5 - Chapter 5. AN INTRODUCTION TO MYOLOGY Muscles are the...

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Chapter 5. AN INTRODUCTION TO MYOLOGY Muscles are the means by which parts of the body are moved. Along with the skeleton, they provide support for the body and play a role in resisting gravity, thereby contributing to stance. If we use the muscles supporting the neck and head as an example, there is provision for lowering and raising these structures, for moving them to the left or right, and for fixing the anterior parts in a state of rigidity. Up-and-down movement may be strictly in the medial plane, where lowering corresponds to flexing , thus bringing the head closer to the trunk; and raising is equivalent to extension , or taking the head further away from the trunk. When the neck is turned to the left or right from the medial plane, or midline, the term abductor is applied to the muscles responsible for this maneuver (Figure 5-1). The muscles that draw the structures back toward the midline are called adductors . To review an earlier definition, muscles that help straighten a limb around a joint and cause distal parts to move away from the body are extensors . Those that cause distal parts to rotate closer to the body are flexors (Figure 5-2). Figure 5-1 . Abduction . In this illustration of an approaching horse, the head and neck are drawn to the right from the midline as the right forelimb is lifted. If only one set of the bilaterally-paired muscles (see Figure 5-4) contract, this abduction may occur to either side, depending upon whether or not the limb is supporting the animal’s weight. 2007 version – page 37
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Earlier, an exercise involving bending and straightening the elbow was used to illustrate flexion and extension. Now hold your right arm straight out in front and horizontal so that you are looking forward at your hand. Swing your arm so it moves 90° to the right and is now directly up from your side. The muscles responsible for rotating the arm around the shoulder are abductors. Swing the arm back to where you started and you are putting the adductors to work. If you have the flexibility, reach back with your left hand onto your right shoulder blade and attempt to palpate the abductor while repeating the exercise. The adductor can be palpated on the front of the shoulder. Quadrupeds have less ability for such extreme limb rotation as was just described for the human forelimb. Even so, our pelvic limb is much less mobile than the arm and shoulder in terms of adduction and abduction. In animals such as giraffes that splay their forelimbs to bring the shoulders closer to the ground to aid in drinking, a similar abduction is used. In addition, the legs are usually extended forward and outward rather than simply splayed (Figure 5-3), to increase stability and efficiency of this maneuver. Another general principle to grasp with
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell.

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5 - Chapter 5. AN INTRODUCTION TO MYOLOGY Muscles are the...

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