of muscle homology was related to the theory that nerve 

 and muscle have a fixed relationship and in fact are joined 

 trom the very beginning. It is now known that these struc- 

 tures arise separately and that the nerves grow out to 

 reach their end structure. Experimental work shows that 

 the innervation can be altered without loss of proper func- 

 tion. Comparative studies indicate that the pattern of nerves 

 is plastic and reflects the loss, subdivision, or fusion of mus- 

 cles. Peculiar innervations are the result of migration of a 

 muscle to a new position and relationship after its nerve has 

 reached it. In spite of this plasticity, the branchial plexus 



TABLE 14- 1 TERMS USED TO DESCRIBE THi ACTIONS OF MUS- 

 CLES OR THE CATEGORIES OF MUSCLES HAVING 

 A COMMON ACT/ON 



flexors muscles which tend to close the angle between two bones 

 extensors muscles which increase the angle between two bones 



abductors muscles which draw the bone away from the median line 

 of the body or midline of the limb 



adductors muscles which draw the bone toward the median line of 

 the body or limb 



rotators rotate bones around a central axis. The supinator rotates 

 the radius in relation to the ulna so that the palm is turned up; 

 the pronator teres rotates these bones so that the palm (the ven- 

 tral surface) is turned down and the radius and ulna are slightly 

 crossed- 



clevators or levators muscles which raise or lift a part; the levator 

 scapulae raises the scapula 



depressors pull down or lower as in the case of the jaw muscles 

 which open the mouth ("depressor mandibulae") 



constrictor rings of muscle fibers which close openings or decrease 

 diameters as in the case of the sphincter muscles of the gut or the 

 constrictor colli, a dermal muscle of the neck 



and pattern of nerves of the rabbit (Figure 14-12) can be 

 compared readily with that of the cat or man, and more 

 superficially with the reptile or bird. 



The comparative anatomy of 

 shoulder muscles 



As an example of comparative myology the muscles of 

 the shoulder region of the mammal can be compared with 

 those of a reptile and an amphibian. 



Repfile The musculature of Iguana is much like that of 

 Sphenodon, Lacerta, or Alligator. In Iguana (Figure 14-13) the 

 dorsal extrinsics consist of a trapezius, which has an ante- 

 rior pars clavotrapezius (including the sternomastoideus) 

 and a posterior pars spinotrapezius, and a latissimus dorsi. 

 Beneath this layer is a large levator scapulae and a two- 

 parted sternohyoideus (including the sternothyroideus). A 

 serratus muscle, attached to the suprascapular cartilage, 

 draws the girdle backward, while three slips of muscle, the 

 rhomboideus (whose fibers extend from the body wall up- 

 ward and posterolaterally to the inner surface of the supra- 

 scapula), draws it forward and down. Ventrally the pectoralis 

 has an anterior and a main division. 



On the shoulder, as viewed laterally, are the clavicular 

 and scapular divisions of the deltoid. With this layer re- 

 fle.\ed (Figure 14-14) the teres minor (proscapulohumeralis 

 anterior) and supracoracoideus are revealed. From the pos- 

 terior and inner margin of the suprascapula and scapula 

 arises a teres major. 



As viewed ventrally (Figure 14-15), with the pectoralis 

 reflexed, the supracoracoideus is seen to arise from much of 

 the coracoid. A part of it is covered by an insertion tendon 

 of the sternomastoid part of the trapezius. Behind and partly 



^MLi 14-2 EMBRYOLOGICAL DEVELOPMENT OF THE PROXIMAL MUSCLES OF THE FORELIMB 



deltoid division 



dorsal premuscular aniage 



deltoideus 

 teres minor 



subscapular division : 



latissimus dorsi division : 



triceps division 



subscapularis 

 teres major 



latissimus dorsi 

 extensor antibrachialis 



three heads of triceps 



ventral premuscular aniage ; 



supracoracoid division ; 



ventral brachial division ; 



supraspinatus 

 infraspinatus 



coracobrachialis 

 - biceps 

 ■ brachialis 



pectoral division : 



pectoralis 

 pectoantibrachialis 



432 



THE MUSCULAR SYSTEM 



