516 NERVES OF THE ARM. 



The Radial or Musculo- Spiral 



Is the largest of the nerves of the arm. It passes from the 

 axillary plexus downward, backward, and outward, under the 

 triceps muscle to the external side of the os humeri. In this 

 course, it gives off several muscular branches to the different 

 portions of the triceps. It also frequently gives off a large 

 branch called spiral cutaneous, which passes downwards on 

 the outside of the olecranon, to the back of the fore-arm, and 

 continues to the back of the hand, furnishing many branches 

 which terminate in the integuments. It then proceeds down- 

 wards between the supinator radii longus and the brachialis 

 internus. Immediately after passing the articulation of the 

 elbow, it divides into two branches, one denominated the *SV 

 perficial or Radial, and the other the Profound or Posterior In- 

 terosseous. The Superficial soon joins the radial artery, and 

 proceeds downwards, sending branches to the contiguous mus- 

 cles. In its course about the middle of the arm it passes 

 beneath and across the tendon of the supinator longus, and 

 proceeds between it and the tendon of the extensor carpi radi- 

 alis longior ; it soon after divides into two branches, which are 

 principally distributed to the thumb and fore-finger, and also 

 to the integuments. In the middle third of the arm only is this 

 branch in relation with the radial artery ; it is there found 

 lying to the outer side of the vessel. 



The Profound branch pierces the supinator brevis muscle, 

 proceeds to the back of the fore-arm under the radial extensor, 

 and continues to the back of the wrist and hand. In this 

 course it divides into two branches, which are distributed to 

 all the muscles on the back of the fore-arm muscles, and the 

 integuments. One branch runs down as far as the posterior 

 part of the wrist joint which it supplies. It there forms a long 

 gangliform swelling as is common with the nerves which sup- 

 ply the joints of the extremities. 



The Internal Cutaneus 



' Is the smallest of the nerves which proceed from the axillary 

 plexus. It descends in the course of the basilic vein, and very 



