498 NERVOUS SYSTEM. 



to those on the palmar side of the hand, about the root of the 

 thumb. 



7. The Nervus Radialis, or Musculo-Spiralis, arises from 

 the upper portion of the brachial plexus, but in such a way, 

 that filaments from almost every part of the latter run into it. 

 It is a large trunk which winds spirally around the os humeri, 

 between the triceps muscle and the bone, by entering the fis- 

 sure between the first and the'third head of the triceps. It 

 appears on the outside of the os humeri, between the brachialis 

 internus and the triceps muscle; running for some inches in 

 contact with their intermuscular ligament. While beneath the 

 triceps, it sends several branches to its heads. There are three 

 principal trunks afterwards from this nerve. 



a. The Ramus Superficialis Dorsalis is sent from it on a 

 line with the point of the deltoid muscle. This branch then 

 goes just below the skin, parallel with and over the external 

 ridge of the os humeri; it, of course, crosses the origin of the 

 muscles of the external condyle. It continues superficial on 

 the posterior external edge of the supinator radii longus mus- 

 cle, and terminates in the integuments on the back of the 

 hand. 



The continued trunk of the muscular spiral goes in the inter- 

 stice between the extensor muscles, and the brachialis internus, 

 and, at the external condyle, divides into the other two 

 branches, from which filaments proceed to the contiguous 

 heads of the muscles. 



b. The Ramus Profundus Dorsalis perforates the supinator 

 brevis muscle, getting beneath the radial extensors to the back 

 of the fore-arm; it is then distributed in numerous filaments to 

 the muscles on the back of the fore arm, some of its branches 

 reaching to the wrist. 



c. The Ramus Superficialis Anterior seems to be a continua- 

 tion of the main trunk of the nerve, and descending at the an- 

 terior margin of the supinator radii iongus muscle, it joins 

 with the radial artery and continues in its company to a short 

 distance below the middle of the radius.* Here it crosses the 

 bone obliquely beneath the tendon of the supinator longus, 



* This position gives the whole nerve the name of radial. 



