. The human body and health : an elementary text-book of essential anatomy, applied physiology and practical hygiene for schools . quadricepsextensor. I, hipbone. THE ARRANGEMENT OF THE MUSCLES 207 The Arrangement of the Muscles. — In some regions ofthe body only a single layer of muscles covers the bones,while in other parts as many as three layers a muscle is attached to two different bones, butone end may befastened to the skinor another muscle,and in some casesone end of the mus-cle is divided intoseveral parts. Theend of the musclefixed on the bonewhich it moves iscalled the
. The human body and health : an elementary text-book of essential anatomy, applied physiology and practical hygiene for schools . quadricepsextensor. I, hipbone. THE ARRANGEMENT OF THE MUSCLES 207 The Arrangement of the Muscles. — In some regions ofthe body only a single layer of muscles covers the bones,while in other parts as many as three layers a muscle is attached to two different bones, butone end may befastened to the skinor another muscle,and in some casesone end of the mus-cle is divided intoseveral parts. Theend of the musclefixed on the bonewhich it moves iscalled the insertion^while the other endis the origin. Themasseter^ or chewingmuscle, has its originon the cheek boneand its insertion onthe mandible. In some cases themuscle fibers joindirectly to the peri-osteum of the bone, but in others a tough white cord,called a tendon, apparently continuous with the musclefibers, occurs at either or both ends of the muscle to con-nect it to the bone. The largest is the Achilles tendon,which feels like a hard cord a half inch thick just abovethe heel. Several tendons may be felt in the Fig, 138. — Some of the muscles used in breath-ing, a, muscles under the scapula; e, car-tilage part of the rib; ?i, humerus; m,muscles pulling on the ribs when the armsare thrown backward; n, intercostal muscles;r, rib; s, sternum. 208 THE MUSCULAR SYSTEM Kinds of Muscles. —In reference to the action effected,the muscles are divided into five groups : 1. the flexorsare those which bend a limb; they occupy the palm sideof the hand and arm, and the back of the leg (Fig. 136) ;2. the extensors straighten a limb, and therefore lie on the side of the limbsopposite to the flexors(Fig. 132) ; 3. the ab-ductors pull a limb or apart of a limb outwards,as when the arms arestretched outward oneither side ; *4. the ad-ductors oppose the ab-ductors, as seen in theaction of the pectoralmuscles of the chestwhen drawing the armacross the breast; sphi7icters are
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