The Skeletal System

The Skeletal System: Comprehensive Notes

Bone Terminology

• os: A bone (e.g., os penis).

• osteo-: Prefix relating to bone.

• Osteoblasts: Cells that form bone.

• Osteocytes: Mature bone cells; osteoblasts that have become surrounded by bone matrix.

• Ossification: The process of hardening of the matrix, turning it into bone.

• Lacunae: Tiny spaces within the bone matrix where osteocytes reside.

• Canaliculi: Microscopic canals that connect lacunae and allow communication between osteocytes and nutrient exchange.

Bone Characteristics

• The second hardest natural substance in the body.

• Composed of specialized cells embedded in an extracellular matrix.

Functions of Bones

• Support: Provides the framework for the body.

• Protection: Guards vital organs (e.g., skull protects the brain, ribs protect the heart and lungs).

• Leverage: Acts as levers for muscles, enabling movement.

• Storage: Stores minerals (primarily calcium and phosphorus) and adipose tissue (in yellow bone marrow).

• Blood Cell Formation (Hematopoiesis): Produces blood cells in the red bone marrow.

Bone Structure

• Cancellous Bone (Spongy Bone):

  • Light and spongy in appearance.

  • Composed of tiny "spicules" of bone that appear randomly arranged.

  • Contains many spaces filled with bone marrow.

  • Spicules are arranged to withstand the specific forces the bone is subjected to.

• Compact Bone (Cortical Bone):

  • Heavy and dense.

  • Composed of tiny, tightly compacted cylinders of bone, known as Haversian systems.

  • Forms the outside layer of all bones and the shafts of long bones.

  • Haversian Systems (Osteons):

    • Run lengthwise with the bone.

    • Consist of concentric layers of ossified bone matrix around a central Haversian canal.

    • Haversian canals contain blood vessels, lymph vessels, and nerves.

    • Canaliculi connect the osteocytes within the concentric layers to the Haversian canal for nutrient and waste exchange.

• Periosteum:

  • A membrane that covers the outer surfaces of bones (except articular surfaces).

  • Has an outer layer of fibrous tissue for protection and attachment of tendons/ligaments.

  • Has an inner layer of osteoblasts (bone-forming cells) responsible for bone repair and growth in width.

• Endosteum:

  • A membrane that lines the hollow interior surfaces of bones, such as the medullary cavity.

  • Contains osteoblasts and osteoclasts.

Bone Cells

• Osteoblasts: Cells that are responsible for forming new bone tissue by secreting osteoid, which then mineralizes.

• Osteocytes: Mature bone cells, developed from osteoblasts that have become entrapped within the bone matrix they secreted. They maintain the bone tissue.

• Osteoclasts: Large, multinucleated cells that resorb (break down) bone tissue, playing a crucial role in bone remodeling and calcium homeostasis.

Blood Supply to Bone

• Volkmann's Canals:

  • Channels that run through the bone matrix at right angles to the Haversian systems.

  • Contain blood vessels that join and connect the Haversian canals and the periosteum, allowing extensive blood supply throughout compact bone.

• Nutrient Foramina:

  • Large channels found in large (and long) bones.

  • Carry blood vessels and nerves into and out of the bone marrow.

  • On X-rays, they may sometimes resemble a fracture of the bone cortex, requiring careful differentiation.

Bone Formation

• Endochondral Bone Formation:

  • The most common method of bone formation.

  • Involves the replacement of a cartilage template by bone tissue.

  • Bone first develops in a primary growth center in the diaphysis (shaft) of the bone during fetal development.

  • Secondary growth centers develop in the epiphysis (ends) of the bone shortly before or after birth.

  • Growth in length occurs at the epiphyseal plates (growth plates) located between the diaphysis and epiphysis.

• Intramembranous Bone Formation:

  • Bone develops from fibrous tissue membranes, without a cartilage precursor.

  • Occurs only in certain skull bones (e.g., formation of flat bones of the skull).

Bone Shapes

• Long bones: Longer than they are wide, primarily support weight and facilitate movement (e.g., femur, humerus).

• Short bones: Cube-shaped, provide stability and some movement (e.g., carpal bones, tarsal bones).

• Flat bones: Thin, flattened, and often curved, provide protection or broad surfaces for muscle attachment (e.g., scapula, skull bones).

• Irregular bones: Have complex shapes that do not fit into the other categories, often with multiple processes (e.g., vertebrae, pelvis).

Bone Marrow

• Red Bone Marrow:

  • Hematopoietic tissue: Responsible for producing all blood cells.

  • Most common type in younger animals.

• Yellow Bone Marrow:

  • Primarily adipose connective tissue (fat).

  • Most common type in adult animals.

  • Can revert to red marrow and resume hematopoietic function if the body needs increased blood cell production (e.g., in cases of severe blood loss).

Common Bone Features

• Articular Surfaces:

  • Smooth areas of compact bone at the ends of bones where they come into contact with other bones to form joints.

  • Covered by a smooth, thin layer of hyaline cartilage called articular cartilage to reduce friction.

  • Examples include:

    • Condyle: A large, rounded articular surface (e.g., distal femur).

    • Head: A spherical articular surface (e.g., head of femur, head of humerus).

    • Facet: A flat, articular surface (e.g., on vertebrae).

• Processes:

  • Lumps, bumps, and other projections on a bone.

  • Usually sites for muscle or ligament attachment.

  • Names depend on location:

    • Spinous process of a vertebra.

    • Trochanter on the femur.

    • Tubercle on the humerus.

    • Tuberosity on the ischium.

    • Spine on the scapula.

    • Crest on the tibia.

    • Wing on the atlas.

• Holes and Depressed Areas:

  • Foramen: A hole in a bone, through which something important (like a blood vessel or nerve) passes (e.g., foramen magnum).

  • Fossa: A depressed or sunken area on the surface of a bone, typically occupied by muscles or tendons (e.g., olecranon fossa).

Bones of the Skeleton

• Axial Skeleton:

  • Located along the central axis of the body.

  • Includes bones of the head and trunk: skull, hyoid bone, spinal column, ribs, and sternum.

• Appendicular Skeleton:

  • Comprises the bones of the limbs (forelimbs and hindlimbs).

• Visceral Skeleton:

  • Bones formed in soft organs (viscera), not connected to the main skeleton.

Axial Skeleton Details

• Skull:

  • Composed of $37-38$ separate bones.

  • Most bones are united by jagged, immovable, fibrous joints called sutures.

  • The mandible (lower jaw) is connected to the skull by a freely movable temporomandibular joint (TMJ), which is a synovial joint.

  • External Bones of the Cranium (visible on surface):

    • Occipital bone ($1$): Single bone forming the base of the skull.

      • Foramen magnum: Large opening in its center where the spinal cord exits the skull.

      • Occipital condyles ($2$): Articulate with the first cervical vertebra (atlas).

    • Interparietal bones ($2$): Small bones located between the parietal and occipital bones.

    • Parietal bones ($2$
      ): Form the dorsolateral walls of the cranium.

    • Temporal bones ($2$): Located ventral to the parietal bones, forming the lateral walls of the cranium.

      • Contain the middle and inner ear structures.

      • Form the TMJ joints with the mandible.

      • External acoustic meatus: Visible from the outside, leads to the ear canal.

    • Frontal bones ($2$): Form the forehead region and a portion of the eye socket.

      • In horned breeds of cattle, the horn core develops here.

      • Contain the paranasal (frontal) sinus.

  • Bones of the Ear (Ossicles): Smallest bones of the body, located in the middle ear.

    • Malleus (hammer), Incus (anvil), Stapes (stirrup).

    • Function: Transmit vibrations from the tympanic membrane (eardrum) across the middle ear cavity to the cochlea, where receptor cells convert vibrations into nerve impulses.

  • Bones of the Face (Landmark / External):

    • Incisive bones ($2$)

    • Nasal bones ($2$): Form the bridge of the nose.

      • Skull shapes related to nasal bone length: Dolichocephalic (long-nosed), Brachycephalic (short-nosed).

    • Maxillary bones ($2$): Form the upper jaw and house the upper teeth.

    • Lacrimal bones ($2$): Small bones forming part of the medial orbital wall.

    • Zygomatic bones ($2$): Form part of the cheekbone (zygomatic arch).

    • Mandible ($1$ or $2$): The lower jaw bone.

  • Bones of the Face (Hidden / Internal):

    • Palatine bones ($2$): Form the caudal portion of the hard palate.

    • Pterygoid bones ($2$): Support the pharynx.

    • Vomer bone ($1$): Forms part of the nasal septum.

    • Turbinates ($4$) / Nasal conchae: Scroll-like bones in the nasal cavity, increasing surface area.

• Hyoid Bone (Hyoid Apparatus):

  • A single U-shaped bone located in the neck.

  • Composed of several portions united by cartilage.

  • Function: Supports the tongue and larynx, aiding in swallowing.

• Spinal Column (Vertebral Column):

  • Consists of a series of individual bones called vertebrae.

  • Extends from the skull to the tip of the tail.

  • Composed of $5$ distinct regions, denoted by a letter and number (e.g., C + #):

    • Cervical (C + #): Neck region.

    • Thoracic (T + #): Chest region.

    • Lumbar (L + #): Abdominal region.

    • Sacral (S + #): Pelvic region, fused into the sacrum.

    • Coccygeal (Cy + #): Tail region.

  • General Vertebra Structure:

    • Body: The main, ventral, spool-shaped portion, which supports weight.

    • Arch: Dorsal to the body, forming the vertebral foramen.

    • Spinal Canal: Formed by the alignment of vertebral arches, through which the spinal cord passes.

    • Intervertebral Disks: Cartilaginous disks that separate the bodies of adjacent vertebrae, providing cushioning and flexibility.

    • Processes ($3$ types):

      • Spinous processes (dorsal projection).

      • Transverse processes (lateral projections).

      • Articular processes (cranial and caudal, for articulation with adjacent vertebrae).

  • Cervical Vertebrae:

    • Located in the neck region.

    • Nearly all mammals have $7$ cervical vertebrae.

    • $C1$ = Atlas: The first cervical vertebra, ring-shaped, articulates with the occipital condyles to allow nodding movements.

    • $C2$ = Axis: The second cervical vertebra, has a large peg-like dens that projects cranially into the atlas, allowing rotational head movements.

  • Thoracic Vertebrae:

    • Located dorsal to the thorax.

    • The number of thoracic vertebrae varies among and within species and corresponds to the number of pairs of ribs.

    • Anticlinal vertebra: A specific thoracic vertebra where the dorsal spinous process points straight up instead of caudally or cranially.

      • Dogs and cats: $T11$

      • Horses: $T16$

      • Ruminants: $T13$

      • Swine: $T10$

  • Lumbar Vertebrae:

    • Located dorsal to the abdominal region.

    • Number can vary among and within species.

    • Are large and bulky to support the weight of the abdomen.

  • Sacral Vertebrae:

    • Fuse to form a single, solid structure called the sacrum.

    • The number of fused vertebrae varies among species.

    • Forms the sacroiliac joint with the pelvis.

  • Coccygeal Vertebrae:

    • The bones of the tail.

    • The number varies significantly both among and within species (e.g., long-tailed vs. short-tailed breeds).

    • Their appearance can vary even within an individual animal's tail, becoming smaller and simpler distally.

• Ribs:

  • Flat bones that form the lateral walls of the thorax.

  • The number of pairs of ribs equals the number of thoracic vertebrae.

  • Dorsal heads articulate with the thoracic vertebrae.

  • Costal cartilage: The ventral ends of the ribs, composed of hyaline cartilage.

  • Costochondral junction: The area where the bony rib meets the costal cartilage.

  • Sternal ribs (True ribs): Ribs whose cartilage joins directly to the sternum.

  • Asternal ribs (False ribs): Ribs whose cartilage joins the costal cartilage of the rib cranial to them, forming the costal arch.

  • Floating ribs: Asternal ribs whose cartilage does not attach to other cartilage or the sternum; their ventral ends terminate in the abdominal muscles.

• Sternum (Breastbone):

  • Composed of a series of rod-like bones called sternebrae.

  • Forms the floor of the thorax.

  • Manubrium: The first, most cranial sternebra.

  • Xiphoid process: The last, most caudal sternebra, often ending in a cartilaginous xiphoid cartilage.

Appendicular Skeleton Details

• Thoracic Limb (Front Leg):

  • Has no bony connection to the axial skeleton; it is connected by muscles and ligaments.

  • Bones include: Scapula, Humerus, Ulna, Radius, Carpal bones, Metacarpal bones, Phalanges.

  • Scapula (Shoulder Blade):

    • A flat, triangular bone.

    • Features a prominent spine of the scapula, a longitudinal ridge.

    • Forms the socket portion of the shoulder joint with the humerus.

    • Glenoid cavity: A shallow, concave articular surface that receives the head of the humerus.

  • Humerus (Upper Arm Bone):

    • The long bone of the brachium (upper arm).

    • Proximal end is the ball portion (head) of the shoulder joint.

    • Features tubercles (processes) where muscles attach.

    • The shaft extends to the distal end to form the elbow joint with the radius and ulna.

    • Olecranon fossa: A depression on the caudal aspect of the distal humerus that accommodates the olecranon process of the ulna during elbow extension.

    • Condyles: Distal articular surfaces.

      • Trochlea: Articulates with the ulna.

      • Capitulum: Articulates with the radius.

    • Epicondyles: Non-articular knobs on the sides of the condyles for muscle and ligament attachment.

  • Antebrachium (Forearm):

    • Ulna:

      • Forms the major portion of the elbow joint.

      • Olecranon process: The large, proximal projection forming the point of the elbow, serving as a lever arm for triceps muscle attachment.

      • Trochlear notch: A curved articular surface that fits with the trochlea of the humerus.

      • Its shape parallels that of the radius.

    • Radius:

      • The main weight-bearing bone of the antebrachium.

      • Articulates proximally with the humerus and ulna, and distally with the carpal bones.

  • Carpus (Wrist in Humans, Knee in Horses):

    • Composed of two parallel rows of carpal bones.

    • The proximal row often has named bones.

    • The distal row typically has numbered bones (medial to lateral).

  • Metacarpal Bones:

    • Extend distally from the distal carpal bones to the proximal phalanges.

    • Numbered from medial to lateral.

    • Numbers vary by species:

      • Horses: Have one large cannon bone (metacarpal III) and two small, non-weight-bearing splint bones (metacarpal II on the medial side, metacarpal IV on the lateral side). No remnant of I or V.

      • Cattle: Two metacarpal bones (III and IV) are fused into one large bone.

      • Dogs and Cats: Typically have $5$ metacarpal bones.

  • Specific Species Forelimb Digits (Phalanges):

    • Equine Thoracic Limb Bones:

      • Possess $1$ digit with $3$ phalanges:

        • Long pastern bone (proximal phalanx).

        • Short pastern bone (middle phalanx).

        • Coffin bone (distal phalanx).

      • Also have $3$ sesamoid bones:

        • $2$ proximal sesamoids (palmar/plantar to the fetlock joint).

        • $1$ distal sesamoid, also known as the navicular bone, located deep within the hoof capsule.

    • Canine and Feline Thoracic Limb Bones:

      • Digit I (dewclaw): Usually has $1$ proximal phalanx and $1$ distal phalanx.

      • Digits II-V: Each typically has a proximal, middle, and distal phalanx.

      • Have sesamoid bones associated with their phalanges.

      • Ungual process: A bony projection of the distal phalanx that is surrounded by the claw.

    • Thoracic Limb Bones of Cattle:

      • Have two fused metacarpal bones (metacarpal III and IV).

      • Possess four digits: $2$ weight-bearing digits (III and IV) and $2$ smaller dewclaws (II and V).

      • Have sesamoid bones: $2$ proximal and $1$ distal for each main digit.

• Pelvic Limb (Hind Leg):

  • Directly connected to the axial skeleton via the sacroiliac joint.

  • Bones include: Pelvis (Ilium, Ischium, Pubis), Femur, Patella, Fabellae, Tibia, Fibula, Tarsal bones, Metatarsal bones, Phalanges.

  • Pelvis:

    • Composed of $3$ fused bones on each side, forming the os coxae:

      • Ilium: The largest and most cranial bone.

      • Ischium: The caudalmost bone.

      • Pubis: The most ventral bone.

    • The two halves of the pelvis are joined ventrally at the pelvic (or pubic) symphysis.

    • Acetabulum: The large, cup-shaped socket that articulates with the head of the femur, forming the hip joint.

    • Obturator foramina: Large holes in the pelvis that lighten its weight and allow passage for nerves and blood vessels.

  • Femur (Thigh Bone):

    • The long bone of the thigh.

    • The head of the femur fits into the acetabulum of the pelvis.

    • Features prominent trochanters (e.g., greater trochanter) that serve as attachment points for hip and thigh muscles.

    • The distal end forms the stifle joint (knee) with the tibia and patella.

    • Trochlea: An articular groove on the distal femur that contains the patella.

  • Sesamoid Bones of the Pelvic Limb:

    • Patella (Kneecap):

      • The largest sesamoid bone.

      • Formed in the distal tendon of the quadriceps femoris muscle.

      • Protects the tendon and increases the mechanical advantage of the quadriceps muscle.

    • Fabella:

      • Two small sesamoid bones located in the proximal gastrocnemius muscle tendons of dogs and cats.

      • Not present in cattle or horses.

  • Tibia (Shinbone):

    • The main weight-bearing bone of the lower leg.

    • Forms the stifle joint (knee) with the femur proximally and the hock joint (ankle) with the tarsal bones distally.

    • Features a tibial tuberosity (proximal cranial prominence) and a tibial crest (cranial ridge).

  • Fibula:

    • A thin, complete bone of the lower leg that parallels the tibia.

    • Generally not weight-bearing.

    • Primarily serves as a muscle attachment site.

    • Lateral malleolus: A knob-like process at its distal end, forming the lateral prominence of the ankle.

  • Tarsus (Hock or Ankle):

    • Composed of two rows of tarsal bones.

    • The proximal row has named bones.

    • The distal row has numbered bones (medial to lateral).

    • Calcanean tuberosity of the fibular tarsal bone: A prominent projection forming the point of the hock.

  • Metatarsal Bones:

    • Are almost exactly the same as the metacarpal bones in structure and arrangement.

    • Major species-specific differences:

      • Dogs and cats: Typically have $4$ metatarsal bones (II-V), often lacking metacarpal I.

      • Horses: Possess $1$ large metatarsal bone (cannon bone) and $2$ small vestigial metatarsal bones (splint bones).

  • Pelvic Limb Phalanges:

    • Almost exactly the same as the thoracic limb phalanges.

    • Exceptions:

      • Dogs and cats: Usually only have $4$ digits (II-V), often lacking the dewclaw equivalent on the hindlimb.

Visceral Skeleton Details

• Bones that form in soft organs (viscera) and are not connected to the rest of the skeleton.

• Examples include:

  • Os cordis: Bone in the heart of cattle, providing support for heart valves.

  • Os penis: Bone in the penis of some male carnivores (e.g., dogs, cats, raccoons) that aids in copulation.

  • Os rostri: Bone in the snout of pigs, giving rigidity for rooting behavior.

Joints (Articulations)

• Areas where two or more bones meet.

Joint Terminology

• Joint capsule: Fibrous tissue enclosing synovial joints.

• Joint cavity (Joint space): The space within the joint capsule.

• Synovial membrane: Lines the joint capsule and produces synovial fluid.

• Synovial fluid: Lubricates the joint and nourishes articular cartilage.

• Ligaments: Strong fibrous bands connecting bones, stabilizing joints.

Types of Joints

• Fibrous Joints (Synarthroses):

  • Immovable joints.

  • Bones are firmly united by fibrous tissue.

  • Examples: Sutures of the skull, the joint between equine splint bones and the cannon bone.

• Cartilaginous Joints (Amphiarthroses):

  • Slightly movable joints.

  • Bones are united by cartilage.

  • Allow for slight rocking movements.

  • Examples: Intervertebral disks between vertebral bodies, the pubic symphysis joining the two halves of the pelvis, the mandibular symphysis joining the two halves of the mandible.

• Synovial Joints (Diarthroses):

  • Freely movable joints.

  • Characteristics:

    • Articular surfaces on the bones, covered by articular cartilage.

    • A joint cavity (joint space), which is the fluid-filled space between the articular cartilages.

    • A synovial membrane that lines the joint capsule (except over articular cartilage).

    • Synovial fluid within the joint cavity for lubrication and nutrient supply.

    • Reinforced by ligaments for stabilization.

Synovial Joint Movements

• Flexion: Decreases the angle between two bones.

• Extension: Increases the angle between two bones (opposite of flexion).

• Abduction: Moves an extremity away from the medial plane.

• Adduction: Moves an extremity toward the medial plane (opposite of abduction).

• Rotation: A twisting movement of a part on its own axis (e.g., turning the head).

• Circumduction: The movement of an extremity where the distal end moves in a circle, while the proximal end remains relatively stable (e.g., swinging an arm in a circle).

Types of Synovial Joints

• Hinge Joints (Ginglymus Joints):

  • One joint surface swivels around another, allowing movement in one plane only.

  • Primarily permit flexion and extension movements.

  • Examples: Elbow joint, atlanto-occipital joint (between skull and C1).

• Gliding Joints (Arthrodial Joints):

  • Characterized by flat articular surfaces that allow a rocking motion of one bone on another.

  • Primary motion is flexion and extension, but some abduction and adduction may be possible.

  • Example: Carpus (wrist/knee).

• Pivot Joints (Trochoid Joints):

  • One bone rotates around another.

  • Permit rotation movement only.

  • Example: Atlanto-axial joint (between C1 and C2, allowing head rotation).

• Ball-and-Socket Joints (Spheroidal Joints):

  • Consist of a spherical head (ball) of one bone fitting into a cup-like depression (socket) of another bone.

  • Allow for all $6$ types of synovial joint movements (flexion, extension, abduction, adduction, rotation, circumduction).

  • Examples: Shoulder joint, hip joint.