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Forelimb - Anatomy & Physiology (view source)
Revision as of 17:01, 10 January 2011
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==Common structures of the Proximal Forelimb and Shoulder==
==Common structures of the Proximal Forelimb and Shoulder==
===Scapula===
===Scapula===
The Scapula forms the basis of the shoulder region, providing points of attachment of extrinsic and intrinsic muscles. It is held in place by a synsarcosis of muscles and does not form a conventional articulation with the trunk.
The Scapula forms the basis of the shoulder region, providing points of attachment of extrinsic and intrinsic muscles. It is held in place by a synsarcosis of muscles and does not form a conventional articulation with the trunk. In ungulates, the dorsal border is extended by a scapular cartilage, which enlarges the area for muscle attachment. This ossifies with age. The bone is roughly triangular, with a prominent '''spine''' that can be palpated through the skin.
:The spine:
::defines '''Infraspinous''' and '''Supraspinous''' '''Fossae''', inhabited by muscles accordingly
:serves as a point of attachment for the '''trapezius''' muscle
:culminates in the '''acromion''' in all but the horse and pig
Medial muscle attachment consist mostly of the '''subscapularis''', with the '''serratus ventralis''' attaching dorsally. The Scapula articulates with the humerus at the '''glenoid cavity'''.
===Clavicle===
===Clavicle===
The Clavicle is all but absent in most domestic species, with the notable exception of the avian skeleton. In the dog and cat, a remnant of bone may remain embedded in the fibrous intersection in the brachiocephalicus, which may prove misleading in radiographic images.
The Clavicle is all but absent in most domestic species, with the notable exception of the avian skeleton. In the dog and cat, a remnant of bone may remain embedded in the fibrous intersection in the brachiocephalicus muscle, which may prove misleading in radiographic images.
===Humerus===
===Humerus===
The Humerus is the long bone of the forearm, articulating with the scapula to form the shoulder and the radius and ulna to form the elbow. In situ, it lies obliquely along the ventral thorax and is more horizontal in larger species.
The Humerus is the long bone of the forearm, articulating with the scapula to form the shoulder and the radius and ulna to form the elbow. In situ, it lies obliquely along the ventral thorax and is more horizontal in larger species.
The head of the humerus comprises '''greater''' and '''lesser tubercles''', separated by an '''intertubercular groove''' through which runs the tendon of the '''biceps brachii'''. The shaft of the humerus takes on a characteristically twisted shape via a groove carrying the '''brachialis''' and '''radial nerve'''. Laterally, the '''deltoid tuberosity''' is palpable through the skin and connects to the head of the humerus via a ridge and merges distally with the '''crest of the humerus'''. Distally, the humerus culminates in a '''condyle''' which articulates to form the elbow.
:In large animals, it meets the radius via a '''trochlea'''
:In dogs and cats, it articulates with the radius medially via a trochlea and the ulna laterally via a '''capitulum'''
:Caudally, all species show an '''olecranon fossa''' which articulates with the '''olecranon''' of the ulna
Medial and lateral epicondyles provide attachment for flexors and extensors of the carpus and digits.
===Shoulder Joint===
===Shoulder Joint===
The shoulder joint links the humerus and the scapula at the '''glenoid cavity''', which is much smaller than the head of the humerus. While structually it is a [[Joints - Anatomy & Physiology#Types of Joints|'''ball and socket''']] joint, it functions as a [[Joints - Anatomy & Physiology#Types of Joints|'''hinge joint''']] due to extensive muscling around the articulation. The joint capsule is enlarged and extends under the tendon of the biceps, acting as a synovial sheath to protect the tendon.
The shoulder joint links the humerus and the scapula at the '''glenoid cavity''', which is much smaller than the head of the humerus. While structurally it is a [[Joints - Anatomy & Physiology#Types of Joints|'''ball and socket''']] joint, it functions as a [[Joints - Anatomy & Physiology#Types of Joints|'''hinge joint''']] due to extensive muscling around the articulation. The joint capsule is enlarged and extends under the tendon of the biceps, acting as a synovial sheath to protect the tendon.
It is important to distinguish the shoulder/scapulohumeral joint from the entirely muscular connection (synsarcosis) between the forelimb and the trunk. This latter connection is sometimes called the '''girdle muscles''', although this is a problematic term, because many of its constituent muscles do not attach to a limb girdle muscle. At Cambridge University, it has for some time been given the name '''omothoracic junction''', but this term has not entered common usage.
It is important to distinguish the shoulder/scapulohumeral joint from the entirely muscular connection (synsarcosis) between the forelimb and the trunk. This latter connection is sometimes called the '''girdle muscles''', although this is a problematic term, because many of its constituent muscles do not attach to a limb girdle muscle. At Cambridge University, it has for some time been given the name '''omothoracic junction''', but this term has not entered common usage.
===Elbow Joint===
===Elbow Joint===
Movement of the elbow joint is restricted to the [[Planes and Axes - Anatomy & Physiology|sagittal plane]]. It is bounded medially and laterally by collateral ligaments betweek the humerus and radius, caudally by the olecranon ligament between the humerus and olecranon, and further enforced by the annular radial ligament. The elbow is a compound joint including:
Movement of the elbow joint is restricted to the [[Planes and Axes - Anatomy & Physiology|sagittal plane]]. It is bounded medially and laterally by collateral ligaments between the humerus and radius, caudally by the olecranon ligament between the humerus and olecranon, and further enforced by the annular radial ligament. The elbow is a compound joint including:
*The [[Joints - Anatomy & Physiology#Types of Joints|'''hinge joint''']] between the humerus and the radius and ulna
*The [[Joints - Anatomy & Physiology#Types of Joints|'''hinge joint''']] between the humerus and the radius and ulna.
*The [[Joints - Anatomy & Physiology#Types of Joints|'''pivot joint''']] between the radius and ulna
*The [[Joints - Anatomy & Physiology#Types of Joints|'''pivot joint''']] between the radius and ulna.
*The point of the elbow, or the '''olecranon''' is formed by the anconeal process of the ulna inserting into the olecranon fossa of the humerus
*The point of the elbow, or the '''olecranon''' is formed by the anconeal process of the ulna inserting into the olecranon fossa of the humerus.
While in the human the radius and ulna are separated by an interosseus space and articulate only at their extremities, allowing for significant capability of supination and pronation, these movements are much more limited in domestic animals due to the gradual fusing of the two bones. The extreme case is exhibited by the horse.
While in the human the radius and ulna are separated by an interosseus space and articulate only at their extremities, allowing for significant capability of supination and pronation, these movements are much more limited in domestic animals due to the gradual fusing of the two bones. The extreme case is exhibited by the horse.
The radius forms the shaft-like rod of the distal limb, which is bowed to varying degrees amongst species. It articulates proximally with the distal humerus, caudally with the ulna, and distally with the carpus. Medially on the distal articular process, a '''styloid process''' projects, which is mirrored laterally by the ulna.
===Ulna===
===Ulna===
The Ulna's greatest contribution to functional anatomy is in the formation of the '''Olecranon''', or the point of the elbow, which gives rise to the attachment of the '''triceps'''. The olecranon articulates with the humerus via its anconeal process. Distally (where unfused), the '''lateral styloid process''' articulates with the '''ulnar carpal bone'''.
The Ulna's greatest contribution to functional anatomy is in the formation of the '''olecranon''', or the point of the elbow, which gives rise to the attachment of the '''triceps''' muscle. The olecranon articulates with the humerus via its anconeal process. Distally (where unfused), the '''lateral styloid process''' articulates with the '''ulnar carpal bone'''.
===Carpal bones===
===Carpal bones===
Carpal bones comprise two rows:
Carpal bones comprise two rows:
*Proximally, (mediolaterally), radial, intermediate, ulnar and accessory bones
*Proximally, (mediolaterally), radial, intermediate, ulnar and accessory bones. The accessory bone serves as a landmark for palpation.
*Distally, bones are numbered 1-5, though 5 is always fused with 4.
*Distally, bones are numbered 1-5, though 5 is always fused with 4
A small [[Bones - Anatomy & Physiology#Types of Bone|'''sesamoid''']] bone embedded in the medial tissues of the joint can sometimes be mistaken as a chip fracture.
===Metacarpal bones===
===Metacarpal bones===
The number of metacarpals varies widely among species, as the demand for their function changes: '''plantigrade''', or flat-footed, animals requiring the full complement of five metacarpal bones; the number is reduced in the upright stature of '''digitigrade''' animals such as the dog and cat, and shows the extreme in '''unguligrades''' like the horse, which depends entirely on the third metacarpal bone for its stature.
The number of metacarpals varies widely among species, as the demand for their function changes: '''plantigrade''', or flat-footed, animals requiring the full complement of five metacarpal bones; the number is reduced in the upright stature of '''digitigrade''' animals such as the dog and cat, and shows the extreme in '''unguligrades''' like the horse, which depends entirely on the third metacarpal bone for its stature.
Colloquially, the third metacarpal of the horse is known as the '''canon''' bone, and the vestigial 2 and 4 as '''splint bones'''
==Joints of the Distal Forelimb==
==Joints of the Distal Forelimb==
*The '''carpometacarpal joint''' between the distal carpal bones and the proximal metacarpals
*The '''carpometacarpal joint''' between the distal carpal bones and the proximal metacarpals
The joint is a synovial joint, comprised of a common outer fibrous capsule and three inner synovial pouches, one for each joint. Numerous ligaments add to the stability of the joint and ensure movement is largely limited to the [[Planes and Axes - Anatomy & Physiology|sagittal plane]], although no collateral ligaments exist in the dog between the radius and the proximal metacarpals. This allows a very small amount of rotation.
The joint is a synovial joint, compring a common outer fibrous capsule and three inner synovial pouches, one for each joint. Numerous ligaments add to the stability of the joint and ensure movement is largely limited to the [[Planes and Axes - Anatomy & Physiology|sagittal plane]], although no collateral ligaments exist in the dog between the radius and the proximal metacarpals. This allows a very small amount of rotation.
===Metacarpal Joint===
===Metacarpal Joint===
The metacarpal joint is defined by the presence of palmar [[Bones - Anatomy & Physiology#Types of Bone|'''sesamoids''']], which allow the flexor tendons to pass over the sharp change in angle presented by the joint. These are paired on each digit, with the exception of the first digit where it exists.
The metacarpal joint is defined by the presence of palmar [[Bones - Anatomy & Physiology#Types of Bone|'''sesamoids''']], which allow the flexor tendons to pass over the sharp change in angle presented by the joint. They are paired on each digit, with the exception of the first digit where it exists.
===Extrinsic Musculature===
===Extrinsic Musculature===
These muscle are responsible for joining the forelimb to the trunk, forming a synsarcosis rather than a conventional joint. Collectively, they act to transfer the weight of the body to the forelimbs as well as stabilize the scapula.
These muscle are responsible for joining the forelimb to the trunk, forming a synsarcosis rather than a conventional joint. Collectively, they act to transfer the weight of the body to the forelimbs as well as stabilize the scapula.
'''''Trapezius''''':
:Innervated by: Accessory n.
:Origin: mid-dorsal raphe and supraspinous ligament
:Insertion: spine of the scapula
:Body: two parts, cervical and thoracic separated by aponeurosis
:Action: raises scapula against the trunk and swings cranially to advance the limb
'''''Brachiocephalic m.''''':
:Innervated by: Accessory n.
:Two parts separated by the clavicle where it exists
:Origin: clavicle or vestigial fibrous intersection
:Insertion: several places on the head and neck
:Actions:
::advances the limb and extends the shoulder joint when limb is in motion
::draws head and neck ventrally when limb is fixed
'''''Trapezius''''': Accessory n.
'''''Omotransversarius''''':
:Innervated by: Accessory n.
:Origin: transverse processes of the atlas
:Insertion: acromion and adjacent scapula
:Action: advancing the limb
'''''Brachiocephalic m.''''': Accessory n.
'''''Latissimus dorsi''''':
:Innervated by: local branch of brachial plexus
:The '''broadest muscle of the back'''
:Origin: thoracolumbar fascia
:Insertion: teres tuberosity of the humerus
:Actions:
::antagonist to the brachiocephalic m.
::cranial fibers strap scapula to the chest
::retracts free limb and flexes shoulder joint
::draws trunk forward over the fixed limb
'''''Omotransversarius''''': Accessory n.
'''''Pectoral mm.''''':
:Innervated by: brachial plexus
'''''Latissimus dorsi''''': local branch of brachial plexus
:Two superficial parts, cranial and caudal
:Origin: cranial sternum
:Insertion:
::cranial ('''descending'''): crest of the humerus distal to the deltoid tuberosity
::caudal ('''transverse'''): covers elbow joint to insert on the medial fascia of the forearm
:Action: adduct the forelimb, assist in protraction and retraction
:One deep part ('''pectoralis profundus'''), with cranial and caudal parts
:Origin: ventral sternum and adjacent cartilage
:Insertions:
::cranial (subclavius): supraspinatus m.
::caudal (pectoralis ascendens): lesser tubercle of the humerus
:Actions:
::slinging trunk between forelimbs
::may also retract free limbs
::draw trunk forward when limb is fixed
'''''Serratis ventralis''''': branch of brachial plexus
'''''Serratis ventralis''''':
:Innervated by: branch of brachial plexus
:Origin: C4 to 10th rib
:Insertion: medial scapula and scapular cartilage
:Action:
::supporting the weight of the trunk reinforced by strong fascia
::cervical portion can retract the limb
::caudal portion can advance the limb
'''''Rhomboids''''': brachial plexus, although in some species, dorsal spinal nerves can innervate
'''''Rhomboids''''':
:Innervated by: brachial plexus, although in some species, dorsal spinal nerves can innervate
:Origin: median connective tissue from poll to withers, lying deep to the trapezius
:Insertion: dorsal border and adjacent scapula
:Action: retracting the limb, may also raise limb
===Intrinsic Musculature===
===Intrinsic Musculature===