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 humeral insertion is just below that of the medial division of the supraspinatus. 
Part of 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 vertebre. 
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 scapule.? 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 limb 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 almost 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 lamellze derived from the dorso-scapular ligament. 
Relations.—Superficially, the brachiocephalicus, 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. 
*It has been commonly stated that these muscles form a sort of sling in which the trunk is 
uspended. This is not quite correct as the two muscles do not meet ventrally. The arrange- 
wall is admirable, since the pull of the thorax on the muscles presses the scapulz against the body 
