BMS 201- Comparative Anatomy

Acromion (on scapula)

• not present in horses

• cats have a projection on theirs (suprahamate process)

Clavicle

• not present in most quadripeds

• cats can have a vestigial one

• the humans clavicle pushes the scapula laterally (so bipeds have a more horizontal scapula and quadrupeds have a more vertical one, which allows their thoracic limbs to do 60% of their weight-bearing)

Humeruses and femurs

Larger animals usually have shorter and stouter humeruses and femurs than smaller animals

Supratrochlear foramen (humerus)

• missing in cats and large species

• cats have a supracondylar foramen instead

Radius & ulna

• separate bones, but fused in cattle

• even more fused in horses

• this means that cattle and horses can’t supinate or pronate

Intermedial and radial carpal bones

fused in dogs (“intermedioradial carpal bone”)

1st carpal bone

• inconsistent in horses and cows

2nd and 3rd carpal bones

fused in cows

Accessory carpal bone

• palpable in all species

• pisiform bone in humans

Proximal and distal rows of carpal bones

more organized in horses, cows, and dogs than in humans

Horses’ metacarpals/tarsals

• metacarpal/tarsal 3 is weight-bearing (“cannon bone”)

• metacarpals/tarsals 2 and 4 are “splint bones”

Cows’ metacarpals/tarsals

• metacarpals/tarsals 3 and 4 are fused and bear weight

• metacarpal/tarsal 5 is immature but can be present

*as the number of metacarpals decreases, their stoutness increases

Dogs’ digit 1

• “pollex”

• thumb

• only has a proximal and distal phalanx

Dogs’ sesamoid bones

• 1 proximal sesamoid on digit 1

• 2 proximal sesamoids each on digits 2-5

• 1 dorsal sesamoid on digits 2-5

Sesamoids bones in horses and cows

• 2 proximal sesamoids at metacarpophalangeal joint

• 1 distal sesamoid on palmar surface of distal interphalangeal joint (navicular bone)

• NO dorsal sesamoids

Horses: Long pastern (P1)

proximal phalanx of digit 3

Horses: Short pastern (P2)

middle phalanx of digit 3Ho

Horses: Coffin bone/pedal bone (P3)

distal phalanx of digit 3

Navicular bone

distal sesamoid on palmar surface of distal interphalangeal joint

Fetlock joint

metacarpophalangeal joint + proximal sesamoids

Pastern joint

Proximal interphalangeal joint

Coffin joint

distal interphalangeal joint + navicular bone

Sacroiliac joint

• more cranial in dogs than in horses and cows

• so the hip joint is more directly underneath the sacroiliac joint in larger animals (allows for more weight-bearing)

Tuber coxae

• flared more laterally in horses and cows than in dogs (so more palpable)

^not really palpable in dogs

Iliac crest

• convex and palpable in dogs

• concave and not palpable in horses and cows

Third trochanter

more pronounced in horses

Tibia & fibula in dogs

separate

Tibia & fibula in horses

• proximal fibula is separate from tibia and ½ the length

• distal tibia (lateral malleolus) is fused with tibia

Tibia & fibula in cows

• proximal fibula is tear-shaped and fused to tibia

• distal lateral malleolus of fibula is separate from tibia

Horses’ tarsal bones

• 1st and 2nd are fused

• central and 3rd are larger than in any other species

Cows’ tarsal bones

• central and 4th tarsal bones are fused

• 2nd and 3rd are fused

Humans tarsal bones

most similar to dogs

Metatarsals in humans

• metatarsal 1 is the largest

Dogs’ pelvic limbs

dogs don’t have a 1st digit on their pelvic limbs

Digitigrade posture

• carnivores

• only digits touch the ground

Unguligrade posture

• ruminants

• only top of digits touch the ground (protected by hooves)

• related to speed

Plantigrade posture

• humans, bears

• whole plantar surface touches ground

• related to balance and long-distance traveling

Metatarsal bones

longer than metacarpal bones

Passive Stay Apparatus of thoracic limbs

• arrangement of anatomical parts that allows horses to stay standing longer than other animals with minimal muscular effort

• otherwise, joints would flex or overextend

• serratus ventralis is predominant weight-bearing muscle

• elbow is stabilized by proximal tendons of antebrachial extensors and flexors, and by collateral ligaments

^elbow joint requires less stabilization force than shoulder joint

• carpal joint is stabilized by the lacertus fibrosus transmitting the pull of the biceps to the extensor carpi radialis

• suspensory apparatus prevents overextension of fetlock (made up of interosseus muscle and sesamoidean ligaments)

• accessory (check) ligament stabilizes fetlock as well

Passive Stay Apparatus of pelvic limbs

• arrangement of anatomical parts that allows horses to stay standing longer than other animals with minimal muscular effort

• otherwise, joints would flex or overextend

• overextension of fetlock is prevented by suspensory apparatus and by superficial digital flexor tendon

• pastern joint is stabilized

• stifle and hock joints stabilized by locking mechanism of stifle and by the reciprocal mechanism performed by peroneus tertius and superficial digital flexor

Stifle lock

• occurs when a horse rests the pelvic limb and extends stifle joint

• patella rotates. moving the medial patellar ligament parapetellar cartilage caudally

• “fixes” the stifle and prevents flexion

• horse shifts weight often to switch which stifle is locked

Reciprocal mechanism

• performed by the peroneus tertius and superficial digital flexor

• ensures that the movements of the stifle and hock joints are always in concert (when one flexes, the other does too, etc)

Rupture of peroneus tertius tendon

• occurs due to hyper/over-extension of hock joints

• disrupts reciprocal apparatus, flexion of stifle and hock

Vertebra in cats and dogs

C7, T13, L7, S3

13 rib pairs

*same number of pairs of ribs as thoracic vertebrae

Vertebra in horses

C7, T18, L6, S5

18 rib pairs

*same number of pairs of ribs as thoracic vertebrae

Vertebra in cows

C7, T13, L6, S5

13 rib pairs

*same number of pairs of ribs as thoracic vertebrae

Vertebra in humans

C7, T12, L5, S5

12 rib pairs

Caudal vertebra

• number varies by species but is around 20

• humans have 3-5 (“coccyx,” “tailbone”)