DEEP EXTENSORS OF THE FINGEHS. 223 



means of a thin tendon in common with the extensor cornmunis. The 

 tendon in which it ends occupies a groove between the radius and ulna, 

 passing through a ring in the annular ligament appropriated to itself, and 

 joins the fourth digital tendon of the common extensor. 



The extensor carpi ulnaris, the most internal of the muscles descending on 

 the back of the forearm, arises from the external condyloid eminence of the 

 humerus by the common tendon and from a prolongation of that structure ; 

 from the posterior border of the ulna, below the anconeus, for about the 

 middle third ; and from the fascia of the forearm. The muscular fibres 

 derived from these sources incline somewhat inwards, and end in a tendon, 

 which runs through a special groove in the carpal end of the ulna and a 

 separate sheath in the annular ligament, and is inserted into the base of the 

 metacarpal bone of the little finger on its posterior aspect. 



The deep- seated muscles on the back of the forearm are five in number, 

 the supiuator brevis, the three extensors of the thumb and the extensor of 

 the index finger. 



The supinator radii brevis arises from the external lateral ligament of the 

 elbow-joint, and from the annular ligament of the radius; also from a rough 

 depression below the sigmoid cavity of the ulna, and from the outer border 

 of that bone for two inches. The fibres of the muscle, derived from these 

 points of attachment, as well as from a tendinous expansion on the surface, 

 pass obliquely round the upper part of the radius, covering it closely except 

 at the inner bide, and are inserted into that bone for rather more than a 

 third of its length, passing down to the upper margin of the insertion of 

 the pronator teres. 



The posterior interosseous nerve passes through the fibres of this muscle. 



The extensor ossis metacarpi pollicis (abductor longus pollicis, Alb.) 

 arises from an elongated depression on the radial side of the posterior 

 surface of the ulna below the origin of the supinator brevis, from a similar 

 extent of surface of the radius below the insertion of the same muscle, and 

 from the interosseous membrane between. Thence descending obliquely 

 outwards it ends in a tendon, which passes in company with the extensor 

 primi internodii pollicis through the groove on the outer border of the 

 lower extremity of the radius, and is inserted into the base of the meta- 

 carpal bone of the thumb. 



The upper part of this muscle is concealed by the common extensor, but it becomes 

 superficial beneath, and together with the next muscle crosses the tendons of the 

 radial extensors of the carpus, conceals the insertion of the supinator longus, and, 

 beneath the extremity of the radius, crosses the radial artery. 



The extensor primi internodii pollicis (extensor minor pollicis, Alb.) 

 lies close to the lower border of the extensor ossis metacarpi, and is much 

 smaller than that muscle ; it arises from the interosseous ligament and the 

 radius near the middle of the forearm ; its tendon accompanies that of the 

 extensor ossis metacarpi through the same compartment of the annular 

 ligament, and passes on to be inserted into the proximal end of the first 

 phalanx. 



The extensor secundi internodii pollicis (extensor major pollicis, Alb.), 

 much larger than the extensor primi internodii, which it overlaps, arises 

 immediately below that muscle from the lower half of the shaft of the ulna, 

 and from the interosseous ligament at its lower end for about an inch. Its 



