Applied anatomy and kinesiology; the mechanism of muscular movement . acromion. Beingcovered by a muscle that usually contracts at the same time it isnot easy to study the supraspinatus on the normal living body. PECTORALIS MAJOR. A large fan-shaped muscle lying immediately beneath the skinover the front of the chest. Origin.—The inner two-thirds of the anterior border of theclavicle, the whole length of the sternum, and the cartilages ofthe first six ribs, near their junction with the sternum. Insertion.—By a flat tendon about 3 inches wide into the ridgethat forms the outer border of the bic
Applied anatomy and kinesiology; the mechanism of muscular movement . acromion. Beingcovered by a muscle that usually contracts at the same time it isnot easy to study the supraspinatus on the normal living body. PECTORALIS MAJOR. A large fan-shaped muscle lying immediately beneath the skinover the front of the chest. Origin.—The inner two-thirds of the anterior border of theclavicle, the whole length of the sternum, and the cartilages ofthe first six ribs, near their junction with the sternum. Insertion.—By a flat tendon about 3 inches wide into the ridgethat forms the outer border of the bicipital groove of the humerus,extending from just below the tuberosities nearly down to theinsertion of the deltoid. V \ PECTORALIS MAJOR 89 Structure.—^The fibers arise directly from the bone and convergeto join the tendon of insertion. Near its insertion it is twistedthrough 180 degrees, the lower part passing beneath to be insertednear the head of the humerus while the fibers from the claviclepass across them on the outside and join the humerus lower Fig. 48.—Deltoid and pectoralis major. (Gerrish.) Action.—The pull of the uppermost fibers of the pectoralis majordiffers from that of the anterior deltoid only in having an origin alittle farther to the front and an insertion a little higher. As weobserve the pull of the different strands in turn passing down-ward it is plain that when the arm is at the side the whole muscleis in a position to pull it forward, the upper fibers tending to raiseit and to pull at a better angle as the arm swings forward while the 90 MOVEMENTS OF THE SHOULDER-JOINT lower fibers pull at a small angle that grows smaller as the armadvances, the most of the force acting to pull the head of thehumerus out of its socket. When the arm is first raised to hori-zontal the angle of pull is greater and a point can be found nearthe front horizontal where the pectoralis major pulls at a right angle,the upper part acting directly fonvard and the l
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