Anatomical Directional Terms

Anatomical Directional Terms

• Dorsal: Toward the spine, including the corresponding surfaces of the head and tail.
• Ventral: Toward the belly, including the corresponding surfaces of the head and tail.
• Cranial: Toward the head.
• Caudal: Toward the tail.
• Rostral: Within the head, toward the muzzle.
• Medial: Toward the midline or the median plane.
• Lateral: Away from the midline, toward the side or flank.
• Proximal: Toward the junction with the body, primarily applies to limbs and appendages.
• Distal: Away from the junction, primarily applies to limbs and appendages.
• Anterior: In a forward direction.
• Posterior: In a backward direction.
• Superior: Toward the highest point of the animal (toward the sky).
• Inferior: Toward the lowest point of the animal (toward the ground).
• Axial: Toward the axis of a central digit or the axis of a limb.
• Abaxial: Away from the central axis of the limb.
• Superficial: Toward the surface.
• Deep: Away from the surface.
• Peripheral: Structures or organs distant from the point of origin.

Body Segments and Cavities

• Body segments include cranial, cervical, thoracic, lumbar, sacral, and coccygeal.
• Cranial Cavity: Space within the skull housing the brain.
• Oral Cavity: The mouth, bordered by the lips rostrally and bony/soft tissue structures laterally, dorsally, and ventrally.
• Nasal Cavity: Dorsal to the oral cavity, housing specialized respiratory structures.
• Thoracic Cavity: Chest cavity housing the lungs, heart, and other organs, spanning the thoracic segment and defined by the ribs.
• Abdominal Cavity: Caudal to the thoracic cavity, housing most internal organs, spanning the lumbar segment, partly covered by the ribcage cranially, and is the largest body cavity.
• Diaphragm: A thick muscle separating the thoracic and abdominal cavities, facilitating lung inflation and air expulsion.
• Pelvic Cavity: Spans the sacral segment, housing parts of the gastrointestinal, urinary, and reproductive tracts, mostly defined by the bony borders of the pelvis.

Recumbency Terms

• Supine: Also known as dorsal recumbent, the animal is lying on its back.
• Prone: Also known as ventral recumbent, the animal is lying on its belly.
• Lateral Recumbency: Lying on one side; can be right lateral (right side down) or left lateral (left side down).

Integument and its Functions

• Common Integument: Includes haired skin and specialized structures, completely enclosing the body with seamless connectivity to mucous membranes at body openings.
• Mucous Membranes: Specialized tissue secreting mucus to defend the body from external invaders.
• Functions of Integument:
  • Providing a physical barrier against the outside world.
  • Thermoregulation via sweat and hair.
  • Preventing desiccation and excessive water uptake.
  • Synthesizing vitamin D.
  • Protecting against harmful sun damage.

Epithelial Tissue Types Based on Cell Shape

• Squamous: Cells wider than they are tall.
• Cuboidal: Cells with approximately equal width and height.
• Columnar: Cells taller than they are wide.

Epithelial Tissue Classification Based on Cell Layers

• Simple: One cell layer thick.
• Stratified: Multiple cell layers thick.
• Pseudostratified: Technically one cell layer thick but appears multi-layered due to nuclei at different levels; generally a specialized epithelium.

Epidermis Layers (Superficial to Deep)

• Stratum corneum
• Stratum lucidum
• Stratum granulosum
• Stratum spinosum
• Stratum basale

Stratum Basale

• Closely adhered to the underlying dermis.
• Contains actively dividing cells with a thick cuboidal or columnar shape.

Stratum Granulosum

• Cells begin producing keratin granules.

Keratin

• A specialized protein that strengthens tissues like hair, hoof, and horn.

Stratum Lucidum

• Cells are flattened and lack a nucleus.
• Appears homogenous due to dispersed keratin protein granules.

Stratum Corneum

• The most superficial layer, densely packed with keratin.
• Cells are flat and dead.

Dermis

• Deep layer of the skin mostly made of collagen.
• Collagen: Major protein making up connective tissue.
• Contains elastic fibers that give the skin stretchiness and the ability to regain its form.
• Contains hair follicles, sebaceous glands, vessels, and nerves.

Dermal-Epidermal Interface

• Connects the epidermis via interdigitations or finger-like projections for a tight fit.
• Deep dermal projections and ridges create epidermal contours, giving us fingerprints and dog nose prints.

Mandible

• The lower jaw bone.
• The only mandible bone of the mammalian skull.
• Houses the lower teeth.
• The right and left halves unite at the mandibular symphysis rostrally.
• Parts of the mandible:
  • Body: Horizontal part where lower teeth attach.
  • Ramus: Vertical part that holds no teeth.
  • Condyloid (Articular) Process: Smooth process articulating with the mandibular fossa on the skull.
  • Coronoid Process: The large prominence at the tip of the ramus where chewing muscles attach.

Teeth

• Incisors: Most rostral teeth, used to divide food; absent in ruminants.
• Canines: Well-developed in carnivores, deeply embedded in the bone.
• Premolars: Rostral cheek teeth.
• Molars: Caudal cheek teeth.
• Carnassial Teeth: Specialized premolars/molars for shearing in carnivores.
• Diastema: A large space between teeth, seen in many herbivores.
• Dental Pad: In ruminants, a tough cushion where other species have upper incisors.
• Deciduous Teeth: "Baby teeth" or "milk teeth," usually smaller and fewer than permanent teeth.
• Eruption: Appearance of teeth through the gums.
• Diphyodont: An animal with a set of deciduous teeth.
• Eruption and shedding of deciduous teeth: Used to determine the age of juveniles.
• Typically only have molars as part of permanent set of teeth

Other Skull Structures

• Paranasal Sinuses: Cavities within the skull filled with maze-like pockets, lined with mucous membranes, and communicate with the nasal cavity.
• There are 6 paranasal sinsuses; caudal, rostral, ventral, palatine and frontal- the one associated with horns in ruminants
• Hyoid Apparatus: 5 bones connected by cartilage suspending the tongue and larynx from the skull.
• Larynx: Made of cartilage pieces, acts as gatekeeper to the respiratory tract.

Vertebrae

• Intervertebral Foramen: Opening between vertebrae for spinal nerves to exit.
• Intervertebral Discs: Fibrocartilaginous discs between vertebral bodies.
• Vertebral Arch: Dorsal part of the vertebra, dorsal aspect of the foramen.
• Vertebral Body: Thick rounded ventral portion, convex cranially, concave caudally.
• Vertebral Foramen: Space between vertebral arch and body, where the spinal cord is located.
• Vertebral Canal: Canal formed by vertebral foramina.
• Dorsal Spinous Process: Dorsal projection off of the vertebral arch.
• Transverse Process: Lateral extension off of the vertebral arch.

Vertebral Regions:

• Cervical: Always 7.
• Thoracic: Varies in number depending on species.
• Lumbar: Varies in number depending on species.
• Sacral
• Coccygeal

Cervical Vertebrae: C1 (Atlas)

• Supports the head.
• Articulates with the occipital condyles of the skull via the atlanto-occipital joint, allowing flexion/extension (“yes” motion).
• Lacks a body or spinous process and has wings.

Cervical Vertebrae: C2 (Axis)

• Has a very large and long dorsal spinous process.
• Dens: A peg-like cranial process that articulates with the atlas at a pivot joint ("no" motion).

Other Cervical Landmarks

• C3-C7 have a transverse foramen through the transverse process.

Thoracic Vertebrae

• Where ribs articulate.
• Have elongated dorsal spinous processes and short transverse processes.

Lumbar Vertebrae

• Vary in number depending on the species. Examples:
  • Carnivore: 7
  • Equine: 6 (sometimes 5)
  • Bovine: 6
  • Sheep: 6-7
  • Pig: 6-7
  • Chicken: 14 (lumbosacral)

Sacral Vertebrae

• Fused to form the sacrum.
• Have dorsal and ventral foramina where spinal nerves exit.
• Lateral “wings” articulate with the hip bones at the sacroiliac joint.
• Dorsal spinous processes fused in ruminants.
• Coccygeal Vertebrae are the tail, varies in number, becomes progressively smaller distally, eventually losing the spinal canal.

Thorax Composition

• Bony cavity formed by the ribs, sternum, and thoracic vertebral bodies.
• Protects thoracic organs such as the heart, lungs, trachea, and esophagus.

Ribs (Costae)

• Long, curved bones forming the lateral wall of the thorax, arranged in pairs.
• Articulate with two vertebrae at a time.
• Named for the caudal vertebra they articulate with.
• Rib Anatomy:
  • Head: Articulates with the body of the cranial vertebrae. -Tubercle: Articulates with the transverse process of the caudal vertebrae.
  • Body.
  • Ventral Costal Cartilage: How the rib attaches to the sternum ventrally, connecting caudally to form a “costal arch.”

Sternum

• Made of three parts:
  • Manubrium: Cranial most, projects in front of the first ribs.
  • Several sternebrae segments: Articulate with the costal cartilage.
    • Carnivores= 8 bones
    • Horse= 6 bones
    • Ruminants = 7 bones
  • Xiphoid Cartilage: Caudal-most, projects out between the costal arches.

Rib Categories

• Vertebrosternal ("True Ribs"): Directly articulate with the sternum via costal cartilage (first 8 or so).
• Vertebrochondral ("False Ribs"): Indirectly articulate with the sternum via articulation with other ribs’ costal cartilage.
• Vertebral ("Floating Ribs"): No articulation with the sternum or other ribs (most caudal).

Clavicle

• May or may not exist.
• Connects the axial skeleton and forelimbs when functional.
• Vestigial in cats and dogs.
• Non-existent in horses.

Scapula

• Flat and roughly triangular, lying over the craniodorsal part of the thorax in quadrupeds.
• Held in place by muscles rather than the clavicle.
• Basis of the shoulder region.
• Spine: Divides the lateral surface into the supraspinous fossa and infraspinous fossa.
• Scapular Cartilage: Enlarged area for muscle attachment, becomes more ossified with age.

Humerus

• A long bone that lies obliquely against the ventral part of the thorax.
• Shorter and more robust in cattle and horses.
• Humeral Anatomy: - Head: Proximal, articulates with the glenoid fossa of the scapula. - Greater Tubercle: Lateral process, large, forms the point of the shoulder. - Deltoid Tuberosity: Process on the lateral side that joins the greater tubercle via a prominent ridge. - Olecranon Fossa: Distal, caudal depression where the olecranon fits. - Radial Fossa: Distal, cranial depression.

Ulna and Radius

• The ulna is caudal to the radius.
• Held together by ligaments or fused in some species.
• In animals that can supinate, the distal radius rotates around the ulna (somewhat in dogs, more so in cats).
• The ulna is only partially present in horses, ruminants, and pigs. - Olecranon: At the proximal end of the ulna, forming the point of the elbow. -Head: At the proximal end of the radius, articulating with the humerus at the elbow.

Distal Forelimb (Manus)

• Similar structure to the hindlimb in most animals.
• Carpus
• Metacarpus
• Phalanges
• Sesamoid bones

Carpus

• Joint made up of multiple short, cuboidal bones arranged in two rows: - Proximal Row: Radial, intermediate, ulnar, accessory (projects caudally). - Distal Row: Carpal bones 1-4 (5 is usually fused to #4).
• Convex on the cranial aspect, irregular on the caudal aspect.

Metacarpal Comparative Anatomy

• Herbivores have reduced numbers of metacarpal bones.
• In horses, only MCIII bears weight, with tiny MCII and MCIV on either side.

Metacarpus/Metacarpal Bones

• Bones immediately distal to the carpus.
• Number varies between species.
• In cats and dogs, each separate metacarpal can be palpated.
  • Carnivores: Have all 5 metacarpal bones, with MCI being the most medial (dew claw).

Phalanges

• Distal to the metacarpals, make up the digits.
• 3 phalangeal bones within each digit, named for their location:
• Proximal (P1, or first phalanx)
• Middle (P2, or second phalanx)
• Distal (P3, or third phalanx)