MUSCLES OF THE THORACIC LIMB 



323 



muscles which are not rarely fused. They arise on the extensor epicondyle of the 

 humerus and the lateral ligament of the elbow joint. The larger, superficial belly 

 (M. extensor digiti tertii et quarti) terminates on a tendon which divides at or 

 near the carpus into two branches, which are inserted into the third phalanges 

 of the third and fourth digits, blending with the corresponding tendons of the com- 

 mon extensor. The posterior belly (M. extensor digiti quinti) terminates by a 

 tendon which fuses with that of the common extensor for the fifth digit. 



The ulnaris lateralis or extensor carpi ulnaris corresponds to the flexor carpi 

 externus of the horse, but is an extensor of the carpus. It is a large flat muscle 

 which lies on the outer surface of the ulna. It arises on the ex- 

 tensor epicondyle of the humerus, and is inserted into the proxi- 

 mal end of the fifth metacari:)al and the accessory carpal bone. 



The ulnaris medialis or flexor carpi medius (s. ulnaris) 

 consists of two quite distinct heads. The larger, humeral 

 head arises on the flexor epicondyle, while the smaller, super- 

 ficial ulnar head arises on the posterior l)order of the ulna. 

 The tendons of the two entl together on the accessory 

 carpal bone. 



The radialis volaris or flexor carpi internus (s. radialis) 

 arises on the flexor epicondyle of the humerus and is inserted 

 l)y a bifid tendon into the second and third metacarpal bones. 



The tendons of the foregoing muscles are provided with 

 synovial sheaths at the carpus. 



The pronator teres is a round muscle, which is situated 

 superficially on the inner border of the proximal part of the ra- 

 dius. It arises on the flexor epicondyle of the humerus, and is 

 inserted into the internal border of the radius. Its action is to 

 flex the elbow and rotate the forearm inward. 



The superficial digital flexor is situated superficially on the 

 posterior and inner surfaces of the forearm. It arises on the 

 flexor epicondyle of the humerus and terminates near the 

 carpus on a tendon which passes downward outside of the 

 carpal canal and receives below the carpus two reinforcing 

 bands, one from the accessory carpal, the other from the 

 sesamoid bone at the inner side of the carpus. Below this 

 it divides into four branches, which are inserted into the 

 second phalanges of the second, third, fourth, and fifth digits. 



The deep digital flexor has humeral, ulnar, and radial 

 heads, the latter arising from the inner border of the radius. 

 They unite on a common tendon which passes down through 

 the carpal canal, gives off a branch to the first digit, and 

 divides into four branches. These perforate the tendons of 

 the superficial flexor and are inserted into the third phalanges 

 of the second to the fifth digits. The tendons are provided 

 with synovial sheaths from the middle of the metacarpus downward, and are 

 held in place by three digital annular ligaments. 



The palmaris longus accessorius (?) is a small muscle which arises from the 

 deep flexor below the middle of the forearm and ends by two tendons which unite 

 with those of the superficial flexor for the third and fourth digits. 



The supinator is a short, wide muscle which arises on the humerus beneath the 

 external lateral ligament, crosses the anterior surface of the radius, and is inserted 

 into the inner 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). 



Fig. 234.— Volar Mus- 

 cles OF Fore 

 Paw OB' Dog. 

 a, -Abductor pol- 

 licis brevis et opponens 

 pollicis; h, flexor polli- 

 cis brevis; c, adductor 

 pollicis; d, adductor 

 digiti secundi; e, ad- 

 ductor digiti quinti; /, 

 flexor digiti quinti 

 brevis; g, abductor 

 digiti quinti; h, in- 

 terossei; 1, accessory 

 carpal bone; 2, first 

 digit; S-6, sesamoids 

 of metacarpo-phalan- 

 geal joints. (Ellen- 

 berger-Baum, .\nat. d. 

 Hundes.) 



