Part 210

8. The =axillary= or =circumflex nerve= (N. axillaris) arises behind the musculo-cutaneous. It passes downward and backward on the inner face of the subscapularis and disappears between that muscle and the subscapular artery.

9. The =long thoracic nerve= (N. thoracalis longus) is wide and thin. It passes backward across the surface of the serratus magnus, to which it is distributed. The branches which enter the muscle are given off both upward and downward in fairly regular fashion.

10. The =thoraco-dorsal nerve= (N. thoracodorsalis) passes upward and backward across the subscapularis muscle to ramify in the teres major and the latissimus dorsi.

11. The =external= (or subcutaneous) =thoracic nerve= arises by a common trunk with the ulnar. It runs backward and downward across the inner face of the tensor fasciæ antibrachii, communicates with the anterior pectoral branches, and gives twigs to the deep pectoral muscle. It then runs backward in company with the external thoracic or “spur” vein, gives branches to the latissimus dorsi and the deep pectoral, and ramifies in the panniculus and the skin of the abdominal wall. It communicates with perforating branches of the intercostal nerves. A branch from it, accompanied by a large perforating intercostal branch, winds around the lower border of the latissimus dorsi and ramifies in the panniculus on the outer surface of the arm.

The term posterior thoracic or pectoral nerves (Nn. pectorales caudales) may be used to include 8, 9, and 10.

SUPRASCAPULAR NERVE

The =suprascapular= (Fig. 441) is a large nerve derived chiefly, if not exclusively, from the sixth and seventh cervical components of the brachial plexus. It passes between the supraspinatus and subscapularis muscles and turns around the distal fourth of the anterior border of the scapula to reach the supraspinous fossa. It gives branches to the supraspinatus and continues backward and upward into the infraspinous fossa, where it supplies the infraspinatus, deltoid, and teres minor muscles.

The direct relation of this nerve to the scapula renders it liable to injury, the result of which may be paralysis and atrophy of the muscles supplied by it.

MUSCULO-CUTANEOUS NERVE

The =musculo-cutaneous nerve= (Fig. 441) arises close behind the suprascapular, and is derived chiefly from the part of the brachial plexus which is supplied by the seventh and eighth cervical nerves. It descends across the outer surface of the brachial artery, below which a great part of the nerve unites with the median to form the loop previously mentioned. It gives off a branch which enters the upper part of the belly of the coraco-brachialis, passes downward and forward between the two parts of that muscle or between the muscle and the bone, and divides into branches which enter the biceps brachii. It contributes one of the nerves to the pectoral muscles. In some cases this nerve sends a branch to join the cutaneous branch of the median.

AXILLARY NERVE

The =axillary nerve= (Figs. 441, 520), also termed the circumflex, derives its fibers from the eighth cervical and first thoracic roots of the brachial plexus. It runs downward and backward across the lower part of the subscapularis and dips in between that muscle and the subscapular artery at the level of the shoulder joint. Continuing outward in the interval between the teres minor and the long and external heads of the triceps, it reaches the deep face of the deltoid and divides into several divergent branches. The =muscular branches= supply the teres major, capsularis, teres minor, infraspinatus, deltoid, and mastoido-humeralis. The =cutaneous branch= (N. cutaneus brachii lateralis) runs downward and a little forward across the external head of the triceps and ramifies on the fascia on the front of the forearm and on the superficial pectoral muscle.

RADIAL NERVE

The =radial nerve= (Figs. 441, 443, 520), also called the musculo-spiral, is sometimes the largest branch of the brachial plexus. Its fibers are derived from the seventh and eighth cervical and first thoracic roots of the plexus. It passes downward and backward over the inner surface of the subscapular artery and the teres major. In this part of its course it is related in front to the ulnar nerve, which separates it from the brachial vein. It detaches a branch to the tensor fasciæ antibrachii, passes outward in the interval between the teres major and the long and internal heads of the triceps, and gains the musculo-spiral groove of the humerus. Accompanied by a branch of the deep brachial artery, it runs obliquely downward and outward in the groove, covered externally by the external head of the triceps and the extensor carpi radialis, and reaches the flexion surface of the elbow joint. In this part of its course it gives off a muscular branch which ramifies in the long and external heads of the triceps and in the anconeus, and a =cutaneous nerve= (N. cutaneus antibrachii dorsalis); branches of the latter emerge below or through the external head of the triceps and ramify on the dorso-lateral surface of the forearm. At the elbow the nerve descends with the anterior radial vessels on the joint capsule between the brachialis and extensor carpi radialis, and supplies branches to the extensor carpi and the common or anterior extensor of the digit and (inconstantly) to the brachialis. Below the elbow joint the nerve detaches a large branch which passes back to the flexor carpi externus, and terminates by small branches which descend on the radius to enter the radial and ulnar heads of the common extensor, the lateral extensor, and the oblique extensor of the carpus. Thus the radial nerve innervates the extensors of the elbow, carpal and digital joints, and supplies also the flexor carpi externus.

FIG. 520.—CUTANEOUS NERVES OF RIGHT FORE LIMB OF HORSE, EXTERNAL FACE.

a, Cutaneous branch of axillary nerve; b, cutaneous branches of radial nerve; c, posterior cutaneous branch of ulnar nerve; d, cutaneous branch of median nerve; e, superficial branch of ulnar nerve; f, external metacarpal nerve; g, anastomotic branch connecting internal and external metacarpal nerves; 1, external head of triceps; 2, extensor carpi radialis; 3, anterior extensor; 4, flexor carpi externus. (After Ellenberger, in Leisering’s Atlas.) ]

ULNAR NERVE

The =ulnar nerve= (Figs. 441, 442, 443, 520) arises with the median from the thoracic components of the brachial plexus. It descends between the brachial artery and vein, accompanied for a short distance by the radial nerve. It then crosses the vein and continues behind the latter along the anterior border of the tensor fasciæ antibrachii and dips under that muscle near the elbow. Here it is joined by the ulnar vessels and passes downward and backward over the internal epicondyle of the humerus. In the forearm it crosses obliquely the deep face of the ulnar head of the flexor carpi medius and descends under the deep fascia with the vessels, at first on the ulnar head of the perforans, and then between the middle and external flexors of the carpus. Near the level of the accessory carpal bone it divides into two terminal branches, superficial and deep. It gives off two principal collateral branches. The =cutaneous branch= (Ramus cutaneus palmaris) is detached just before the nerve passes under the tensor fasciæ antibrachii; it runs downward and backward on that muscle under cover of the posterior superficial pectoral, becomes superficial below the elbow and ramifies on the posterior surface and both sides of the forearm. The =muscular branch= is given off at the elbow joint and divides to supply the flexors of the digit and the middle flexor of the carpus. Of the two terminals, the =superficial branch= (Ramus superficialis) emerges between the tendons of insertion of the external and middle flexors of the carpus and ramifies on the dorso-lateral aspect of the carpus and metacarpus. The =deep branch= (Ramus profundus), after a very short course, unites under cover of the tendon of the flexor carpi medius with the outer branch of the median nerve to form the external metacarpal or volar nerve.

FIG. 521.—CROSS-SECTION OF FOREARM OF HORSE ABOUT THREE INCHES (CA. 8 CM.) BELOW THE ELBOW JOINT.

The deep fascia is designated by red line. N, A, V, Interosseous vessels and nerve. ]

MEDIAN NERVE

The =median nerve= (Figs. 441, 442, 447, 520, 521) derives its fibers chiefly from the first thoracic root of the brachial plexus. It is usually the largest branch and it accompanies the chief arterial trunks to the distal part of the limb. It descends over the inner face of the brachial artery, which it crosses obliquely, and continues down the arm in front of the artery. Near its origin it is joined by a large branch with the musculo-cutaneous nerve, thus forming a loop in which the artery appears to be suspended. Near the elbow it crosses obliquely over the artery (posterior radial) and lies behind it on the internal lateral ligament. Below the joint it again crosses the artery and lies behind the radius and the lower part of the long internal lateral ligament.

FIG. 522.—DISSECTION OF DISTAL PART OF RIGHT FORE LIMB OF HORSE, VIEWED FROM BEHIND.

1, Lateral cartilage; 2, 3, 4, ligaments from first phalanx to third sesamoid, bulb of heel, and lateral cartilage. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

At the elbow the nerve is covered by the posterior superficial pectoral muscle and is crossed by the large oblique connection between the cephalic and brachial veins. The variable and often plexiform venous arrangement renders approach to the nerve here undesirable. It can be reached best by an incision just behind the lower part of the long internal lateral ligament; here the nerve lies under the deep fascia in the furrow between the radius and the anterior border of the flexor carpi internus, overlapped by the latter. In some cases the nerve retains its position in front of the artery at the elbow, and in others it crosses the deep face of the artery.

Passing beneath the internal flexor of the carpus, the nerve continues downward in the forearm with the posterior radial vessels, and divides at a variable distance above the carpus into two branches; these are the internal volar or metacarpal nerve and the branch which unites with the ulnar to form the corresponding external nerve.

At the proximal part of the forearm the nerve runs almost straight downward along the posterior border of the long internal lateral ligament, while the artery here inclines somewhat backward. Thus the nerve is superficial to the artery for a short distance, then lies in front of the latter to about the middle of the region, where it inclines a little backward and arrives at the interval between the internal and middle flexors of the carpus. The division may occur about the middle of the region, but commonly takes place in the distal third or fourth.

The collateral branches are as follows: