I308 HUMAN ANATOMY. 



humeral head of the triceps and externally by the humerus. Having traversed 

 this space it winds around the surgical neck of the humerus and reaches the outer 

 aspect of the shoulder. 



Branches. — These are : (a) the artiadar, {b) the cutaneous and (r) the 

 muscidar, 



a. The articular branches are usually two in number. The upper arises near the origin of 

 the circumflex and the lower during the passage of the nerve through the quadrilateral space. 

 They supply the anterior inferior portion of the capsular ligament of the shoulder. A third 

 articular branch is described as passing up the bicipital groove, supplying a twig to the upper 

 end of the humerus and one to the neighboring portion of the capsular ligament of the shoulder. 



b. The cutaneous branch (n. cutaneus brachii lateralis) arises as a common trunk with the 

 nerve to the teres minor. It becomes superficial between the long head of the triceps and the 

 posterior border of the lower third of the deltoid and is distributed to the integument over the 

 posterior half of the deltoid and the posterior surface of the upper half of the arm. 



One or two cutaneous filaments are derived from the muscular branches to the deltoid. 

 They pierce the deltoid and are distributed to the skin over the lower portion of that muscle. 



c. The muscular branches (rr. musculares) innervate {aa) the teres minor and (bb) the 

 deltoid. 



aa. The nerve to the teres minor arises from the circumflex at the posterior margin of the 

 quadrilateral space and enters the middle of the posterior inferior border of the muscle which it 

 supplies. 



bb. The deltoid branches comprise the largest portion of the nerve and consist of its termi- 

 nal fibres. The terminal portion of the circumflex forms a bow, with its convexity in contact 

 with the deep surface of the deltoid, extending around the upper part of the humerus almost as 

 far forward as the anterior margin of the deltoid muscle. It gradually diminishes in size as the 

 result of the departure of a series of twigs which enter and supply the fasciculi of the deltoid. 



' Variations. — The circumflex may receive very few or no fibres from the sixth cervical nerve. 

 It may pierce the subscapularis and may supply that muscle. It may give origin to the nerve to 

 the teres major and has been observed to furnish filaments to the long head of the triceps and 

 to the infraspinatus. 



Practical Considerations. — The circumflex nerve is frequently paralyzed 

 from injuries to the shoulder, as in birth palsies when pressure is made in the axilla. 

 It undergoes special strain in dislocations of the shoulder, the nerve being stretched 

 over the head of the humerus and often lacerated. Other branches of the brachial 

 plexus may be injured in this dislocation. Since the circumflex passes around 

 the humerus at about the level of the surgical neck it is sometimes damaged 

 in fractures in that situation. The most prominent symptom in paralysis of this 

 nerve is loss of the rotundity of the shoulder from atrophy of the deltoid muscle. 

 As the circumflex winds around the posterior surface of the humerus and reaches 

 the anterior part of the deltoid muscle from behind, incisions for reaching the 

 shoulder joint, as in excisions, should be made anteriorly, since only the terminal 

 branches of the circumflex will then be divided ; paralysis of the deltoid is thus 

 prevented. 



i6. The Musculo-Spiral Nerve. 



The musculo-spiral nerve (n. radialis) (Fig. iioo), the larger terminal branch of 

 the posterior cord, is in fact the continuation of the latter. Its component fibres are 

 derivatives of the sixth, seventh and eighth, and sometimes of the fifth, cervical 

 nerves and it is distributed to the muscles and integument of the extensor surface of 

 the arm, forearm and hand. After separating from the circumflex, it passes down- 

 ward behind the axillary artery and over the surface of the latissimus dorsi and teres 

 major muscles. Accompanied by the superior profunda artery, it turns backward on 

 the inner aspect of the arm and, entering the musculo-spiral groove and traversing 

 the interval between the internal and long and the external head of the triceps, 

 reaches the lateral aspect of the arm. It then takes a forward course through the 

 external intermuscular septum and becomes an occupant of the cleft between the 

 brachioradialis and the brachialis anticus. Continuing in this space as far as the level 

 of the external condyle of the humerus the nerve divides into its terminal branches, 

 Xh^ posterior interosseous and the radial (Fig. 1095)- 



