MUSCLES, THEIR ACTION. 39 



the gases which are developed in them. The flute-player of 

 antiquity kept his cheeks distended by means of a leather 

 strap. Thus there is a constant struggle between the con- 

 tractility and the extensibility of the muscles. But if, during 

 the contraction of certain muscles, as the flexors of the arm, 

 the antagonistic muscles, the extensors, are relaxed and do 

 not oppose the movement, they regulate it nevertheless by 

 virtue of a property named muscular tonicity, which gives to 

 their tissues even when not contracted a certain power of 

 resistance. Thus when a group of muscles is paralyzed, the 

 antagonistic muscles cause by their contraction a jerking 

 movement which has no regularity. 



In contracting, the muscles act like levers upon the. bones, 

 and therefore just so much less powerfully as they are placed 

 obliquely to the bone. Notwithstanding the larger part of 

 the muscles are attached to the bones at an acute angle, and 

 their direction is very oblique in regard to the lever they 

 are to move. The result is a great loss of force, but this 

 loss is compensated by an increase of volume in the muscles, 

 that is in the number of fibres of which they are composed. 



Most of the muscles are subject also to deviations or 

 reflections around the joints. Some even take a direction 

 perpendicular to their original one in turning round bony 

 hooks or in the grooves of the pulleys. The apophyses, or 

 the protuberances to which they are attached, permit them 

 to move at a greater angle and a more favourable one than 

 the initial angle, and this angle gradually increases as the 

 bone obeys the force applied to it; and lastly, the relative 

 direction of the muscle as regards the bone varies according 

 to the attitude. These dispositions of the muscles are always 

 adapted to the kind of movement to be executed, to the 

 extent or rapidity required, or to the force demanded; and 

 they are always so combined as to produce the maximum of 

 useful result. So in flexing the fore-arm, in elevating the 

 arm, the bones act as levers of the third order. The biceps, 

 anterior brachial, and deltoid muscles act as very short 

 levers upon the arm, and their initial direction is almost 

 parallel to the bone, but it soon becomes almost perpen- 

 dicular to it. In this case extent and rapidity of movement 



