328 THE MUSCLES 



brevis, the interosseous recurrent artery wliicli runs through the deeper filires of the 

 muscle, the elbow and upper radio-ulnar joints; on its outer border, the extensor 

 carpi uhxaris. 



Deep I^ayer 



The deep layer consists of five muscles: the supinator radii Itrevis; extensor 

 ossis metacarpi pollicis; extensor l)revis pollicis; extensor longus pollicis; and the 

 extensor indicis. Four of these arise from the bones of the forearm, and one 

 only from the humerus also. 



1. SUPINATOR RADII BREVIS 



The supinator radii brevis — named from its action and size in comparison 

 with the brachio-radialis (= supinator longus) — is a rhomboidal sheet of muscular 

 fibre curved upon itself into a somewhat cylindrical shai)e to wrap round the 

 up})er third of the radius. 



Origin. — (1) Lower and back part of external condyle; (2) the external 

 lateral ligament of the elbow-joint; (3) the orbicular ligament; (4) the triangular 

 depression below the lesser sigmoid cavity of the ulna, especially along its posterior 

 margin, which forms the upper part of the external border of the ulna. 



Insertion. — (1) The back of the neck of the radius; (2) the anterior and 

 outer surfaces of the radius above and at the upjjer border of the oblique 

 line. 



Structure. — Its origin is partly fleshy and partly by a strong aponeurosis which 

 covers the upper half of the muscle and gives attachment to some of the muscles in 

 the superficial layer. The line of origin runs downwards and somewhat inwards 

 from the external condyle to the outer border of the ulna, while that of insertion 

 runs downwards and outwards from the tul^ercle of the radius to the imjiression 

 for the pronator radii teres. 



Between these lines the fleshy fibres run in parallel curves Avrapping round the 

 upper third of the radius. The muscle is divided into a small superficial and more 

 extensive deep plane by the posterior interosseous nerve Avhich perforates it on its 

 way to supply the muscles at the back of the forearm. The line of insertion is 

 broken at the tubercle of the radius l:)y a notch in which lies the bursa in front of 

 the attachment of the biceps tendon. 



Nerve-supply. — From the posterior cord of the brachial plexus (through the 

 sixth cervical nerve) by branches derived from the posterior interosseous division 

 of the musculo-spiral nerve, which are given of? from this nerve as it perforates the 

 muscle. 



Action. — To supinate the radius upon the ulna. By its contraction the 

 muscle unwraps itself from the radius; the tubercle of the radius is drawn for- 

 wards, and the outer border of the bone backwards; and if the radius at the 

 commencement of action be in the position of complete pronation, it Avill be 

 caused by this muscle to revolve about the axis which passes through the centre of 

 its head and the middle of the lower extremity of the ulna through nearly 1S0°. 



The action of this muscle will, unlike that of the biceps, be unaft'ected In' the 

 position of the elbow. 



This completes the list of muscles by which the rotation of the radius upon the 

 ulna is effected. The biceps and supinator brevis are the su})inators; the pronator 

 teres and (juadratus, the pronators. It should be remembered, however, that 

 ordinary pronation and supination are effected by a more complicated movement 

 than simple rotation of the radius, which would cause the hand to revolve around 

 the axis of the little finger. The usual rotation, the axis of which movement 

 passes through the line of the middle finger, involves a slight flexion and extension 

 of the elbow with some movement of the shoulder-joint. 



Relations. — Superficially, tlie l)iceps, brachio-radialis, pronator radii teres, 

 extensor car])i radialis brevior, extensor comnu^nis digitorum, extensor minimi 



