Applied anatomy and kinesiology; the mechanism of muscular movement . ers will move the arm; forward, then outwarddiagonally, then sideward, and finally backward, as we pass fromfront to rear as before. By using two rubber bands to represent twostrands with origins separated at different distances we can seehow the combined action of different parts raises the arm at everypossible angle and also guides its motion in a definite direction;by holding the humerus up to the horizontal plane we can see howvarious parts pull forward or back upon it. 86 MOVEMENTS OF THE SHOULDER-JOINT Using a non-elas
Applied anatomy and kinesiology; the mechanism of muscular movement . ers will move the arm; forward, then outwarddiagonally, then sideward, and finally backward, as we pass fromfront to rear as before. By using two rubber bands to represent twostrands with origins separated at different distances we can seehow the combined action of different parts raises the arm at everypossible angle and also guides its motion in a definite direction;by holding the humerus up to the horizontal plane we can see howvarious parts pull forward or back upon it. 86 MOVEMENTS OF THE SHOULDER-JOINT Using a non-elastic cord to represent a portion of the muscle,noting its length when the arm is at the side and again when it israised to horizontal, we can see how far the muscle must contractto raise it as far as the joint permits. It is easy to demonstrate inthis way that the middle part shortens less to lift the arm through90 degrees than the others, the figures being approximately 1§and 2 inches, from which the fibers of the middle part would appear to be about 3 inches long. and rear fibers and the frontabout 4 inches. Isolated action of the deltoid,as described by Duchenne, liftsthe arm just as the above studyof conditions would lead us toexpect; it is raised to the great-est height by the most anteriorfibers, and when the electricterminals are moved along themuscle from front to rear the armswungs to the rear and graduallylowers, the posterior fibers beingable to lift it backward but 45degrees. Anatomists who judgedof 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 thesc
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