286 THE MUSCLES 



would thus obtain a consiflorablo mechanical advantage. This extreme obliquity, 

 however, actually places the muscle in a condition of mechanical disadvantage. 

 Lastly, the biceps and brachialis anticus are Ijoth of them good exam])les of the 

 third order of lever, when they act by their insertion close to the fulcrum of 

 the elbow to flex the forearm, the centre of gravity of which is much farther from 

 the joint. 



The direction of the tendon at its point of insertion is of great importance in 

 estimating the effect of the contraction of any muscle. It is rarely perjiendicular 

 to its lever. There is, therefore, in nearly every case, a considerable loss of 

 meclianical advantage; but it should be remembered that this is more than 

 counterbalanced by important gains. In the first place, the range and rapidity 

 of movement are greatly increased for a given contraction of the muscle; and in 

 the second place a power is also produced by which the articular ends of the bones 

 connected by the muscle are pressed together, so that the tendency to dislocation 

 of the joint is diminished. A third advantage is the compactness given to the 

 limb bv the tendons being placed in close apposition to the bones, an object which 

 could not be attained unless their insertions were very oblique. 



Take the case of the brachio-radialis flexing the forearm. In the adjoining 

 diagram let E be the elbow, B E the humerus. E R the radius, and B R the line of 

 the ])racliio-radialis. To estimate how much of the contracting force of the muscle 

 is actually expended in flexing the forearm, all that is necessary is to take any 

 point K in B R, to draw K A perpendicular to E R, and form a rectangle R A K C, 

 on R A. If the force of the muscle acting along R B be represented by the line 

 R K, then R C w'ill represent that portion of the force which tends to flex the 



Fig. 260. 



elbow, and R A the portion which is devoted to compression of the head of the 

 radius against the capitellum of the humerus, so as to strengthen the elbow-joint. 

 Again, let E R' be the new position of the forearm after the brachio-radialis has 

 contracted. The arc of the circle through w^hich it has moved will be indicated by 

 R R'. a small arc drawn with radius R E, and centre E. Now join R' to B, and 

 draw another small arc R' D with centre B and radius R' B. R D l^eing the difi"er- 

 ence between B R and B R', Avill indicate the amount by which the length of the 

 muscle has diminished during its contraction, and R R' Avill represent the space 

 through whi<;h the lower end of the radius has moved. A comparison of the 

 lengths of R D and R R' will show how great has been the gain in range of move- 

 ment by the (jblique insertion of B R. If the muscle had been inserted at right 

 angles to E R, it would, in order to produce the same effect, have had to contract 

 through a space equal in length to R R'. With its oblique insertion a contrac- 

 tion equal in length to R D hiis sufficed. The gain in range (and therefore in 

 rapidity of movement, for by contracting through the short distance it causes a 

 simultaneous motion through the longer space) may be roughly represented by 

 the fraction j^ j^ . In muscles in which the insertion of the tendon is at a very 

 small angle, the loss of power is proportionately greater, and the gain in speed and 

 range enormous. As instances of this may be mentioned the insertions of the 

 tendons of the i)halanges, which ])roduce the comparatively feeble, but exceedingly 

 niml)le, movements of tlie fingers with a very small contraction of the forearm 

 muscles. 



In speaking of the ilirection of the muscles, it should clearly be understood that 



