770 THE NERVOUS SYSTEM. 



and passes downward and outward to the lower border of that muscle. It then 

 winds backward in company with the posterior circumflex artery, through a 

 quadrilateral space bounded above by the Teres minor, below by the Teres major, 

 internally by the long head of the Triceps, and externally by the neck of the 

 humerus, and divides into two branches. 



The upper branch winds round the surgical neck of the humerus, beneath the 

 Deltoid, with the posterior circumflex vessels, as far as the anterior border of that 

 muscle, supplying it, and giving off cutaneous branches, which pierce the muscle 

 and ramify in the integument covering its lower part. 



The lower branch, at its origin, distributes filaments to the Teres minor and 

 back part of the Deltoid muscles. Upon the filament to the former muscle an oval 

 enlargement usually exists. The nerve then pierces the deep fascia, and supplies 

 the integument over the lower two-thirds of the posterior surface of the Deltoid, as 

 well as that covering the long head of the Triceps. 



The circumflex nerve, before its division, gives off an articular filament, which 

 enters the shoulder-joint below the Subscapularis. 



The Musculo-cutaneous Nerve (Fig. 413) (external cutaneous or perforans Cas- 

 serii} 1 supplies some of the muscles of the arm and the integument of the fore- 

 arm. It arises from the outer cord of the brachial plexus, opposite the lower border 

 of the Pectoralis minor, receiving filaments from the fifth, sixth, and seventh 

 cervical nerves. It perforates the Coraco-brachialis muscle, passes obliquely 

 between the Biceps and Brachialis anticus to the outer side of the arm. and. a little 

 above the elbow, winds round the outer border of the tendon of the Biceps, and, 

 perforating the deep fascia, becomes cutaneous. This nerve, in its course through 

 the arm, supplies the Coraco-brachialis, Biceps, and the greater part of the 

 Brachialis anticus muscles. The branch to the Coraco-brachialis is given off 

 from the nerve close to its origin, and in some instances, especially in early life, 

 as a separate filament from the outer cord of the plexus. The branches to the 

 Biceps and Brachialis anticus are given off after the nerve has pierced the Coraco- 

 brachialis. The nerve also sends a small branch to the bone, which enters the 

 nutrient foramen with the accompanying artery, and a filament, from the branch 

 supplying the Brachialis anticus, to the elbow-joint. 



The cutaneous portion of the nerve passes behind the median cephalic vein, and 

 divides, opposite the elbow-joint, into an anterior and a posterior branch. 



The anterior branch descends along the radial border of the forearm to the 

 wrist, and supplies the integument over the outer half of the anterior surface. At 

 the wrist-joint it is placed in front of the radial 'artery, and some filaments, 

 piercing the deep fascia, accompany that vessel to the back of the wrist, supplying 

 the carpus. The nerve then passes downward to the ball of the thumb, where it 

 terminates in cutaneous filaments. It communicates with a branch from the radial 

 nerve and the palmar cutaneous branch of the median. 



The posterior branch passes downward along the back part of the radial side 

 of the forearm to the wrist. It supplies the integument of the lower third of the 

 forearm, communicating with the radial nerve and the external cutaneous branch 



7 O 



of the musculo-spiral. 



The musculo-cutaneous nerve presents frequent irregularities. It may adhere 

 for some distance to the median and then pass outward, beneath the Biceps, 

 instead of through the Coraco-brachialis. Frequently some of the fibres of the 

 median run for some distance in the musculo-cutaneous and then leave it to join 

 their proper trunk. Less frequently the reverse is the case, and the median sends 

 a branch to join the musculo-cutaneous. Instead of piercing the Coraco-brachialis, 

 the nerve may pass under it or through the Biceps. Occasionally it gives a fila- 

 ment to the Pronator teres, and it has been seen to supply the back of the thumb 

 when the radial nerve was absent. 



The Internal Cutaneous Nerve (Fig. 413) is one of the smallest branches of the 

 brachial plexus. It arises from the inner cord in common with the ulnar and 



1 See foot-note, page 726. 



