acromial process 

 suprccoracoideus 



ABC 



Figure 14-1 1. Three stages in the development ot the supracoracoideus of the opossum. (After Cheng, 1955) 



concealed by the supracoracoideus are the two heads of the 

 biceps muscle and the fleshy belly of the coracobrachialis 

 brevis. The coracobrachialis longus arises from the inner 

 surface of the coracoid, behind the level of the glenoid fossa 

 (socket for humerus). The coracobrachialis longus inserts 

 with the biceps and on the proximal end of the humerus 

 adjacent to its origin. On the inner aspect of the pectoral 

 girdle is a large subscapularis which is joined with the teres 

 major (Figure 14-14). 



The upper arm muscles consist of the brachialis, biceps, 

 and triceps. The latter has three heads: lateralis, longum, 

 and medialis. 



The forearm muscles are simpler than those of the mam- 

 mal (Figure 14-13). The dorsal layer consists of an anterior, 

 two-parted extensor carpi radialis, a middle (and super- 

 ficial) extensor digitorum, and a posterior extensor carpi 

 ulnaris. The e.xtensor carpi radialis, which includes the su- 

 pinator, inserts along the length of the radius. Below the ex- 

 tensor digitorum is the abductor and extensor poUicis brevis, 

 which inserts both at the base and further out on the first 

 digit. Associated with this muscle, and arising from the 

 region of the ulnare, are short extensors (and interosseus 

 muscles) of the digits II to V. 



On the ventral aspect of the forearm (Figure 14-15), from 

 front to back margin, are a flexor carpi radialis, a palmaris 

 longus, one or more heads of a flexor digitorum, and a 

 flexor carpi ulnaris. The flexor radialis (includes the pro- 

 nator teres) inserts distally on the radius and on the radiale. 

 The flexor ulnaris is bound to the extensor ulnaris and in- 

 serts on the pisiforme bone and through this movable ele- 

 ment on the rear margin of the hand. The palmar fascia is 

 thick and covered externally with distinct short fle.xors for 

 the digits. With the exception of that of the fifth digit, these 

 radiate from the region of the pisiform. 



With the palmaris reflexed and the palmar sheet opened, 

 the three heads of the flexor digitorum profundus and its 

 tendon are seen. Involved in the origin of the heads are the 

 bellies of the palmaris and flexor ulnaris, which arise from 

 the inner condyle of the humerus, and a strip along the 

 ulna anterior to the ulnar origin of the flexor ulnaris. From 

 the palmar part of the tendon of the flexor digitorum pro- 

 fundus, branches extend to all five digits. On the palmar 

 expansion of the tendon are seen thin muscle bellies collec- 

 tively identified as the lumbricales. Below the flexor digi- 

 torum is a strongly developed pronator layer, on which is 

 seen a separate division of the profundus which inserts on 

 the radiale along with the flexor carpi radialis. The prox- 

 imal part of the pronator layer, is semiseparate, while the 

 distal part is a typical pronator quadratus. 



Amphibian The muscles of the amphibian are more dif- 

 ficult to separate from the enclosing connective tissue than 

 those of the higher forms. Among the salamanders there is 

 much variation in the details of the musculature and some 

 variation in the presence and absence of particular muscles, 

 just as among reptiles or mammals. As a large and easily 

 dissected type, Cryptobranchus will be described. 



The dorsal extrinsics (Figure 14-16) are the trapezius 

 and latissimus. The former has two parts whose fibers tend 

 to cross in reaching their insertion on the base of the scapula 

 and the coracoid. The bellies of both the trapezius and 

 latissimus begin some distance from the midline where the 

 segmental myotomes are well marked. The insertion of the 

 latissimus is along an arched line across the belly of the 

 triceps and extending to the margin of the deltoid insertion. 

 Below this superficial layer of muscles is an anterior levator 

 scapulae and a posterior serratus. 



SKELETAL MUSCULATURE • 433 



