THE MUSCLES OF THE SHOULDER GIRDLE 295 



(M. cleido-mastoideus) partly overlaps the other (M. cleido-transversarius), which 

 lies dorsal to it. The former is attached to the mastoid process and the occipital 

 bone by a broad tendon which fuses at its terminal part with that of the splenius 

 and longissimus capitis et atlantis; it is also attached to the tendon of insertion of 

 the sterno-cephalicus by aponeurosis. The dorsal part is attached to the transverse 

 processes by four fleshy digitations. The belly of the muscle is adherent superfi- 

 cially to the cervical fascia and the cutaneous muscle, and deeply to the omo- 

 hyoideus. In front of the shoulder its deep face may present a tendinous inter- 

 section of variable development. ^ Here the muscle becomes wider, covers the 

 shoulder joint, passes between the brachialis and biceps, and is inserted by means 

 of a wide tendon which it shares with the superficial pectoral muscle. 



Relations. — Superficially, the skin, cervical fascia, the parotid gland, the cuta- 

 neus, the brachialis, and branches of the cervical nerves; deeply, the splenius, 

 longissimus capitis et atlantis, rectus capitis ventralis major, omo-hyoideus, ser- 

 ratus ventralis, anterior deep pectoral and biceps muscles, the inferior cervical 

 artery, the prescapular lymph glands, and branches of the cervical nerves. The ven- 

 tral edge of the muscle forms the dorsal boundary of the jugular furrow. The 

 dorsal border may be in contact with the cervical trapezius, or be separated from 

 it by a variable interval. 



Blood-supply. — Inferior cervical, carotid, and vertebral arteries. 



Nerve-supply. — Spinal accessory, cervical, and axillary nerves. 



The pectoral fascia is a thin membrane covering the surface of the pectoral 

 muscles, to which it is, for the most part, closely attached. It detaches a layer 

 which passes between the superficial and deep pectorals. At the posterior edge of 

 the triceps another layer is given off, which passes on the lateral surface of this 

 muscle to blend with the scapular fascia; the deeper layer becomes continuous with 

 the subscapular and cervical fasciae. 



The pectoral muscles form a large fleshy mass which occupies the space be- 

 tween the ventral part of the chest-wall and the shoulder and arm. They are 

 clearly divisible into a superficial and a deep layer, ^he superficial layer may be 

 subdivided into two parts by careful dissection; the deep layer is clearly made up 

 of two muscles. 



2. Superficial pectoral muscle (M. pectoralis superficialis) . 



(a) Anterior superficial pectoral muscle (pars descendens).^ — This is a short, 

 thick, somewhat rounded muscle, which extends from the manubrium sterni to 

 the front of the arm. 'It forms a distinct prominence on the front of the breast, 

 which is easily recognized in the living animal. 



Origin. — The cariniform cartilage of the sternum. 



Insertion. — (1) The curved line of the humerus with the brachiocephahcus; (2) 

 the fascia of the arm. 



Action. — To adduct and advance the limb. 



Structure.-^The belly of the muscle is convex on its superficial face, but deeply 

 it is flattened where it overlaps the posterior superficial pectoral. Here the two 

 muscles are usually closely attached to each other, and care must be exercised in 

 making the separation. The tendon of insertion blends with that of the brachio- 

 cephalicus and with the fascia of the arm. At the middle hne of the breast a fur- 

 row occurs between the two muscles; laterally, another furrow, containing the 

 cephalic vein, lies between the rnuscle and the brachiocephahcus. 



Relations.— Super ficially, the skin, fascia, and panniculus; deeply, the pos- 



1 This is regarded as a vesti^ of the clavicle. On this basis the portion of the muscle from 

 the vestige to the arm represents the clavicular part of the deltoid and perhaps the clavicular part 



of the pectoralis major of man. 



2 Also termed the pectoralis inticus-QT pars clavicularis. 



