the other

(eg. atlantoaxial joint, C1-C2 vertebrae)

ellipsoid joint – one convex oval one concave oval surface

allows biaxial movement

(eg. metacarpophalangeal joints)

ball-and-socket joint – one ball-like one cup-like surface

allows multiaxial movement (shoulder and hip)

AXIAL SKELETON – bones arranged around the body’s longitudinal axis
SKULL – most joints are immovable fibrous joint called sutures
Cranium (braincase) – surrounds and protects brain

frontal bones

parietal bones

interparietal bones

temporal bones

occipital bone

sphenoid bone

ethmoid bone

Facial Bones – create cavities containing special senses (nasal cavity,

orbits, mouth); assist in eating

nasal bones

incisive bones

maxillary bones

zygomatic bones

lacrimal bones

pterygoid bones

palatine bones

nasal turbinates (conchae)

vomer bone

mandible – only moveable skull bone (articulates with temporal)

temporomandibular joint – synovial hinge joint

Special Features of Skull
sinuses – cavities in some bones lined with mucous membranes and connected to the nasal cavity; produce mucus

help moisturize and warm inhaled air

act as resonating chambers frontal, sphenoid, ethmoid, and maxillary sinuses

sinusitis – inflammation of sinuses

nasal septum – midsagittal division of nasal cavity

formed by vomer and ethmoid bones and septal cartilage

orbits – eye sockets

hard palate – separates nasal cavity from mouth

formed by incisive, maxillary and palatine bones

horn processes – extensions of frontal bones
VERTEBRAL COLUMN (spine) – flexible rod that surrounds and protects the

spinal cord, and serves as an attachment site for the head, thoracic cage,

and hind limbs
Structure of a Typical Vertebra
vertebral body – solid, ventral portion

articulates with adjacent vertebral bodies

intervertebral discs – fibrocartilage discs between vertebral bodies; have soft, pulpy center which acts as a cushion

form cartilaginous joints which are slightly moveable

intervertebral disc disease – can degenerate or be damaged

by excessive stress

inner pulp squeezes out (herniates) dorsally and

compresses spinal cord or nerve roots

vertebral arch – dorsal to the body

vertebral foramen – hole in arch which contains spinal cord

spinal canal – formed by all vertebral foramina lined up

intervertebral foramina – openings in spinal canal between

2 adjoining vertebrae; allows passage of spinal nerves

vertebral processes

transverse processes (2) – lateral; muscle attachment sites

spinous process (1) – dorsal; muscle and ligament attachment site

nuchal ligament – thick, elastic ligament on dorsal spinous

processses from thoracic vertebrae to head

helps to support the head

cranial and caudal articular processes (2 of each) – sites of

intervertebral articulation; joint surfaces called facets

intervertebral joints – synovial gliding joints

Divisions of Vertebral Column
Cervical vertebrae – neck

usually 7

atlas vertebra (C1) – holds up skull

ring of bone with 2 large ala

articulates with skull– synovial hinge joint (nodding)

axis vertebra (C2) – dens (peg-like process) articulates with atlas

atlantoaxial joint – synovial pivot joint (“no” joint)

Thoracic vertebrae – chest

usually 13-18

body and transverse processes have facets for articulation with ribs

Lumbar vertebrae – lower back

usually 5-7

largest, thickest vertebrae

Sacrum (3-5 fused vertebrae) – attachment site for pelvic girdle

Coccygeal vertebrae – tail

highly variable numbers between and within species

Thoracic Cage – encloses and protects organs of thoracic cavity
Ribs usually 1 pair for each thoracic vertebra

dorsal end articulates with vertebral body and transverse process

synovial gliding joints that allow the thoracic cage to

expand when breathing

sternal ribs – attach to sternum via costal cartilages

asternal ribs - don’t attach to sternum

attach to costal cartilages of other ribs

floating ribs – most caudal ribs with no ventral attachment
intercostal space – space between adjacent ribs
Sternum (breastbone) – ventral midline

made up of rod-like bones called sternebra

manubrium – cranial sternebra

xiphoid – caudal sternebra

xiphoid cartilage – hyaline cartilage attached to the

caudal xiphoid

function as levers for movements of skeletal muscles

Scapula (shoulder blade) – doesn’t articulate with the axial skeleton

strongly attached to the thorax by muscles

Humerus (brachium of forelimb)

proximal end articulates with scapula (scapulohumeral joint)

shoulder joint – synovial ball-and-socket joint

distal end articulates with radius and ulna

elbow joint – synovial hinge joint

Ulna (antebrachium of forelimb)

proximal end articulates with medial humerus

distal end connected lateral carpus

(the ulna doesn’t reach the carpus in the horse)

Radius (antebrachium of forelimb)

main weight-bearing bone of antebrachium

proximal end articulates with lateral humerus

distal end articulates with medial carpus

(radius and ulna also articulate with each other, allowing some

rotational movement of the forepaw)

Carpals (carpus) – carpal joint (“knee” of horse or cow)

2 rows of short bones forming 3 joint regions

radiocarpal joint – proximal row articulates with radius

synovial ellipsoid or hinge joint

intercarpal joint – proximal row articulates with distal row

synovial gliding joints

carpometacarpal joint – distal row articulates with metacarpals

mainly synovial gliding joints

Metacarpals – bones of forepaw

numbered 1-5 starting medially

cats and dogs have all 5

ruminants – metacarpals 3 and 4 fused to bear weight

metacarpals 2 and 5 are remnants

horse – metacarpal 3 (cannon bone) bears all weight

metacarpals 2 and 4 are remnants (splint bones)

metacarpophalangeal joints (distal articulations with phalanges)

cats and dogs – synovial ellipsoid joints

horse – synovial hinge joint (fetlock)

has 2 sesamoids caudally (proximal sesamoids)

Phalanges (singular phalanx) – digits of forepaw

digits numbered 1-5 starting medially

most digits have 3 phalanges – proximal, middle, and distal

cat and dog – 5 digits, most with 3 phalanges each

digit 1 (dewclaw) has 2 phalanges

ruminants – 4 digits

digits 3 and 4 bear weight and have 3 phalanges each

digits 2 and 5 bear no weight and are remnants with

reduced phalanges

horse – 1 digit

digit 3 bears weight and has 3 phalanges

distal phalanx is the coffin bone

navicular bone – sesamoid at caudal P2-P3 joint
PELVIC LIMB – hind limb
Pelvis – 2 coxal bones (os coxae)

pubic symphysis – 2 coxae articulate with each other

slightly moveable cartilaginous joint

sacroiliac joints – coxae articulate with sacrum

synovial gliding joint

the coxal bones form from 3 embryonic bone which fuse
ilium – craniodorsal portion

forms sacroiliac joint

ischium – caudal portion

pubis – ventromedial portion forms pubic symphysis

obturator foramen – large hole that makes pelvis lighter

acetabulum – deep, rimmed socket which articulates with femur formed where ilium, ischium, and pubis join

coxofemoral (hip) joint – synovial ball-and-socket joint
hip dysplasia – poorly formed, loose hip joint

leads to wear-and-tear at joint (osteoarthritis)

Femur – hindquarter or thigh region

proximal end articulates with coxal bone

round ligament – anchors head of femur in acetabulum

distal end articulates with tibia and patella

stifle (knee) joint – synovial hinge joint

medial and lateral menisci – fibrocartilage pads

anchored to tibia; add stability to joint

cruciate ligaments – crossed intracapsular ligaments

that add stability to joint

cranial cruciate lig.– prevents tibia sliding

cranially and stifle hyperextension

caudal cruciate lig.- prevent tibia sliding


fabellae – 2 sesamoids on caudal stifle in cat & dog

Patella (kneecap) – sesamoid bone in quadriceps femoris tendon

patellar ligament – connects patella to cranial tibia

patella fits into a groove on the cranial femur

stay apparatus – in horses, additional patellar ligaments and certain

muscles lock the stifle and hock joints so the horse can

stand with very little effort

reciprocal apparatus – in horses, special muscles automatically flex

the hock when the stifle flexes

luxating patella – patella slips medially, causing pain

Tibia – leg or gaskin region

medial, weight-bearing bone of hind leg

proximal end articulates with femur

distal end articulates with tarsals

Fibula – leg or gaskin region

non-weight bearing bone; site of attachment for muscles

proximal end articulates with tibia

cats and dogs – distal end articulates with tibia and tarsus

horse and cattle – fibula is a remnant

Tarsals (hock or tarsus) – tarsal joint or hock joint

2 rows of short bones forming 3 joint regions

tibiotarsal joint – proximal row articulates with tibia

synovial hinge joint

intertarsal joints – proximal row articulates with distal row

synovial gliding joints

tarsometatarsal joint – distal row articulates with metatarsals

mainly synovial gliding joints

Metatarsals – bones of hind paw

numbered 1-5 starting medially

cats and dogs usually have metatarsals 2-5

metatarsal 1 may be a small remnant

ruminants – as with forefoot

horse – as with forefoot

metatarsophalangeal joints (distal articulations with phalanges)

similar to forefeet

Phalanges (singular phalanx) – digits of hind paw

as with forefoot

cat and dog – usually 4 digits (2-5) with 3 phalanges each

remnant of digit 1 may be present with reduced phalanges

ruminants and horse – as with forefoot

highly modified for both walking and flying

strong yet lightweight

fewer bones, fused together for strength

pneumatic bones – many large bones are hollow and air filled

upper and lower jaws covered by beaks

lighter than teeth

both jaws are moveable

large orbits

sclerotic ring – bony plates protecting eye

nasal cavity usually very reduced

1 round occipital condyle (ball-and-socket type joint)

allows greater rotation of the head


birds have more cervical vertebrae than mammals

increased flexibility of the neck

thoracic vertebrae are fewer and fused

synsacrum – lumbar, sacral, and cranial coccygeal vertebrae fuse with the

coxal bones

forms a strong, light plate to articulate with the legs

pygostyle – fused caudal coccygeal vertebrae

attachment site for retrices (tail feathers)

thoracic cage – rigid structure for attachment of flight muscles

ribs – reduced numbers

no costal cartilages (ossified except at a small, cartilaginous joint)

vertebral ribs – articulate with vertebrae

sternal ribs – articulate with sternum


keel – large, bony ridge for attachment of flight muscles
pectoral limb – securely attached to axial skeleton by scapula and coracoid

furcula (wishbone) – fused clavicles

braces the shoulder during flight

major and minor metacarpals

digit I – anchors feathers of the alula (feathers for steering)

digits II, III, metacarpals and ulna anchor remiges (flight feathers of wing)

pelvic limb

ilium – fused to synsacrum

pubis and ischium – not united caudally

open pelvis for laying large eggs

femur (drumstick)

tibiotarsus – tibia fuses with proximal tarsals to form 1 bone

tarsometatarsus – distal tarsals and metatarsals fuse to form 1 bone

digits – most birds have 4

digits have 2-5 phalanges (numbering corresponds to number of joints)

digits II and III point anteriorly

digit I points posteriorly

digit IV may point

posteriorly (parrots)

anteriorly (chickens)

either (owls)

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