592 NERVES OF THE UPPER LIMB. 



Some similarity will be observed between the course and distribution 

 of the median and ulnar nerves. Neither gives any oflFset in the arm. 

 Together they supply all the muscles in front of the forearm and in the 

 hand, and together they supply the skin of the palmar surface of the 

 hand, and impart tactile sensibility to all the fingers. 



Varieties. — One or both heads of the median may be double, and the outer 

 head may pass behind instead of in front of the brachial artery. (Turner.) 

 The ■R'hole nerve has frequently been seen passing behind the brachial artery. 

 (Turner, Gruber.) Gruber has also noticed one case in which the nerve entered 

 the forearm after passing completely over the pronator teres muscle. 



Mtxsculo-spiral nerve. — The musculo-spiral nerve, the largest 

 ofTset from the brachial plexus, occupies chiefly the back part of the 

 limb, and supiDlies nerves to the extensor muscles, as well as to the 

 skin. 



Arising behind the axillary vessels from the posterior cord of the 

 brachial plexus, of which it is the principal continuation and the only 

 one prolonged into the arm, it soon turns backwards into the musculo 

 spiral groove, and, accompanied by the superior profunda artery, pro- 

 ceeds along that groove, between the humerus and the triceps muscle, 

 to the outer side of the limb. It then pierces the external inter- 

 muscular septum, and descends in the interval between the supinator 

 longus and the brachialis anticus muscle to the level of the outer 

 condyle of the humerus, where it ends by dividing into the radial and 

 posterior interosseous nerves. Of these, the radial is altogether a cuta- 

 neous nerve, and the posterior interosseous is the muscular nerve of the 

 back of the forearm. 



The branches of the musculo-spiral nerve may be classified according 

 as they arise on the inner side of the humerus, behind that bone, or ou 

 the outer side. 



A. Internal branches : — 



a. Muscular hranches for the inner and middle heads of the triceps. That 

 for the inner portion of the muscle is long and slender ; it lies by the side of 

 the ulnar nerve, and reaches as far as the lower third of the upper aiTQ. One 

 branch, previously noticed by authors, but more particularly described by Krause, 

 is named by him the ulnar collateral branch. It arises oj^posite the outer border 

 of the latissimus dorsi tendon, and descends within the sheath of the ulnar' 

 nerve, through the internal intermuscular septum, and is distributed to the short 

 inferior fibres of the triceps (Reichert and Du Bois Reymond's Archiv, 186i). 



b. The internal cutaneous branch of the musculo-spiral nerve, commonly united 

 in origin with the preceding, winds backwards beneath the intercosto-humeral 

 nerve, and after supplying filaments to the skin, ends about two inches from the 

 olecranon ; in some instances extending as far as the olecranon. This nerve is 

 accompanied by a small cutaneous arteiy. 



B. Posterior branches : — 



These consist of a fasciculus of imisc^ilar hranches which supply the outer head 

 of the triceps muscle and the anconeus. The branch of the anconeus is slender, 

 and remarkable for its length ; it descends in the substance of the triceps to 

 reach its destination. 



C. External branches : — 



a. The muscular branches supply the supinator longus, extensor carpi radialis 

 longior, (the extensor carpi radialis brevior receiving its nerve from the posterior 

 interosseous,) and frequently give a small branch to the brachialis anticus. 



