Skeletal Function
Support: Internal “scaffold” for the body
Locomotion: Attachments for muscles, operates a system of levers bringing about movement.
Protection: Underlying soft parts (brain in cranium, heart in ribs)
Storage: Essential minerals, Ca2+ and PO43-
Haemopoiesis: Manufacture of blood cells from tissues in the bone marrow
Skeletal Structure
Bone Shape:
- Long= typical of limb bones as well as metacarpus/metatarsus and phalanges. Shaft containing a medullary cavity filled with marrow.
- Flat= outer layer of compact bone, inner layer of cancellous bone but no medullary cavity (skull, scapula, ribs)
- Short= outer layer of compact bone, core of cancellous bone, no medullary cavity (carpal, tarsal)
- Irregular= similar to short bones but less organised in shape, lie in midline, unpaired (vertebrae).
Specialised Bones
Sesamoid: Sesame seed in shape, develop in tendons (sometimes ligaments), runs over an underlying bony prominence. Reduces wear and tear.
Pneumatic: Air-filled spaces (sinuses) reduce weight of bone
Splanchnic: Develops in a soft organ, unattached to rest of skeleton
Bone Development
Ossification (bone-forming process) - intramembranous and endochondral
Osteoblasts: lay down new bone, becomes osteocyte
Osteoclasts: Destroy/Remodel bone
Osteocytes: Cell signalling, modify activity of (osteo)blasts/clasts.
Intramembranous: Flat bones of skull are formed and osteoblasts lay down bone between 2 layers of fibrous connective tissue - no cartilage template.
Endochondral: Replace hyaline cartilage model, within an embryo, with bone.
1) Cartilage model develops (embryo)
2) Primary centres appear in diaphysis/shaft
3) Cartilage replaced. Osteoblasts
4) Secondary centres in epiphyses/ends
5) Osteoclasts remove diaphysis bone (marrow)
6) Cartilage in gap (growth/epiphyseal plate) - bone lengthens in growth
7) “Closes” when growth finishes
- Collar of repair tissue at each surface of broken bones end.
- Bridges gap= external callus, osteogenic layer: periosteum
- Endosteum: internal callus
- Callus: cartilage or spongy bone (blood supply decides)
- Osteoclasts remove dead bone
- Repair: spongy bone of callus is reabsorbed
Skeleton
3 parts: Axial- *Skull *Mandible *Vertebrae *Sternum Appendicular- *Pectoral + Pelvic Limbs *Shoulder *Pelvic Girdles Splanchnic *Os penis within tissue of penis
Tuberosity/Trochanter/Tubercle: Bone protuberances - muscle
Trochlea: Bony structures for tendons to pass, grooves in bone
Condyle: Rounded projection on a bone (articulation)
Epicondyle: Projection of lateral edge of condyle
Foramen: Opening/Passage into or through bone
Fossa: Hollow/Depressed area on bone
Head, Neck + Shaft: Describing long bones
Tendon: Connect muscle to bone
Ligament: Connects bones
Axial Skeleton
Skull
Cranium
Skull:
- House + protect brain
- House special sense organs (eyes, ears, nose, tongue)
- House + provide attachment for parts of the digestive system (teeth, tongue)
- Provide attachment for the hyoid apparatus and muscles of mastication and facial expressions
- Provide a bony cavity for air to enter
- Communication
- Flat bones connected by fibrous joints called sutures*
- Divided into cranium and facial bones*
Cranium:
- 11 bones (occipital, interparietal, parietal, temporal, frontal, sphenoid, ethmoid, ossicles, malleus, incus and stapes)
- Parietal= dorsal and lateral walls of the cranium
- Temporal= below parietal (caudolateral surface of skull), tympanic bulla formed from ventral part (middle ear), external acoustic meatus (opening in tympanic bulla). Cartilages of external ear canal attached here
- Sphenoid= ventral aspect of skull, floor of cranial activity, small foramina penetrate (nerves and blood vessels pass)
- Sagittal Crest= ridge of bone on dorsal midline surface of skull, prominent in muscular dogs
- Zygomatic Arch= arch of bone that projects laterally from the skull (cheekbone)
- Lacrimal= base of orbit (houses the eye) - tears drain from eye to nose
- Frontal= front aspect of cranium, forehead, air-filled chamber (frontal sinus) connecting to nasal chamber
- Occipital= base of skull, caudal aspect, foramen magnum (spinal cord passes). Bony prominences (occipital condyles) - articulate with atlas, jugular processes for muscle attachment.
Axial Skeleton
Nasal Chambers
Mandible
Nasal Chambers:
- Most rostral part=nasal chamber formed by maxilla and nasal bone roof
- Divided lengthways into 2 by the nasal septum (cartilaginous plate)
- Chambers contain rolls= nasal turbinates/conchae, covered in ciliated mucous epithelium
- Ethmoid bone= boundary between nasal and cranial cavities
- Cribriform plate= numerous foramina, olfactory nerves from nasal mucosa to olfactory bulbs of the brain
- Roof of mouth= hard palate formed up of the incisive bone, part of the maxilla and palatine
Mandible:
- Lower jaw
- Contains all lower teeth
- Only moving bone of the skull
- 2 bones joined by cartilaginous joint (mandibular symphysis)
- Vertical (ramus), Horizontal (body)
- Body= sockets for teeth
- Ramus= articulates with rest of skull at temporomandibular joint via the condylar process
- Rounded coronoid process= projects (ramus to temporal fossa)
- Depression on lateral surface ramus (masseteric fossa)
Axial Skeleton
Hyoid Apparatus
Skull Shapes
Hyoid Apparatus:
- Lies in the intermandibular space
- Consists of fine bones and cartilages joined in a trapeze arrangement
- Means by which the larynx and tongue are suspended from the skull
- Articulates with the skull’s temporal region in a cartilaginous joint
Skull Shapes:
- Varies amongst species
- Domestic cat= rounded, “apple-shaped” and doesn’t differ usually
- Domestic dog= differs amongst breeds
- Brachycephalic* Cranium more rounded, nose is short + pushed in (shortening of nasal chambers, hard palate + mandible)
- Mesaticephalic* Normal shape and means “middle”
- Dolichocephalic* Head, particularly the nose, is long + narrow
Axial Skeleton
Vertebrae
Vertebrae:
- Divided into 5 regions= cervical (neck: C7), thoracic (chest: T13), lumbar (loin: L7), sacral (base of spine: S3), coccygeal (tail: Cd 20-23)
- Function= stiffen body axis, maintain posture> enclose + protect spinal cord> shield + protect soft underlying structures of the neck, thorax, abdomen and pelvis
- Body= most dense, load bearing, articulates with neighbouring vertebrae
- Articular Processes= caudal + cranial ends, form joints
- Processes= 3 types present (spinous, ventral, transverse) - muscular attachment
- Between each pair of vertebrae is the intervertebral disc (fibrocartilaginous) > shock absorber + prevents spinal cord damage> composed of “annulus fibrosus” + “nucleus pulposus”
- Linked by ligaments + articulate with 2 types of joint = cartilaginous (bodies of each vertebra) + synovial (between cranial and caudal articular processes)
Cervical= always 7 in the neck of mammals> 1st: Atlas (2 large lateral masses joined by a ventral + dorsal arch)> 2nd: Axis (blade-like spinous, point of muscle attachment) nuchal ligament is the spinous of axis to 1st thoracic> dens of axis fits into vertebral foramen of atlas (pivoting)
Thoracic= usually 13, tall spinous processes + short bodies> articulate with ribs (at costal fovea: head) + (at transverse fovea: tubercle)
Lumbar= usually 7, large bodies, long transverse processes> lumbar muscles attach due to cranioventral angle
Sacral= 3 vertebrae fused to form sacrum in dog + cat> forms a fibrosynovial joint with the wing of the ilium of the pelvic girdle (sacroiliac joint)
Caudal (Coccygeal)= vary in shape + number (length of tail)> first few look like lumbar but get smaller + simpler
Axial Skeleton
Ribs + Sternum Guttural Pouch (Horse)
Ribs:
- Long bones, form lateral thorax walls, rib pairs= thoracic vertebrae, dorsal end thoracic vertebrae articulated
- Movement facilitates lung ventilation
- Structure= dorsal (bone), ventral (costal cartilage)
- Sternum forms breast bone, constructed from sternebrae
Guttural Pouch:
- Lies within the skull of the horse
- Thin mucosal caudoventral pocket (Eustachian tube)
- Holds air
- Found between the base of the skull, atlas (C1), pharynx and the beginning of the oesophagus
Appendicular Skeleton
Forelimbs
- Pectoral + Pelvic limb
- Shoulder + Pelvic girdles
- Forelimb= no bony connection to trunk (only muscles) - absorbs shock
- Differs from primates (bipedalism)= pectoral girdle + clavicle
- Hind limb> bony articulation in pelvic girdle (platform for muscles>propulsive force)
Forelimbs:
- Clavicle: Frequently absent (dog)> described as vestigial, present in cat but doesn’t articulate
- Scapula: Shoulder blade> large, flat bone> lateral surface of trunk> prominent spine down the middle> divides into 2> medial surface is flat + smooth
- Humerus: Long bone of upper forelimb> articulates proximally (scapula, shoulder) + distally (radius/ulna, elbow)
- Radius + Ulna: Long bones> side by side in forearm
- Carpus: 2 rows of short bones> radial, ulnar and accessory carpal bones (proximal row)> radial is most medial, accessory lies on lateral edge, projects caudally
- Metacarpus: 5 small long bones> MC1 is the dew claw, smallest, most medial> proximal= carpal, distal= phalanges
- Digits: Composed of phalanges (long bones)> each digit has 3 phalanges except dew claw which has 2> proximal phalanx articulates metacarpal bone> middle phalanx articulates phalanx above and below> distal phalanx articulates nothing (ungula process, end, part of the claw)
Hindlimbs
Hindlimbs:
- Pelvis: Hind limb connects to the body
- Femur: Head placed in teres/round ligament attaching to acetabular (joint) fossa> long bone forming the thigh> articular head faces medially (articulate acetabulum)> Greater Trochanter + Lesser Trochanter are for muscle attachment> Medial Epicondyle + Lateral Epicondyle articulate with tibia
- Patella: Sesamoid bone (quadriceps femoris muscle)> main extensor of stifle> 2 more sesamoids (fabellae) behind stifle (gastrocnemius)> articulate with femur condyles
- Tibia + Fibula: Long bones form lower leg> lie parallel to one another> proximal expansion to articulate femur> Tibia (largest + medial)> Fibula (thin, long bone + lateral to tibia)
- Tarsus: 7 short bones (tarsal) arranged in 3 rows> 2 bones, proximal row, talus + calcaneus articulate distal tibia and fibula at hock joint> talus is the most medial, proximal to trochlea> calcaneus is lateral, large caudal projection (tuber calcis)
- Metatarsus + Digits: Similar to forepaw> metatarsus= 4 metatarsal bones (some breeds have 5 - small metatarsal 1 or hind dew claw)
Joints
- Bone connects to another > articulation (joint)
- 3 types:
- Fibrous* immovable joint, bones forming them united by dense fibrous connective tissue (ie, sutures in the skull). Teeth attached by bony sockets in jaw bone. Also classed as synarthroses (joint that allows little/no movement)
- Cartilaginous* Limited/No movement, united by cartilage (ie, pubic symphysis connecting the 2 hip bones) - synarthroses. Some classed as amphiarthroses (some degree of movement - limited flexibility of spinal column)
- Synovial* AKA diarthroses (wide range of movement), few synovial joints contain 1+ intra-articular fibrocartilaginous discs/menisci (stifle joint) + (temporomandibular joint) > increase range of movement act as “shock absorbers” (reduce wear and tear)
Movements of Synovial:
- Flexion/Extension: F (bends), E (straightens)
- Abduction/Adduction: Ab (out of normal), Ad (back to normal)
- Rotation: twists on own axis
- Circumduction: movement in a circular pattern of an extremity
- Gliding/Sliding: articular surfaces of joint slide over each other
- Protraction: limb moves cranially (advance limb forward)
- Retraction: limb moves back towards body
