Structure & Function of the Thoracic Limb

Question 1

Describe the role of the forelimb in locomotion.

Answer 1

Forelimb GRF is ~30% higher than in the hindlimb as bodyweight is not evenly distributed (forelimbs support weight of the head)

GRF is exerted in both vertical and horizontal directions
Horizontal force = breaking force (pushes back against the forward motion)

Forelimbs have higher vertical forces to aid in vertical weight and stability
Forelimbs can have a wide variety in appearance between species due to the other adaptations required besides the need for locomotion (eg: grasping, swimming, climbing)

Thoracic girdle:
Unites limbs to trunk
Greatly reduced in cursorial animals (quadrupeds do not need high stability in their thoracic girdle as their forelimbs are mainly used for locomotion)

Forelimb plays a supportive role, therefore:
- loss/reduction of clavicle
- coracoid and part of scapula reduced
- scapula attached via extrinsic muscles (synsarcosis) and slides across the trunk to increase stride length

Extrinsic muscles - join limb to body
Intrinsic muscles - span joints within limb (cannot move the limb relative to the body)

Question 2

Discuss differences in forelimb anatomical structure between the major veterinary species and this relates to their locomotor behaviour.

Answer 2

Joints of the thoracic limb:
- 3 joints creating the elbow
— humeroradial ; humeroulnar ; radioulnar
- 3 joints creating the carpus
— radiocarpal ; intercarpal ; carpometacarpal
- scapulohumeral (shoulder)
- scapulothoracic (synsarcosis)
- metacarpo-phalangeal (fetlock)
- interphalangeal (PIP/DIP) (pastern/coffin)

Shoulder:
Acromion - bony lump at distal end of scapula used for muscle attachment
- a horse’s scapula is elongated in comparison to a dog as an adaptation to increase the length of the limb therefore the length of their step

Shoulder extensors:
- biceps brachii
- brachicephalius
- supraspinatus
when contracted, they extend the limb forward

Shoulder stabilisers:
- subscapularis
- supraspinatus
- teres minor
- infraspinatus
work isometrically to prevent shoulder movement during weight bearing

Shoulder flexors:
- teres major
- deltoid
- triceps brachii (long head only)
when muscles contract, they flex the shoulder

Elbow flexors:
- biceps brachii
- brachialis

Elbow extensors:
triceps - medial head
triceps - long head
triceps - accessory head
triceps - lateral head

Carpus:
2 rows of carpals - proximal row (bones names after bones above them) / distal row (number after which metacarpal bone they are sat on top of)
accessory carpal bone at the back of carpus

Cursorial skeletal specialisations of the antebrachium and manus:
- minimalisation of distal limb mass
- less requirement for manipulation and control

Extensors of the carpus and digits:
(located on the cranio-lateral aspect of the limb)
antebrachial muscles-
- Extensor carpi radialis
- Abductor pollicis longus
- Common digital extensor
- Lateral digital extensor
- Ulnaris lateralis

Flexors of the carpus & digits:
(located of the caudal-medial aspect of the limb)
Antebrachial muscles-
Flexor carpi radialis
Superficial digital flexor
Deep digital flexor
Flexor carpi ulnaris

Interosseus muscle:
- short fibre muscles on the palmar aspect of the metaphalangeal aspect of the dog
- allow a carnivore to retain manipulative ability
- muscles reduce in species that have less needed to move individual digits

• Nerves that cause action of the muscles in the limb arise from the brachial plexus
• Exit the spinal cord and split off into individual nerves that innervate the muscle
• Blood supply enters at the limb, comes out of the trunk at the brachial plexus