Applied anatomy and kinesiology, the mechanism of muscular movement . udgedof the action of muscles solelyby the conditions apparent onthe skeleton or cadaver had fora long time doubted the abilityof the middle deltoid to start theelevation of the arm without theaid of other muscles, because ofits small angle of pull, but Du-chennes experiments on isolatedaction solved the problem defi-nitely, showing that it can do the deltoid contracts fromelectric stimulus it does not liftthe arm as high as the shoulder-joint would permit, because thescapula, being somewhat free to move, is rotated
Applied anatomy and kinesiology, the mechanism of muscular movement . udgedof the action of muscles solelyby the conditions apparent onthe skeleton or cadaver had fora long time doubted the abilityof the middle deltoid to start theelevation of the arm without theaid of other muscles, because ofits small angle of pull, but Du-chennes experiments on isolatedaction solved the problem defi-nitely, showing that it can do the deltoid contracts fromelectric stimulus it does not liftthe arm as high as the shoulder-joint would permit, because thescapula, being somewhat free to move, is rotated downward bythe pull of the deltoid and the weight of the arm, bringing the lowerangle back well toward the spinal column, depressing the acromion,and making the posterior edge of the scapula stand out from thechest wall as in Fig. 38. This downward rotation of the scapulagives the appearance of only a partial movement in the shoulder-joint, even when it has been performed to its full extent, preciselyas in attempts to raise the arm by those whose trapezius and ser-. Fig. 47.—Identity of isolated actionof deltoid (left side), with voluntaryattempt to raise the arm when thetrapezius and serratua magnus arelacking (right side). (Duchenne.) DELTOID 87 ratus are destroyed. Duchenne shows in Fig. 47 an experimentto make this plain. The subject has lost the trapezius and serratusthrough disease. When he tries to raise his arm he can bring intoaction only the deltoid, possibly aided by the supraspinatus. Inthe picture he is trying to raise his right arm and at the same timethe left deltoid is being stimulated by electricity. The effect isthe same on both sides: partial elevation of the arm, downwardrotation of the scapula, and a deep trough between the posteriorborder of the scapula and the back. Notice how the arm and theaxillary border of the scapula have moved away from each other,and recall that in normal elevation of the arm the axillary bordermoves forward because of the pull o
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