DEEP LAYER OF MUSCLES. 271 



outer part of the interosseous membrane ; and by a round distinct slip from 

 the coronoid process of the ulna, internal to the attachment of the brachi- 

 alis anticus. The fleshy fibres descend to a tendon, which is continued 

 beneath the annular ligament, and is inserted into the last phalanx of the 

 thumb. 



On the cutaneous surface of the muscle is the flexor sublimis, with the 

 radial vessels for a short distance interiorly. The muscle lies on the radius 

 and the pronator quadratus. To the inner side is the flexor profundus 

 digitorum. 



Action. It bends both joints of the thumb, but first the distal or un- 

 gual. After the phalanges are drawn downwards the muscle flexes the 

 wrist. 



The PRONATOR QUADRATUS (fig. 81, D ) is a flat muscle, and lies on the 

 lower fourth of the bones of the forearm. It arises from the anterior and 

 inner surfaces of the ulna, where it is somewhat widened, and is inserted 

 into the forepart of the radius for about two inches. 



The anterior surface is covered by the tendons of the flexor muscles of 

 the lingers, and by the radial vessels ; and the posterior surface rests on 

 the radius and ulna with the intervening membrane, and on the interosse- 

 ous vessels and nerve. Along its lower borders is the arch formed by the 

 anterior carpal arteries. 



Action. The end of the radius is moved forwards over the ulna by this 

 muscle, and the hand is pronated. 



The anterior interosseous artery (fig. 81, e) is continued on the front of 

 the interosseous membrane, between the two flexors or in the fibres of the 

 flexor digitorum, till it readies an aperture beneath the pronator quadratus. 

 At that spot the artery turns from the front to the back of the limb, and 

 descends to the back of the carpus, where it ends by anastomosing with 

 the posterior interosseous and carpal arteries. 



Branches. Numerous offsets are given to the contiguous muscles. 



a. One long branch (/), median, accompanies the median nerve, sup- 

 plying it, and either ends in the flexor sublimis, or extends beneath the 

 annular ligament to the palmar arch. 



b. About the middle of the forearm two nutrient vessels of the radius 

 and ulna arise from the artery. 



c. Where it is about to pass through the interosseous membrane it fur- 

 nishes twigs to the pronator quadratus ; and one branch is continued be- 

 neath that muscle to anastomose with the anterior carpal arteries. 



The anterior interosseous nerve is derived from the median (fig. SI. '-'), 

 and accompanies the artery of the same name to the pronator quadratus 

 muscle, the under surface of which it enters. Some lateral branches are 

 distributed by it to the deep flexor muscles. 



Dissection. The attachment of the biceps and brachialis anticus to the 

 bones of the forearm may be now cleaned and examined. 



The insertion of the brachialis anticus takes place by a broad thick 

 tendon, about an inch in length, which is fixed into the coronoid process <>t 

 the ulna, except at the inner edge ; and into the contiguous rough part oi 

 the bone. 



Insertion of the biceps. The tendon of the biceps is inserted into the 

 inner part of the tubercle of the radius, and slightly into the bone behind 

 it. A bursa intervenes between it and the forepart of the tubercle. At 

 its attachment the anterior surface becomes external ; and the opposite. 

 The supinator brevis muscle partly surrounds the insertion. 



