PRONATOR QUADRATUS—BRACHIO-RADIALIS 321 



Fourth Layer 



The fourth layer consists of one muscle — the pronator quadratus. 



PRONATOR QUADRATUS 



The pronator quadratus (fig. 284) is a thin quadrilateral sheet, named from 

 its action and its nearly square shape. 



Origin. — The inner part of the front surface of the lower fourth of the ulna. 



Insertion. — The lower two inches (5 cm.) or rather less of the outer border 

 and the anterior surface of the radius. 



Structure. — The greater part of the muscle consists of fleshy fibres Avhich pass 

 transversely between its two attachments. Its inner third, hoAvever, is covered by 

 a strong aponeurosis which arises from the inner Ijorder of the ulna. The radial 

 insertion is of a somewhat triangular shape, the outer side of the triangle passing 

 obliciuely from the interosseous line downwards, and joining the outer border of the 

 radius a short distance from the base of the styloid process; while the inner side 

 corresponds to the interosseous ridge, and the base crosses the front of the radius 

 above the attachment of the anterior radio-carpal ligament. 



Nerve-supply. — From the outer and inner cords of the brachial plexus 

 (through the eighth cervical and first thoracic nerves) by means of the anterior 

 interosseous Ijranch of the median nerve which terminates by filaments which enter 

 the front of the muscle near its upper border. 



Action. — By drawing the outer border of the anterior surface of the radius 

 towards the inner border of the anterior surface of the ulna, it pronates the radius 

 upon the ulna. 



Relations. — Superficially, the flexor longus pollicis, the flexor carj^i radialis, 

 the flexor profundus digitorum, and the flexor carpi ulnaris, the radial artery, and 

 the ulnar nerve; deeply, the anterior interosseous arter}', the interosseous mem- 

 brane, and the inferior radio-ulnar joint. 



Variations. — The pronator quadratus is sometimes absent. It sometimes gives off slips to the 

 scaplioid or trapezium, the base of the first metacarpal bone, or to the origins of the short thumb 

 muscles. 



EADIAL GROUP OF :\IUSCLES 



Along the radial border, between the groups which clothe the front and back of 

 the forearm, lie three long muscles, one upon the other — viz. the brachio-radialis 

 (or supinator longus); the extensor carpi radialis longior; and the extensor carpi 

 radialis brevior. 



1. BRACHIO-RADIALIS 



• 



The brachio-radialis, or supinator radii longus — named from its attachment 

 to the humerus and radius, and sometimes called the supinator longus from an 

 erroneous view of its action — is a fusiform sheet. 



Origin, — The upper two-thirds of the external condylar ridge, and the front of 

 the external intermuscular septum of the upper arm. 



Insertion. — The base of the stylf)id process of the radius. 



Structure. — Arising by fleshy fibres from the septum, and by short tendinous 

 filires from the condylar ridge, the muscle passes downwards and forwards in penni- 

 form fashion to its tendon which lies first on its deep surface. Becoming free just 

 below the middle of the forearm, the tendon runs directly downwards and expands 

 before its insertion upon a horizontal line at the base of the styloid process of the 

 radius. At first the plane of the muscle is directed outwards and inwards; but 

 lower down the outer surface becomes anterior, and the inner posterior. 



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

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