Page 59 - Adams and Stashak's Lameness in Horses, 7th Edition

Functional Anatomy of the Equine Musculoskeletal System 25

the carpus surrounded by a tendon sheath within a fas­ triceps brachii and the elbow, and then runs between the
cial canal on its way to the proximal phalanx where it ulnar heads of the flexor carpi ulnaris and deep digital
VetBooks.ir digital flexor. From here the nerve runs under the deep
flexor. It supplies these two muscles and the superficial
inserts.
antebrachial fascia, passing between the superficial digi­
Flexor Muscles
tal flexor and the extensor carpi ulnaris and finally
The flexor carpi radialis muscle is related to the medi­ between the latter and flexor carpi ulnaris muscles as
ocaudal surface of the radius (Figure 1.26), extending they near their insertions. A few centimeters proximal to
from the medial epicondyle of the humerus to the base the accessory carpal bone, the ulnar nerve divides into
of the medial splint bone. Caudal to the flexor carpi palmar and dorsal branches.
radialis, the flexor carpi ulnaris muscle consists of an Distal to the elbow the median nerve runs along the
ulnar head from the olecranon and a humeral head from caudal border of the medial collateral ligament of the
the medial epicondyle; it inserts on to the accessory car­ elbow joint, cranial to brachial veins and artery
pal bone. (Figure 1.26). In the proximal part of the forearm, the
Between flexor carpi ulnaris caudally and the lateral median nerve supplies branches to the flexor carpi radi­
digital extensor laterally is the extensor carpi ulnaris alis muscle, the humeral and radial heads of the deep
muscle (formerly ulnaris lateralis) that originates on the digital flexor muscle, and the periosteum of the radius
lateral epicondyle of the humerus caudal to the lateral and ulna. At about the middle of the forearm, the median
collateral ligament of elbow joint. A bursa lies deep to nerve divides into the medial and lateral palmar nerves
the muscle at this location; it often communicates with that remain together in a common sheath before sepa­
the elbow synovial joint. The muscle extends distad to rating close to the carpus. The medial palmar nerve
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insert on the accessory carpal bone and, by means of a descends in the carpal canal; the lateral palmar nerve is
somewhat longer, sheathed tendon, to the proximal end joined by the palmar branch of the ulnar nerve and
of the fourth metacarpal bone. descends within the dense connective tissue of the flexor
The preceding three muscles flex the carpal joint and, retinaculum (Figure 1.18).
through their humeral origins, extend the elbow joint. Just distal to the elbow joint, the common interosse­
Although the extensor carpi ulnaris is morphologically ous artery, a branch of the brachial artery, gives off a
in the extensor group and is supplied by the radial nerve, small caudal interosseous artery and then, continuing as
its attachments make it a flexor of the carpus. the cranial interosseous, passes through the interosseous
The superficial digital flexor muscle has a single space, supplying nutrient arteries to the radius and ulna.
humeral head, which originates from the medial epicon­ Together with the transverse cubital artery (arising
dyle of the humerus and then lies deep to the flexor carpi proximal to the elbow joint), the cranial interosseous
ulnaris and ulnar head of the deep digital flexor (which provides branches to the cranial and medial aspects of
is quite superficial as it originates from the medial sur­ the forearm.
face of the olecranon). The muscle belly of the superfi­ After giving off the common interosseous artery, the
cial digital flexor lies flat against the deep digital flexor brachial artery continues as the median artery between
muscle. Under the proximal part of the flexor retinacu­ the flexor carpi radialis and the radius. In the distal part
lum, the tendon of the humeral head of the superficial of the forearm, the median artery angles caudad and
digital flexor is joined by a flat, wide fibrous band, its gives rise to the proximal radial artery, a small vessel
accessory ligament (really a radial head of the muscle), that courses to the palmar aspect of the carpus. The
which arises on a ridge on the mediocaudal surface of median artery terminates at the distal end of the forearm
the distal half of the radius (Figure 1.32, later in text). by trifurcating into the large medial palmar artery, the
The deep digital flexor muscle has one large humeral much smaller lateral palmar artery, and, medially, the
head and two small heads originating from the ulna (olec­ radial artery (Figure 1.18).
ranon) and radius. The long slender tendon of the ulnar Two median veins accompany the median artery and
head joins the main tendon of the humeral head proximal nerve: a proximal continuation of the lateral palmar
to the carpus just before the combined tendon becomes vein, which ascends caudal to the artery, and a vein
enclosed within the carpal synovial sheath alongside the formed by branches from the caudal antebrachial mus­
SDFT. The tendon of the radial head also joins the main cles, which ascends cranial to the artery.
tendon near this level. The radial head is inconsistently
identifiable; when present, it takes origin from the middle
third of the caudal radius. A synovial pouch from the Elbow Joint
elbow joint capsule protrudes distad beneath the humeral The two principle flexors of the equine elbow (cubi­
head’s origin on the medial epicondyle of the humerus. tal) joint are the biceps brachii and the brachialis, with
lesser contributions made by the extensor carpi radialis
and common digital extensor muscles. The principal
Nerves and Deep Vessels
extensors are the tensor fasciae antebrachii, triceps bra­
The deep branch of the radial nerve descends over the chii, and the anconeus; these are assisted by the flexors
flexor surface of the elbow and supplies motor branches of the carpus and digit.
to the extensor muscles of the forearm and the extensor The terminal part of the biceps brachii muscle crosses
carpi ulnaris. the joint cranially and inserts on the radial tuberosity
The ulnar nerve crosses the medial epicondyle of the and medial collateral ligament. This tendon of insertion
humerus accompanied by the collateral ulnar artery and also gives rise to the lacertus fibrosus, which blends into
vein, descends obliquely across the medial head of the the deep fascia of the extensor carpi radialis (Figure 1.27).