Skeletal System
Skull
Skull morphology varies by species, particularly in facial bone proportions and bone structures:
Types of skulls based on facial proportion:
Dolicocephalic: Long facial compartment (e.g., Collie).
Mesaticephalic: Medium facial compartment (e.g., Beagle).
Brachycephalic: Short facial compartment (e.g., Pug).
Species-specific features:
Dog: Supraorbital foramen absent; orbital ligament completes bony orbit.
Cattle: Frontal bone forms cranial roof; presence of facial tuber; cornual process for horns; orbits complete; no median sagittal crest.
Horse: Facial crest present; complete orbit.
Pig: Os rostri nasal bone unique; incomplete orbit.
Bird: Large, incomplete bony orbit; early suture fusion; single occipital condyle; beak present.
Mandible
Shape and landmarks vary markedly:
Dog: Deep masseteric fossa, angular process present, short diastema, long and wide coronoid process.
Cattle: Wide, long diastema; long but narrow coronoid; concave condyloid head.
Horse: Wide, long diastema; convex condyloid head.
Pig: Short coronoid process; massive body; coronoid nearly equal height to condyloid.
Cat: Angular process present.
Birds: Mandible formed by two fused, thin bones covered by lower beak.
Hyoid Apparatus
Connects skull to tongue and larynx, exhibiting species-specific features:
Dog & Cat: Longest epihyoid; no lingual process; cylindrical stylohyoid & epihyoid.
Cattle: Short lingual process; laterally flattened stylohyoid.
Horse: Long lingual process; epihyoid fused with stylohyoid.
Pig: Epihyoid ligamentous; articulates at nuchal process.
Vertebral Formula and Vertebrae
Vertebral counts show variation:
Animal C T L S Cd
Horse 7 18 6 5 15-21
Cattle 7 13 6 5 18-20
Sheep 7 13 6-7 4 16-18
Goat 7 13 7 4 12
Dog 7 13 7 3 20-24
Pig 7 11-15 6-7 4 20-23
Chicken 14-17 5-7 14-15 - -
Atlas & Axis: Cervical vertebrae with species-specific foramina, e.g., dog lacks alar foramen, pig has unique transverse foramen positioning.
Thoracic Vertebrae: Variation in spinous process height and anticlinal vertebra, which is the vertebra where spinous processes change orientation (e.g., Dog T11, Horse T16).
Lumbar Vertebrae: Differing orientations of spinous and transverse processes influence muscle attachment and flexibility.
Sacral Vertebrae: Degree of fusion varies; birds have fused synsacrum combining lumbar and sacral vertebrae.
Ribs and Sternum
Species differ in rib shape and number of sternal (attached) and asternal (free) ribs.
Birds have uncinate processes overlapping ribs and a prominent keel on the sternum for flight muscle attachment.
Appendicular Skeleton
Scapula: Varies in shape, presence of acromion, and scapular cartilage thickness.
Humerus: Differences in tuberosities and presence of supratrochlear foramen (dog), or intermediate tubercle (horse).
Radius and Ulna: Fusion and relative lengths differ; for example, ulna fused partially to radius in horses but separate in dogs and cats.
Carpals, Metacarpals, and Digits: Fusions common in ruminants and horses (e.g., fused metacarpals forming cannon bones). Digit number correlates with weight-bearing function.
Os Coxae (Pelvic Bone): Shape and tuber positions vary reflecting muscular attachments and locomotion modes.
Femur: Differences include relative height of greater trochanter and presence of trochlear tubercle.
Tibia and Fibula: Degree of fusion marks specialization; horses and ruminants exhibit fibular reduction or fusion whereas pigs retain separate bones.
Tarsals and Metatarsals: Similar fusion patterns as in thoracic limbs; birds show extensive fusion forming tarsometatarsus.
Muscular System
Brachiocephalicus
Composed of species-specific muscle parts reflecting neck and limb function:
Dogs have three parts including cleidomastoideus.
Ruminants and pigs include cleidooccipitalis.
Horses lack cleidooccipitalis but have cleidobrachialis and cleidomastoideus.
Sternocephalicus
Facial parts differ reflecting head-neck movement:
Dogs have sternooccipitalis and sternomastoideus.
In ruminants and goats, both sternomandibularis and sternomastoideus are present.
Horses primarily have sternomandibularis.
Pectorals
Variably developed across species:
Dogs have narrow descending pectoral bands, no subclavius.
Ruminants have poorly developed subclavius.
Horses and pigs have well-developed subclavius.
Birds’ pectoralis powers downstroke of flight; supracoracoideus powers upstroke.
Deltoideus and Triceps Brachii
Deltoideus parts separate in dogs and ruminants; fused in horses and pigs.
Triceps brachii has four heads in dogs but only three in ruminants, horses, and pigs.
Biceps Femoris and Sartorius
Biceps femoris partially or fully fused with superficial gluteal muscle in some species (e.g., cattle, pigs).
Sartorius muscle belly divided variably; dogs and some ruminants show division, horses and cats do not.
Soleus Muscle
Generally absent in carnivores; present and varying in herbivores and pigs.
Digestive System
Lips and Oral Structures
Dogs and horses have highly mobile lips adapted for grasping food.
Ruminant lips less mobile, featuring specialized nasolabial plates.
Pigs form a rostrum with the upper lip supported by os rostrale.
Birds lack lips and instead use their beaks for prehension.
Hard Palate
Reflects dietary adaptation:
Dogs have a ridged median raphe replaced by a ridge.
Ruminants feature a dental pad instead of upper incisors.
Horses show a distinctive margo plicatus separating glandular and non-glandular mucosa.
Pigs have a long, narrow palate with prominent rugae and incisive papilla.
Tongue
Varies in shape and papillae distribution according to feeding habits.
Dogs: broad, mobile with lyssa.
Ruminants: torus linguae present with conical papillae.
Cats: thick, cornified.
Birds: no intrinsic musculature; supported by entoglossal bone.
Dentition and Dental Formula
Permanent dental formulae vary:
Species Permanent Formula Total Teeth
Dog 2(I3/3 C1/1 P4/4 M2/3)2(I3/3C1/1P4/4M2/3) 42
Ruminant 2(I0/4 C0/0 P3/3 M3/3)2(I0/4C0/0P3/3M3/3) 32
Horse 2(I3/3 C0/1 P3(4)/3 M3/3)2(I3/3C0/1P3(4)/3M3/3) 40-42
Pig 2(I3/3 C1/1 P4/4 M3/3)2(I3/3C1/1P4/4M3/3) 44
Cat 2(I3/3 C1/1 P3/2 M1/1)2(I3/3C1/1P3/2M1/1) 30
Birds No teeth —
Carnassial teeth noted in carnivores.
Specialized teeth such as wolf teeth in horses and tusks in pigs highlight functional adaptation.
Ruminants lack upper incisors replaced by a dental pad.
Salivary Glands
Saliva quantity and gland types align with dietary needs:
Ruminants: High saliva production (110–180 L/day).
Horse: ~40 L/day.
Pig: ~15 L/day.
Dogs and cats: have zygomatic glands; parotid ducts open near carnassial teeth.
Birds possess well-developed oral glands for seed digestion.
Esophagus and Stomach
Muscle composition varies: dog and ruminants have fully striated esophagus; horses/cats partial smooth muscle.
Birds have crop for food storage.
Stomach types:
Carnivores: completely glandular.
Horse: composite with margo plicatus.
Ruminants: four-compartmentalized (rumen, reticulum, omasum, abomasum).
Birds: proventriculus (glandular) and ventriculus (gizzard with koilin membrane).
Ruminant Stomach
The rumen forms the largest compartment in adults, lined by papillae.
Reticulum: honeycomb pattern, clinically significant for hardware disease.
Omasum: muscular laminae resembling pages.
Abomasum: true glandular stomach with rugae and torus pyloricus.
Intestine: Jejunum, Colon, and Cecum
Jejunum location varies with abdominal organ distribution.
Colon structure adapted for dietary needs:
Dogs/cats: short ascending colon.
Ruminants: complex spiral colon.
Horses: large segmented great colon.
Pigs: spiral ascending colon with mixed sacculation.
Birds lack colon, having ceca instead.
Cecum size and orientation reflect fermentation habits; large in horses and sacculated uniquely.
Rectum and Cloaca (Birds)
Dogs have anal glands.
Ruminants and horses possess ampulla recti.
Birds have cloaca with divisions for fecal, urinary, and reproductive tracts; includes immune bursa.
Liver and Pancreas
Liver morphology varies in lobation and gall bladder presence:
Dogs have subdivided lobes and visible gall bladder.
Horses lack gall bladder.
Birds usually have two lobes; many lack gall bladder.
Pancreas differences relate to duct arrangement and shape tied to digestive patterns.
Urinary System
Kidney shape varies (bean-shaped to lobated).
Ruminants have lobated kidneys without a renal pelvis, instead having multiple calyces.
Horses possess a heart-shaped right kidney with terminal recess; pigs have fused cortex but unfused medulla.
Birds have lobed kidneys without calyces or pelvis.
Cats have unique subscapular veins.
Respiratory System
Nose
Dogs and cats have planum nasale with philtrum.
Horses lack philtrum and have false nostrils.
Pigs have a planum rostrale.
Birds feature nostrils covered by operculum or cere.
Larynx and Trachea
Epiglottis shape varies from pointed (dog, horse) to rounded (ruminants, pigs).
Birds have no phonation at larynx but have a syrinx located at tracheal bifurcation.
Cartilage rings number and fusion vary; dogs have 42-46 while birds have ~120 complete rings.
Lungs and Air Sacs
Mammals have lobed lungs differing by species.
Birds have non-lobed lungs with multiple air sacs playing primary roles in ventilation.
Cardiovascular System
Heart size and position vary; dogs’ hearts represent ~0.7% of body weight, pigs’ ~0.3%.
Specific structures like os cordis in cattle exist.
Coronary artery branching patterns differ among species, influencing circulation.
Aortic arch branching varies, with ruminants and horses sharing a common brachiocephalic trunk, dogs and pigs having separate vessels.
Reproductive System
Male System
Testes location varies (horizontal in dogs; vertical in ruminants).
Epididymis length ranges widely (dog 5–8 m, stallion 72–81 m).
Accessory glands show species-specific presence and size (e.g., bulbourethral gland prominent in boars).
Penis types:
Vascular with os penis in dogs and cats (with penile spines in tomcats).
Fibroelastic in ruminants and pigs with sigmoid flexures.
Birds mostly lack protrusible phallus except some waterfowl.
Female System
Ovaries vary in size and location; in birds, only left ovary functional except ducks.
Uterus shape reflects reproductive strategy ranging from Y-shaped in dogs to coiled long horns in sows.
Cervical mucosal folds: longitudinal in carnivores and horses; circular in ruminants and pigs.
Birds’ reproductive tract highly specialized for egg formation with distinct segments (infundibulum, magnum, isthmus, uterus, vagina).
Lymphatic System
Large lymphocenters present, with nodes draining head, neck, thorax, abdomen, and limbs.
Ruminants possess hemal nodes containing blood rather than lymph.
Birds have a lymph heart aiding systemic lymph movement in embryos.
Bursa of Fabricius in birds vital for B-lymphocyte maturation; involutes at sexual maturity.
Integumentary System
Skin Glands
Specialized glands tailored for communication and protection:
Pheromone-secreting carpal glands in pigs.
Circumoral glands unique to cats.
Anal sac glands in carnivores.
Various scent and sebaceous glands across species.
Hair Types and Follicles
Hair types differ per species, including bristle, guard, wool, long, and tactile hairs.
Follicle structure varies from compound (dogs) to single (horses, cattle).
Mammary Glands
Number and teat openings vary (dogs 10–12, sows 14, cows 4).
Location ranges from thoracoabdominal in cats to inguinal in ruminants.
Hoof and Claw
Hooves of horses have complex structures including wall, sole, frog, and coronet with functional adaptations to locomotion and circulation.
Ruminants and pigs have cloven hooves; no frog.
Dogs/claws vary in shape; cats have retractile, sickle-shaped claws.
Horns
Permanent epidermal structures over cornual processes in ruminants.
Growth marked by visible rings; polled (hornless) animals exist.
Structural pneumatization occurs by 6 months.
Bird Skin and Feathers
Birds lack sweat glands, but have uropygial gland for feather waterproofing.
Feather structures are highly specialized with main types being contour and down feathers.
Specialized skin folds (patagia) assist wing movement.
Feather anatomy includes calamus, rachis, vane, barbs, and barbules.