The shoulder embraces two sets of muscles: one connects the shoulder-girdle with the trunk and the other the humerus with the shoulder-girdle.

The inner end of the clavicle articulates with the sternum and gives a bony support to the shoulder-girdle at this point. The rest of the shoulder-girdle, comprising the remainder of the clavicle and all of the scapula and bearing, the weight of the whole upper extremity, hangs from and is supported and moved by the muscles which pass from it to the vertebrae posteriorly and to the skull, hyoid bone, and ribs anteriorly.

Fig. 240.   Scapular muscles.

Fig. 240. - Scapular muscles.

Anteriorly the clavicle has attached to its upper inner third the clavicular origin of the sternomastoid muscle; and on its under surface is the subclavius muscle, which arises from the cartilage and anterior end of the first rib. These muscles aid in moving the clavicle. Running from the third, fourth, and fifth ribs to the coracoid process is the pectoralis minor muscle; and on the side of the chest, passing to the posterior edge of the scapula, is the serratus anterior (magnus) muscle. It will be alluded to again.

Posteriorly is the trapezius muscle superficially, and beneath are the omohyoid, levator scapulce, and the two rhomboid muscles.

The trapezius arises from the superior curved line of the occiput, the liga-mentum nuchae , and the spines of the seven cervical and all of the thoracic vertebrae. It inserts into the upper surface of the outer third of the clavicle, acromion process, and spine of the scapula to near its root. Its upper fibres directly aid in sustaining the weight of the upper extremity. It is not infrequently paralyzed, and then falling of the shoulder is marked. It also tends to pull the scapula backward toward the spine, and rotates it.

The levator scapulae arises from the transverse processes of the upper four cervical vertebrae and passes downward to insert into the posterior edge of the scapula between its upper angle and the root of the spine of the scapula.

The scapula is supported largely by this muscle; hence when the trapezius is paralyzed, as occurs in division of its motor nerve, the spinal accessory, this muscle is utilized in counteracting its loss.

The rhomboid muscles arise from the lower part of the ligamentum nuchae and the spines of the seventh cervical and upper five thoracic vertebrae and insert into the posterior edge of the lower three-fourths of the scapula.

The serratus anterior (magnus) muscle (Fig. 202), lies beneath the scapula and arises from nine slips from the outer surface of the upper eight or nine ribs; the second rib receives two slips. It passes backward and upward and inserts into the posterior edge of the scapula from its upper to its lower angle.

The serration attached to the sixth rib is the one that reaches farthest forward on the side of the chest.

The omohyoid muscle arises posteriorly from the upper border of the scapula, just behind the suprascapular notch, and then runs upward and forward to the under surface of the body of the hyoid bone. It is a digastric or two-bellied muscle and its middle tendon is attached by a pulley-like process of the deep cervical fascia to the first rib.