Page 57 - Adams and Stashak's Lameness in Horses, 7th Edition

Functional Anatomy of the Equine Musculoskeletal System 23

synovial spaces of the more distal carpal joints. The mid­ over the insertion of the biceps brachii and then passes
dle carpal synovial sac communicates with the small car­ over the median nerve and brachial artery to join the
VetBooks.ir bones. in its course the median cubital vein may receive a large
brachial vein in the distal part of the brachium. Midway
pometacarpal sac between the third and fourth carpal
branch emerging from between the radius and the flexor
The carpus is flexed by the combined action of the
flexor carpi radialis, flexor carpi ulnaris, and probably carpi radialis muscle.
the oddly named extensor carpi ulnaris. It is extended by The caudal cutaneous antebrachial nerve (from the
the extensor carpi radialis and extensor carpi obliquus ulnar nerve) emerges through the pectoralis transversus to
(a.k.a. abductor digiti I longus) muscles and also by the innervate the skin on the caudal aspect of the forearm.
digital extensors. The thick palmar carpal ligament unit­
ing the palmar aspect of the carpal bones serves to pre­
vent overextension of the antebrachiocarpal and middle Fascia
carpal joints. The deep antebrachial fascia is especially thick. It
invests all of the muscles of the forearm and provides for
attachment of the tensor fasciae antebrachii muscle
Antebrachium
medially, the cleidobrachialis muscle laterally, and the
The antebrachium (forearm) includes the radius and biceps brachii muscle cranially (the latter by means of
ulna and the muscles, vessels, nerves, and skin surround­ the lacertus fibrosus). The deep fascia blends with the
ing the bones. The prominent muscle belly of the exten­ periosteum on the medial radius and with the collateral
sor carpi radialis muscle creates the distinctive bulging ligaments and bony prominences at the elbow.
contour on the cranial antebrachium. A horny cutane­ Intermuscular septa from the deep fascia extend between
ous structure, the chestnut, is present on the medial skin the bellies of the antebrachial muscles.
of the distal one‐third of the forearm. The chestnut is
considered to be a vestige of a carpal pad. 28 Extensor Muscles
There is extensive overlapping among adjacent sen­
sory cutaneous branches of the axillary, radial, muscu­ The extensor carpi radialis is the largest of the exten­
locutaneous, and ulnar nerves in the forearm. The sor muscles of the antebrachium. It arises proximally on
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axillary nerve gives rise to cutaneous branches to the the lateral epicondyle and radial fossa of the humerus
lateral aspect of the brachium and terminates as the cra­ (in common with the tendon of origin of the common
nial cutaneous antebrachial nerve, which courses distad digital extensor) and is attached to the elbow joint cap­
over the extensor carpi radialis muscle (Figure 1.30). sule and the deep fascia in this region. The tendon of the
The lateral cutaneous antebrachial nerve arises from extensor carpi radialis runs nearly the length of the mus­
the superficial branch of the radial nerve as the latter cle, becoming visible on its cranial surface about half­
runs between the extensor carpi radialis and the lateral way on the antebrachium and completely replacing the
head of the triceps brachii (Figure 1.30). In its course the fleshy muscle bellow at the junction of the middle and
lateral cutaneous antebrachial nerve descends to supply distal thirds.
the skin on the craniolateral part of the distal forearm. The extensor carpi obliquus muscle originates broadly
Terminal branches often course as far as the carpus and on the lateral side of the distal radius. Its muscle belly is
proximal metacarpus (Figure 1.20). at first deep to the common digital extensor; then its
The medial cutaneous antebrachial nerve continues tendon crosses that of the extensor carpi radialis super­
distad from the musculocutaneous nerve, coursing over ficially as it angles distad over the carpus toward its
the biceps brachii muscle and then along the deep face insertion on the base of the medial splint bone. On the
of the lacertus fibrosus, and continues into the tendon of medial aspect of the carpus, a bursa underlies the tendon;
the extensor carpi radialis muscle. The nerve is readily it usually communicates with the tendon’s synovial
palpable through the skin as it crosses the cranial edge sheath in older horses (Figure 1.20). 30
and then the medial surface of the lacertus fibrosus, The common digital extensor muscle takes origin
dividing here into two main branches (Figure 1.26). The along with the extensor carpi radialis on the lateral epi­
larger branch accompanies the accessory cephalic vein. condyle and radial fossa of the humerus, with additional
The nerve continues on the dorsomedial aspect of the attachments to the ulna, deep fascia, lateral aspect of the
carpus and metacarpus to the fetlock. The smaller radius, and the lateral collateral ligament of the elbow.
branch runs briefly with the cephalic vein and then Its tendon of insertion, enclosed in its tendon sheath,
courses obliquely across the medial surface of the radius occupies its respective groove on the distal extremity of
where the bone is subcutaneous. This branch of the the radius. A slender tendon from the very small radial
nerve is sensory to the skin as far as the medial carpus. head of the muscle accompanies the main tendon as the
Ascending over the transverse pectoral muscle, the two tendons enter the synovial sheath proximal to the
cephalic vein runs in the groove between the descending carpus. Distal to the carpus, it diverges laterad to unite
pectoral and cleidobrachialis muscles. A small artery with the tendon of the lateral digital extensor.
accompanies the cephalic vein in the groove. Under The lateral digital extensor originates from the radius,
cover of the cutaneous colli muscle, the cephalic vein ulna, and lateral collateral ligament of the elbow joint,
empties into the external jugular vein or occasionally and the intermuscular septum from the deep fascia. It is
into the subclavian vein. The more cranial accessory a relatively weak muscle whose muscle belly lies adjacent
cephalic vein joins the cephalic vein close to the point at to the radius and ulna between the extensor carpi ulnaris
which the latter gives rise to the median cubital vein caudally and the larger common digital extensor muscle
(Figure 1.26). The median cubital courses proximocaudad belly cranially (Figure 1.26). The muscle’s tendon crosses