Applied anatomy and kinesiology; the mechanism of muscular movement . leis 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 direct
Applied anatomy and kinesiology; the mechanism of muscular movement . leis 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 lower part forwardand doA\Tiward. With the arm overhead all parts pull forward anddownward. The position of the insertion enables it to rotate thehumerus inward; the twisting of the tendon gives the upper fibersthe longer and the lower the shorter leverage. Duchennes study of isolated action cleared up several pointsabout the action of the pectoralis major which had hitherto beentopics of dispute and showed for the first time just what the muscle. Fig. 49.—The pectorklis major in action. P, pectoral; D, deltoid; S, serratus magnus. can do and what it cannot do. He shows that it acts like twomuscles, just as the deltoid acts like three and the trapezius likefour. He finds that the upper half of the pectoralis major swingsthe arm forvvard and inward and at the same time lifts the acromionso that it can help the levator and second trapezius in lifting andholding a weight on the shoulder; it presses the arm firmly againstthe side and front of the chest. When the arm is first raised tohorizontal the action of the upper half swings it horizontally for-ward; when it is in vertical position upward the same fibers depressit forward to the horizontal. Isolated action of the lower halfswings the arm fon\-ard and downward, depresses it if elevated,and pulls the head of the humerus strongly out of the glenoid cavity,at the same time lowering the acromion and pressing th
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