106 



PHYSIOLOGY 



CHAP. 



articulation more solid. The more obtuse the angle BAG formed 

 by the two bones, the smaller will be the components M'E, i.e. the 

 force utilised in the movement. The smaller this angle becomes, 

 the greater will be the proportion of the force employed in the 

 movement. 



Since movable bones may be regarded as levers, the laws 



which govern the action of 

 levers can be applied to them. . 

 When the object is to attain 

 considerable speed rather than 

 have great force, the force is 

 applied to the shorter arm of 

 the lever; when, on the con- 

 trary, a high resistance has to 

 be overcome, and less speed 

 of movement is required, the 

 force is applied to the longer 

 arm. In the animal body the 

 arm of the lever to which the 

 force is applied is shorter than 

 that which causes resistance, 

 i.e. the majority of the muscles 

 are inserted nearer the articu- 

 lations than is the centre of 

 gravity of the movable part. 



This arrangement is advan- 

 tageous for the speed of the 

 mo vementjbut disadvantageous 

 owing to loss of force. The 

 loss is, however, compensated 

 by the fact that a less amount 

 of muscular shortening is re- 

 quired to effect a given range 

 of movement (Fig. 67). 



It is important to note that 

 during movement the length 

 of the arm to which the force 

 is applied, and that which 

 carries the weight, often vary 

 in proportion with the range of 

 the movement, so that the load diminishes during work. When, 

 for instance, the body is raised from the bent knee, this movement 

 is accompanied by the unloading of the muscles which actively 

 extend the knee. In this position the arm that carries the load 

 is represented by the horizontal distance of the axis of the knee- 

 joint from the line of gravity of the body, i.e. from the perpen- 

 dicular taken from its centre of gravity. During the rise this 



FIG. 67. Diagram showing the various degrees of 

 muscular shortening required for a given move- 

 ment, according as the lever arm varies for 

 power and for the load. (Luciani.) When the 

 lever AC rotating on the axis A reaches AC', the 

 muscle MM only shortens slightly (Mm'), because 

 the lever arm AM is shorter than that of the 

 load MC ; the muscle MM', on the contrary, has 

 to shorten much more (to Mm") to execute the 

 same movement, because the arm AM' is much 

 longer than that of the load M'C. 



