DISSECTION OF THE ANTERIOR LBIB. 23 



Directions. — The muscles on the back of the fore -arm must now be 

 learnt. These consist of the three flexors of the metacarpus, and the two 

 flexors of the digit. 



The Flexor Metacarpi Internus (Plate 6). This muscle lies along 

 the inner edge of the posterior surface of the radius, where it conceals 

 the posterior radial vessels and the median nerve. It arises from the 

 inner condyle of the humerus, just behind the point of origin of the 

 internal lateral ligament, where it is confounded with the origin of the 

 middle flexor. It terminates inferiorly in a long, slender tendon, which, 

 after passing through a synovial sheath at the inner side of the carpus, 

 is inserted into the head of the inner small metacarpal bone. 



Action. — It is a flexor at the carpal articulations — i.e., it flexes the 

 manus on the fore-arm. 



The Flexor Metacarpi Medius (Plate 6). This muscle descends in 

 contact with the posterior edge of the internal flexor. It has two heads 

 of origin — an anterior and a posterior. It arises by its anterior head 

 just behind the origin of the preceding muscle, and by its posterior 

 head from the upper part of the posterior edge of the olecranon. After 

 a course of three or four inches these two heads unite, and the single 

 inferior tendon is inserted into the upper border of the pisiform bone. 

 The ulnar nerve and vessels pass beneath the posterior or ulnar head of 

 the muscle. 



Action. — The same as the preceding muscle. 



The Flexor Metacarpi Extbrnus (Plates 7 and 8) is situated at the 

 outer side of the back of the fore-arm, having the lateral extensor of the 

 digit (extensor suffraginis) in front of it, while behind it is separated from 

 the last-described muscle by the ulnar division of the deep flexor of the 

 digit (ulnaris accessorius). It arises from the lowest point of the outer 

 I'idge bounding the olecranon fossa. At its lower end it has two inser- 

 tions, viz., (1) into the upper border of the pisiform bone, where it is 

 confounded with the insertion of the middle flexor ; (2) by a cord-like 

 tendon which, after descending in a synovial sheath formed inwardly by 

 the oblique groove on the outer surface of the pisiform bone, is inserted 

 into the head of the external small metacarpal bone. 

 Action.— LWe the preceding two muscles. 



Directions. — The three flexors of the metacarpus surrotmd the flexors 

 of the digit, and they should be cut about their middle and reflected to 

 bring these latter into view. 



The Superficial Flexor of the Digit (flexor pedis perforatus) (Plate 

 6) arises, by a tendon common to it and the deep flexor, from the 

 lower extremity of the ridge bounding the olecranon fossa on the inside. 

 Its muscular belly contains much tendinous tissue, and cannot without 

 difficulty be separated from the deep flexor, on which it rests. At the 

 lower part of the radius its muscular portion is succeeded by a tendon, 



