» The Blood Vessels and Lymphatic Structures of the Forelimb
The axillary artery, the main supply of the limb, enters the axillary space after crossing the cranial border of the first rib, where it may be punctured (p. 528). It descends on the medial aspect of the arm in company with the median and ulnar nerves and shortly becomes known as the brachial artery.
The trunk releases several branches to the muscles of the shoulder and arm, the most prominent being the subscapular artery, which follows the caudal border of the scapula, and the deep brachial artery, which disappears between the heads of the triceps (Fig. 23.8). Just proximal to the elbow joint, lesser cranial and caudal branches (transverse cubital and collateral ulnar arteries, respectively) are detached for the muscles in the forearm (Fig. 23.39/11 and 12). The brachial artery crosses the elbow cranial to the medial collateral ligament, where it can be palpated and the pulse evaluated, through the pectoralis transversus (Fig. 23.40/5). Together with the median nerve it dips under the flexor carpi radialis caudal to the radius and soon gives off the common interosseous artery, which passes through the interosseous space to reach the craniolateral muscles of the forearm.The main trunk, now redesignated the median artery (Fig. 23.41/12), gradually works its way to the caudal surface of the forearm before dividing into three branches above the carpus. The lesser branch (palmar branches of the median and radial artery) contributes the small palmar metacarpal arteries that accompany the interosseus muscle, while the main trunk passes through the carpal canal with the digital flexor tendons (Fig. 23.15B/19). It continues with these in the cannon where it becomes the medial palmar artery, the main artery to the digit and hoof. This inclines axially before splitting into the medial and lateral digital arteries above the fetlock. The digital arteries pass over the abaxial surfaces of the sesamoid bones (where they are palpable) and continue into the digit on each side of the flexor tendons.
The lateral artery is reinforced by the small metacarpal arteries that join above the sesamoid bone (Fig. 23.39/18'). The branches of the digital arteries distal to the fetlock are symmetrical. Dorsal and palmar branches are given off opposite PI, and these supply adjacent structures while forming a circle about the bone. A branch to the digital cushion is detached level with the pastern joint before the digital artery disappears by passing deep to the hoof cartilage. Dorsal and palmar branches detached opposite the middle of PII comport themselves similarly to the branches about PI but also take part in the supply of the dermis of the hoof. The dorsal and palmar terminal branches (to PIII) have been described (pp. 590 and 600); the palmar branches anastomose to form a terminal arch within the bone.
FIG. 23.39 The major arteries (aa.) of the right forelimb: (A) medial view; (B) palmar view. 1, Axillary artery (a.); 2, suprascapular a.; 3, subscapular a.; 4, thoracodorsal a.; 5 and 6, caudal and cranial circumflex humeral aa., respectively; 7, brachial a.; 8, deep brachial a.; 9, collateral radial a.; 10, collateral ulnar a.; 11, transverse cubital a.; 12, common interosseous a.; 13, median a.; 14, radial a.; 15 and 15', medial and lateral palmar aa., respectively; 16 and 16', medial and lateral palmar metacarpal aa., respectively; 17 and 17', medial and lateral digital aa., respectively.
FIG. 23.40 Transverse section of the left elbow. 1, Extensor carpi radialis; 2, brachialis; 3, medial cutaneous antebrachial nerve and cephalic vein lying on lacertus fibrosus; 4, biceps; 5, brachial vessels and median nerve; 6, medial collateral ligament; 7, humerus; 8, flexors arising from medial epicondyle of humerus; 9, ulnar nerve and collateral ulnar vessels; 10, tensor fasciae antebrachii; 11, olecranon; 12, ulnaris lateralis; 13, lateral collateral ligament; 14, common digital extensor.
Most veins of the forelimb are satellite veins, although they are often duplicated or further replicated where they accompany the larger arteries (Fig.
23.42/1). Some superficial veins seek independent courses, and those coming from the hoof have already been mentioned. The superficial veins include the cephalic and accessory cephalic veins, which are prominent and palpable in the forearm (Fig. 23.42/10 and 10'). The cephalic vein is joined to the brachial vein via the median cubital at the elbow and continues to ascend in the groove between the brachiocephalicus and pectoralis descendens, where it is at risk in "staking" injuries. It joins the external jugular vein at the base of the neck.
FIG. 23.41 Transverse section of the right forearm at the level shown in Fig. 23.42. Cr., Cranial; Med., medial; 1, Radius; 2, extensor carpi radialis; 3, common digital extensor; 4, lateral digital extensor; 5, ulnaris lateralis; 6, deep digital flexor; 7, superficial digital flexor; 8, flexor carpi ulnaris; 9, flexor carpi radialis; 10, accessory cephalic vein and medial cutaneous antebrachial nerve (from musculocutaneous);
11, cephalic vein; 12, median artery, veins, and nerve; 13, muscular branches of median vessels; 14, cranial interosseous vessels; 15, ulnar nerve and collateral ulnar vessels.
Two clusters of lymph nodes drain the free part of the limb. The cubital nodes lie on the medial aspect of the humerus just proximal to the elbow joint. They drain more distal parts of the limb and channel their outflow to the axillary nodes. These lie medial to the shoulder joint in the angle between the axillary and subscapular arteries and drain the arm and shoulder, together with a part of the thoracic wall caudal to the limb. Their efferent vessels go to the caudal deep cervical nodes, and thence the lymph flows directly or indirectly to the veins at the thoracic inlet. The superficial cervical nodes are arranged in a long chain that crosses the deep surface of the omotransversarius and brachiocephalicus (see Fig. 18.41/8).
The group consists of many small nodes, and because these are embedded in fat and do not form a firm compact mass, the group is not always easily located. Palpation should be directed to drawing the nodes forward, away from the subclavius against which they lie. The superficial cervical nodes mainly drain skin over the upper part of the limb but also receive some lymph from deeper structures.
FIG. 23.42 Dissection of the medial surface of the right forearm. (The broken transverse line indicates level of section in Fig. 23.41.) 1, Multiple brachial veins; 2, biceps; 3, ulnar nerve and collateral ulnar vessels; 3', caudal cutaneous antebrachial nerve; 4, triceps; 5, flexor carpi radialis, resected; 6, medial cutaneous antebrachial nerve; 7, median nerve and vessels; 8, extensor carpi radialis; 9, flexor carpi ulnaris; 10 and 10', cephalic and accessory cephalic veins, respectively; 11, radius; 12, extensor carpi obliquus; 13, superficial digital flexor; 14, radial artery and vein; 15, accessory carpal bone; 16, medial palmar nerve and vessels.