MUSCULO-SPIRAL NERVE. 259 



the back of the humerus between the inner and outer heads of the triceps ; 

 in tliis position it supplies branches to the triceps and deltoid muscles, and 

 is continued onwards in the groove in the bone to the outer part of the 

 arm, where it divides into its terminal offsets : One of these, which is 

 very small, courses on the musculo-spiral nerve to the front of the elbow, 

 anastomosing with the recurrent radial branch : whilst others continue 

 along the intermuscular septum to the elbow, and join the radial and pos- 

 terior interosseous recurrent branches. 



Branches. Most of the muscular offsets of the vessels descend to the 

 olecranon, supplying the triceps, and communicate with the inferior pro- 

 funda and anastomotic branches of the brachial artery (p. 254) ; and with 

 the recurrent branches of the arteries of the forearm except the anterior 

 ulnar. One slender offset accompanies a branch of the musculo-spiral 

 nerve, and ends in the anconeus muscle below the outer condyl* of the 

 humerus. 



Two or more cutaneous offsets arise on the outside of the arm, and 

 accompany the superficial nerves. 



The musculo-spiral nerve (fig. 70) is the largest trunk of the posterior 

 cord of the brachial plexus (p. 236) and is continued along the back and 

 outer part of the limb to the hand. In the arm the nerve winds with the 

 profunda artery beneath the triceps muscle. At the outer aspect of the 

 arm it is continued between the brachialis anticus and supinator longus 

 muscles to the external condyle of the humerus, in front of which it divides 

 into the radial and posterior interosseous nerves. The nerve gives muscu- 

 lar branches, and the following cutaneous offsets to the inner and outer 

 parts of the limb. 



a. The internal cutaneous branch of the arm (fig. 78, 6 ) is of small 

 size, and arises in the axillary space in common with the branch to the 

 inner head of the triceps ; it is directed across the posterior boundary of 

 the axilla to the inner side of the arm, where it becomes cutaneous in the 

 upper third, and is distributed as before said (p. 250). 



b. The external cutaneous branches springing at the outer side of the 

 limb are two in number : they are distributed in the integuments of the 

 arm and forearm (p. 250). 



c. The muscular branches to the triceps are numerous, and supply all 

 three heads. One slender offset for the inner head, arises in common with 

 the inner cutaneous branch, and lies close to the ulnar nerve till it enters 

 the muscular fibres at the lower third of the arm. Another long and 

 slender branch behind the humerus, appearing as if it ended in the triceps, 

 can be followed downwards to the anconeus muscle. 



d. On the outer side of the limb the musculo-spiral nerve supplies the 

 brachialis anticus in part, and two muscles of the forearm, viz., supinator 

 longus and extensor carpi radialis longior. 



Subanconeus miiscle. A thin fleshy stratum of the under part of the 

 triceps near the elbow has been so named. It is described as consisting 

 of two fasciculi, inner and outer, which are attached above the fossa for. 

 the olecranon, and end in the synovial sac of the joint. A corresponding 

 muscle is placed beneath the extensor of the knee joint. 



Action. It is said to raise the synovial membrane in extension of the 

 joint. 



Directions. As the dissection of the arm has been completed as far as 

 the elbow, it will be advisable to keep moist the shoulder joint until it is 

 examined with the other ligaments. 



