THE MUSCLES OF THE SHOULDER GIRDLE 297 



and thin, but as the muscle is traced forward, it becomes narrower and much 

 thicker. It passes forward and slightly upward in a gentle curve to its insertion. 

 The hmneral insertion is just below that of the medial division of the supraspinatus. 

 Part o"f the fibers are inserted by means of a tendinous band which binds down the 

 tendon of the biceps and is attached to the lateral lip of the intertuberal or bicipital 

 groove, and a small part is attached to the tendon of origin of the coraco-brachialis. 



Relations.- — Superficially, the skin, cutaneus, and superficial pectoral; deeply, 

 the abdominal tunic, the external oblique, the rectus abdominis et thoracis, the 

 brachial vessels, and branches of the brachial plexus of nerves. The external 

 thoracic vein lies along the lateral border. 



Blood-supply. — Internal and external thoracic, inferior cervical, anterior 

 circumflex, and intercostal arteries. 



Nerve-supply. — Pectoral nerves, from the brachial plexus. 



3. Serratus ventralis^ (Figs. 267, 268). — This is a large, fan-shaped muscle, 

 situated on the lateral surface of the neck and thorax. It derives its name from 

 the serrated ventral edge of its thoracic portion. It consists of cervical and tho- 

 racic parts. 



(a) Serratus cervicis. 



Origin. — The transverse processes of the last four or five cervical vertebrse. 

 Insertion. — The anterior triangular area on the costal surface of the scapula 

 (facies serrata) and the adjacent part of the cartilage. 



(b) Serratus thoracis. 



Origin. — The lateral surfaces of the first eight or nine ribs. 



Insertion. — The posterior triangular area on the costal surface of the scapula 

 (facies serrata) and the adjacent part of the cartilage. 



Action. — The two muscles form an elastic support, which suspends the trunk be- 

 tween the two scapulae.^ Contracting together, they raise the thorax; contracting 

 singly, the weight is shifted to the limb on the side of the muscle acting. The two 

 parts can act separately and are antagonistic in their effect on the scapula. The 

 cervical part draws the base of the scapula toward the neck, while the thoracic 

 part has the opposite action; these effects concur in the backward and forward 

 swing of the hmb respectively. With the limb fixed, the cervical part extends 

 (raises) the neck or inclines it laterally. The thoracic part may act as a muscle of 

 forced inspiration. 



Structure.— In the domesticated animals there is no such clear division of 

 the muscle as is found in man and the apes. On account of the difference in action, 

 however, it seems desirable to distinguish the two portions. The serratus cervicis is 

 thick and almpst entirely fleshy. The serratus thoracis has on its superficial face a 

 thick, tendinous layer which may sustain the weight of the trunk when the muscle 

 substance relaxes. The ventral edge presents distinct digitations, the last four of 

 which alternate with those of the obliquus externus abdominis, and are covered by 

 the abdominal tunic. The fourth and fifth digitations extend nearly to the sternal 

 ends of the ribs. The ninth digitation is small and may be absent. Exceptionally, 

 additional digitations may be attached to the tenth or eleventh rib or to the fascia 

 over the intercostal muscles. The fibers converge to the insertion, which is thick 

 and is intersected by elastic lamella derived from the dorso-scapular hgament. 



Relations.— Super&cially, the brachiocephahcus, trapezius, deep pectoral, 

 subscapularis, teres major, latissimus dorsi, cutaneus, the abdominal tunic, the 

 brachial vessels, and the long thoracic nerve; deeply, the splenius, complexus, 



1 Formerly termed the serratus magnus. 



2 It has been commonly stated that these muscles form a sort of sUng in which the trunk is 

 suspended This is not quite correct as the two muscles do not meet ventrally. i he arrange- 

 ment is admirable, since the pull of the thorax on the muscles presses the scapute agamst the body 

 wall. 



