scapula 



limit of 



levator scapulae 



(dashed line) 



teres minor 



suprocoracoideus 



deltoideus 



pectorolis 



subscapularis 

 (subcoracoideus) 



procoracoid socket 



sternocoracoideus 



A B 



Figure 14-14. Muscles of the shoulder joint of /guano. A, deep muscles on outer aspect of girdle 

 B, muscles of inner surface of girdle. 



Ventrally (Figure 14-17) the pectoralis is a single mass; 

 the muscles of the two sides meet along the midline separated 

 only by a line of connective tissue. The sternohyoideus 

 layer does not attach directly to the girdle; it is only an an- 

 terior continuation of the segmental ventral body-wall mus- 

 cles. A lateral division of this series attaches to the inner 

 surface of the scapula; this is the rhomboideus. 



The deltoid has both scapular and clavicular heads; the 

 latter is very large and joined marginally with the teres 

 minor. Section of the teres minor reveals a deeper layer 

 composed of a fan-shaped supracoracoideus, a thin prox- 

 imal head of the biceps and a coracobrachialis brevis. 

 Lateral to the brevis is the coracobrachialis longus, which 

 arises from the margin of the coracoid and inserts along the 

 length of the humerus and along the biceps tendon and, 

 therefore, on the radius and ulna along with the brachialis. 



The upper arm muscles are similar to those of the other 

 groups except that the biceps is reduced to a tendon. The 

 triceps has a single head. What appears to be a medial 

 head arises from the margin of the coracoid rather than 

 from along the humerus as in the reptile and the mammal. 

 From its origin, this head is part of the coracobrachialis 

 longus. Viewing the medial aspect of the girdle, a small 

 subcoracoideus inserts below the triceps head of the coraco- 

 brachialis longus. 



The forearm mu.sculature is simpler than that of the rep- 

 tile. Dorsally (Figure 14-15) there is an extensor carpi 

 radialis, which includes the supinator, a broad extensor 

 digitorum, and a thick extensor carpi ulnaris, joined for 

 much of its length with the extensor digitorum. The extensor 

 carpi radialis inserts along the radius and on the radiale. 

 With the extensor digitorum reflexed, the deeper layer of 

 muscles is exposed. There is an abductor and extensor pol- 



licis brevis, whose belly arises off the end of the ulna. Dis- 

 tally this belly joins a slip of the short extensor of the first 

 digit. The extensor digiti brevis, the short extensor series, 

 arises from the proximal carpals and sends slips to all five 

 digits. 



The ventral musculature (Figure 14-17) consists of a 

 flexor carpi radialis, a palmaris longus, and a flexor carpi 

 ulnaris. Beneath the palmaris is a flexor digitorum pro- 

 fundus of two parts, one inserting on the radiale and first 

 digit, the other on the remaining three digits. The origin of 

 the profundus is along the ulna. Beneath the profundus is a 

 pronator quadratus, which lacks a proximal division. 



Genera/ observations The musculature of the amphibian 

 suggests the embryological stages of the higher forms in the 

 incomplete separation of some of the muscles, particularly 

 those of the forearm. It also suggests specialization in the re- 

 duction of the main biceps belly to a tendon. The idea that 

 there is an evolutionary sequence beginning with the am- 

 phibian and leading through the reptile to the mammal 

 must be replaced by the view that these classes are separate 

 lines developed from a common ancestral form. If one de- 

 fines the amphibian as the ancestor of the reptiles and the 

 mammals, then what should living amphibians be called? 

 Each of these living groups has features which represent 

 specializations and each retains primitive features. 



The homologizing of muscles in these three classes ap- 

 pears relatively easy. The differences also suggest the types 

 of problems encountered, such as the fusion of two muscles, 

 as in the case of the mammalian digastricus— a jaw mus- 

 cle; the separation of one muscle into two, the supracora- 

 coideus; or the loss of a muscle sometimes accompanied by 

 occupation of its region by another. In the course of sub- 



436 • THE MUSCULAR SYSTEM 



