THE ANTERIOR RADIOULNAR REGION 479 



immediately below the tuberosity and oblique line, and extending below to 

 within a short distance of the Pronator quadratus. It also arises from the ad- 

 jacent part of the interosseous membrane and generally by a fleshy slip from the 

 inner border of the coronoid process or from the internal condyle of the humerus. 

 The fibres pass downward, and terminate in a flattened tendon which passes 

 beneath the annular ligament, is then lodged in the interspace between the super- 

 ficial head of the Flexor brevis pollicis and the Adductor obliquus pollicis, and, 

 entering an osseoaponeurotic canal similar to those for the other Flexor tendons, 

 is inserted into the base of the last phalanx of the thumb. 



Relations. By its superficial surface, with the Flexor sublimis digitorum, Flexor carpi 

 radialis, Brachioradialis, and radial vessels; by its deep surface, with the radius, interosseous 

 membrane, and Pronator quadratus; by its ulnar border, with the Flexor profundus digitorum, 

 from which it is separated by the anterior interosseous vessels and nerve. 



The Pronator quadratus (m. pronator quadratus) (Figs. 358 and 367) is a 

 small, flat, quadrilateral muscle, extending transversely across the front of the 

 radius and ulna, above their carpal extremities. It arises from the oblique 

 ridge on the lower part of the anterior surface of the shaft of the ulna; from the 

 lower fourth of the anterior surface and the anterior border of the ulna; and from 

 a strong aponeurosis which covers the inner third of the muscle. The fibres pass 

 outward and slightly downward, to be inserted into the lower fourth of the anterior 

 surface and anterior border of the shaft of the radius. 



Relations. By its superficial surface, with the Flexor profundus digitorum, the Flexor 

 longus pollicis, Flexor carpi radialis, and the radial vessels; by its deep surface, with the radius, 

 ulna, and interosseous membrane. 



Nerves. All the muscles of the superficial layer are supplied by the median nerve, excepting 

 the Flexor carpi ulnaris, which is supplied by the ulnar nerve. The Pronator teres and the 

 Flext>r carpi radialis derive their supply primarily from the sixth and seventh cervicals; the 

 Palmaris longus from the eighth cervical; the Flexor sublimis digitorum from the seventh and 

 eighth cervical and first thoracic, and the Flexor carpi ulnaris from the eighth cervical and first 

 thoracic nerves. Of the deep layer, the Flexor profundus digitorum is supplied by the seventh 

 and eighth cervicals and first thoracic through the ulnar and anterior interosseous branch of the 

 median. The remaining two muscles, the Flexor longus pollicis and Pronator quadratus, are 

 also supplied by the eighth cervical and first thoracic through the anterior interosseous branch 

 of the median. 



Actions. These muscles act upon the forearm, the wrist, and hand. The Pronator teres 

 helps to rotate the radius upon the ulna, rendering the hand prone; when the radius is fixed it 

 assists the other muscles in flexing the forearm. The Flexor carpi radialis is one of the Flexors 

 of the wrist; when acting alone it flexes the wrist, inclining it to the radial side. It can also 

 assist in pronating the forearm and hand, and, by continuing its action, in bending the elbow. 

 The Flexor carpi ulnaris is one of the flexors of the wrist; when acting alone it flexes the wrist, 

 inclining it to the ulnar side (adducting -the wrist), and, by continuing to contract, it bends the 

 elbow. The Palmaris longus is a tensor of the palmar fascia. It also assists in flexing the wrist 

 and elbow. The Flexor sublimis digitorum flexes first the middle and then the proximal pha- 

 langes. It assists in flexing the wrist and elbow. The Flexor profundus digitorum is one of 

 the Flexors of the phalanges. After the Flexor sublimis has bent the second phalanx, the Flexor 

 profundus flexes the terminal one, but it cannot do so until after the contraction of the superficial 

 muscle. It also assists in flexing the wrist. The Flexor longus pollicis is the flexor of the 

 distal phalanx of the thumb. When the thumb is fixed it also assists in flexing the wrist. The 

 Pronator quadratus helps to rotate the radius upon the ulna, rendering the hand prone. 



Applied Anatomy. When a finger is amputated so that the fibrous sheath of the Flexor ten- 

 dons is divided in a region in w r hich it is firm and dense, the tendon contracts but the theca does 

 not, and the rigid theca constitutes a permeable passage to the palm. If the parts should be 

 infected the theca will draw pus toward the palm. Hence, it is best to close the theca by sutures. 

 "Over the terminal phalanx, and over the joint between the middle and terminal phalanges, 

 there is no fibrous sheath. In front of the metacarpophalangeal joint it is scarcely evident. 

 Over the first and second (proximal and middle) phalanges, and in front of the joint between 

 these bones, the fibrous sheath is well marked, and appears as a rigid tube when cut across. 

 As the sheath crosses the metacarpophalangeal and first interphalangeal joints it is adherent 



