THEIR ACTION AS LEVERS 285 



of the nmsc'le will 1)e to eoni})ress the structures lyinf;; in its concavity. The buc- 

 cinator and alxloniinal muscles are good examples of tliis form of muscle. Or if 

 the surface upon which the curved slieet lies is concave, the muscle will tend l)y 

 its contraction to lift up the soft parts u})on its deep or convex surface. As an 

 instance of this we have the influence of the platysma myoides upon the fascia and 

 other stru(!tures which overlie the great vascular diannels of the neck and the 

 apices of the lungs. 



In estimating the effect of muscles acting upon the levers formed by the bones, 

 each of which moves upon a joint as its fulcrum, three points have to be taken into 

 consideration: — (1) the order of lever; (2) the distance from the fulcrum, of the 

 points of application of the force at the insertion of the muscle, and of the resist- 

 ance to be overcome; and (3) the direction in which the force is applied. It will 

 be found that whereas in most levers employed in mechanics the object aimed at 

 is what is called mechanical advantage, i.e. by the application of a small force to 

 overcome a greater resistance; in the human body the object is, on the contrary, 

 by the exertion of a great force through a small space to overcome a small resist- 

 ance, but at the same time to cause motion through a much greater space, and with 

 a much greater speed. In the three orders of levers, the fulcrum is placed either 

 between the power and the resistance (first order); or at one end, with the power 

 at the other, and the resistance to be overcome between (second order); or at one 

 end with the power in the middle, and the resistance to be overcome at the other 

 end (third order). The power required to overcome the resistance varies inversely 

 with the distance of its point of application from the fulcrum. Where this distance, 

 or the arm of the lever as it is called, is short compared with the arm at w^hich the 

 resistance acts, then the power has to be greater than the resistance, and vice versa. 

 In the first order of levers, if the power and resistance act parallel to one another, 

 there may or may not be a gain of mechanical advantage, according as the arm at 

 which the powder acts is greater or less than the arm at which the resistance acts. 

 In the second order of levers there must necessarily be mechanical advantage; and 

 in the third order the power must be greater than the resistance, as it has to act 

 upon a shorter arm, so there will be what may be termed mechanical disadvantage. 



In the human body there is hardly a single muscle which forms a good example 

 of a lever of the second order, for the simple reason that mechanical advantage 

 is of so much less importance than a wide range of movement with its attendant 

 rapidity. The best example that can be given is that of some of the muscles Avhich 

 depress the mandible, e.g. the anterior belly of the digastric, which is inserted into 

 the extremity of the lever formed by that bone, while the resistance to be overcome 

 — viz. the tonic contraction of the masseter, temporal, and internal pterygoid — is 

 exerted at a point much nearer to the fulcrum upon which the mandible turns. 



The muscles which are inserted upon the tendo Achillis are often given as an 

 example of this order of lever, when by their contraction they raise the heel and 

 lift up the body, the weight of which acts upon the lever of the foot through the 

 ankle-joint. This would be a good example if another person were to lay hold of 

 the tendo Achillis, and by drawing upon it to raise the individual from the ground. 

 But the fact that the other end of the muscle springs from a j^art of the mass to 

 be raised alters the conditions, and, as a matter of fact, the muscle, instead of 

 obtaining any mechanical advantage in its action, has to contract with a force four 

 or five times as great as the weight to be raised. 



It will be found that most of the muscles belong either to the third order, in 

 which there is necessarily mechanical disadvantage, or to the first order; and in 

 this case, the arm at which the power acts is usually the shorter, so that the power 

 has to be greater than the resistance. In the movements of the forearm about the 

 elbow as a fulcrum, the triceps acting with the very short arm afforded l)y the pro- 

 jection of the olecranon and overcomiug a resistance experienced by the hand at 

 the other end of the radius and ulna, will be a lever of the first order. The 

 brachio-radialis (supinator longus) when used to flex the forearm acts at the lower 

 end of the radius, so as to overcome the weight of the forearm and hand, which 

 will act through a centre of gravity, which is about the middle of the radius, and 

 therefore much nearer to the fulcrum at the elbow. It therefore forms a lever of 

 the second order. If its angle of insertion were not so very small, the muscle 



