MUSCLES OF THE THORACIC LIMB 375 
dorsal surface and the medial border of the radius. (A pouch of the capsule of the 
elbow joint lies under the tendon of origin.) Its action is to rotate the forearm 
outward (supination). 
The pronator quadratus consists of a thin layer of fibers which cross the volar 
surface of the radius and the interosseous ligament of the forearm, except at the two 
extremities of the latter. The fibers extend from the medial border of the radius 
backward and outward to the interosseous border of the ulna. It rotates the fore- 
arm inward (pronation). 
The palmaris brevis (?) is a very small muscle, which arises on the tendon of 
the superficial digital flexor at the carpus, and is inserted at the fifth metacarpo- 
phalangeal joint into the sheath and annular ligament of the deep flexor. 
The lumbricales are three very delicate muscles, which arise on the volar sur- 
face of the tendons of the deep flexor, and are inserted into the first phalanges of the 
third, fourth, and fifth digits. 
The abductor pollicis brevis et opponens pollicis, a very small pale muscle, 
arises on the fibrous band which connects the superficial flexor tendon with the 
medial carpal sesamoid, and ends on the distal end of the first metacarpal bone and 
the first phalanx of the first digit. It abducts the first digit. 
The flexor pollicis brevis arises on the volar carpal ligament over the second 
metacarpal bone, and ends on the volar sesamoid of the first digit. 
The adductor pollicis, situated lateral to the preceding, is the largest of the 
thumb muscles. It arises between the preceding and the second interosseous 
muscle, and is inserted into the first phalanx of the first digit. 
The adductor digiti secundi is situated between the second interosseous 
muscle and the adductor digiti quinti. It arises on the volar carpal ligament, and 
ends on the first phalanx of the second digit. 
The adductor digiti quinti arises close to the preceding muscle, and passes 
outward to end on the first phalanx of the fifth digit. 
The flexor digiti quinti arises on the ligament connecting the accessory carpal 
to the third and fourth metacarpal bones, crosses the corresponding interosseous 
muscle, and ends on the fifth digit with the next muscle. 
The abductor digiti quinti is larger than the two preceding muscles; it arises 
on the accessory carpal bone, and ends on the lateral sesamoid of the fifth digit and 
on the lateral ligament. 
There are four interossei which lie on the volar surface of the metacarpus. 
They are well developed and fleshy. They arise on the distal row of the carpus 
and on the proximal ends of the metacarpals. Each divides distally into two 
branches, which are inserted by small tendons on the corresponding sesamoid 
bones, and detach slips to the extensor tendons. 
The chief facts with regard to the synovial membranes in connection with the muscles of the 
forearm and manus are as follows: The tendons of the extensor carpi radialis frequently have a 
synovial sheath at the distal part of the forearm, and there may be one or two burs at the carpus. 
The tendons of the extensor digitalis communis have a synovial sheath which extends from the distal 
part of the forearm to the carpus or to the proximal part of the metacarpus. The tendons of the 
extensor digitalis lateralis have in about half of the cases a synovial sheath at the distal part of the 
forearm and at the carpus. The tendon of insertion of the extensor carpi obliquus has a synovial 
sheath. The tendon of insertion of the ulnaris lateralis sometimes has a bursa between it and the 
styloid process of the ulna. The tendon of insertion of the flexor carpi radialis has a synovial 
sheath. There is a bursa between the two tendons of the flexor carpi ulnaris at their insertion; 
it usually communicates with a subfascial bursa which lies on the accessory carpal bone. The 
superficial digital flexor has a bursa under its origin; this communicates with another bursa which 
lies between the deep digital flexor and the flexor carpi ulnaris. The tendons of the digital flexors 
for the second to the fifth digits have a common sheath for each digit which begins at the distal 
part of the metacarpus. The tendon of the deep flexor for the first digit has a synovial sheath. 
1 Movements of individual digits are specified with regard to the axis of the manus (paw), 
and not to the median plane of the body. 
