268 PHYSIOLOGY AND HEALTH. 



CHAPTER VI 



Muscles. Shape. Attachment. Situation. Swell in Action. 

 Arrangement. Antagonism. Cooperation. 



621. THE muscles are of various shapes, and their fibres 

 are arranged in various directions, to suit the wants of the 

 places where they are to operate, and the convenience of 

 distribution. On the limbs they are long and roundish; on 

 the trunk they are mostly flat. Sometimes the fibres are 

 arranged in the shape of a fan, the broad end being fixed 

 to an immovable bone ; and, at the other end, all the fibres 

 are gathered into one point, and are fixed to a movable bone, 

 as in the deltoid, (Fig. XL1X. &,) and the trapczius, (Fig. 

 L. ,) ( 618, 619.) When the fibres of one side of this 

 fan-shaped muscle act, they draw the bone in that direction ; 

 and when the fibres of the opposite side act, they draw the 

 bone in the opposite direction ; and when they all act together, 

 they draw it in a line with the middle fibres. 



622. The diaphragm ( 273, p. 124) is a muscle, and 

 acts by the same contractile power as the muscles of the 

 arms and legs. But, unlike them, it is attached to no joint, 

 and moves no bone. It is fixed to the bottom of the ribs, 

 and is arched up into the chest. (Fig. XVII.) Its fibres, 

 like the spokes of a wheel, extend from the circumference 

 to the centre ; and, when they contract, they draw the centre 

 and the whole arch downward towards the line of attach- 

 ment of the outer edge. This descent of the arch leaves 

 room in the chest for the expansion of the lungs and the 

 inspiration of air. The heart ( 206, p. 94) is a hollow, 

 muscular bag ; when its fibres contract, they lessen the cavity 

 within and expel the contents of blood. The muscular coat 

 of the oesophagus, ( 23, p. 17,) the stomach, ( 28, p. 20,) 

 and of the alimentary canal, winds about them, and is 

 attached to no fixed point. It presses upon the contents of 

 these organs, and aids the digestive operation in the stomach, 

 and carries the contents onward through the channel. 



