94 PHYSIOLOGY OF MUSCLES AND NERVES. 



of its own action. If two or more of the muscles as- 

 sume a state of activity at the same time, then the action 

 will be the resultant of the separate tensions of each, 

 and this may also be found by the law of the parallelo- 

 gram of forces.* 



There is yet another way in which the work per- 

 formed by the muscles is conditioned by their attach- 

 ment to the bones. The latter must be regarded as 

 levers which turn on axes, afforded by the sockets. 

 They usually represent one-armed, but sometimes two- 

 armed levers. Now, the direction of the tension of 

 the muscles is seldom at right angles to that of the 

 moveable bone lever, but is usually at an acute angle. 

 In this case, again, the whole tension of the muscle 

 does not take effect, but only a component, which is at 

 right angles to the arm of the lever. Now, it is notice- 

 able that in many cases the bones have projections 

 or protrusions at the point of the attachment of the 

 muscles, over which the muscle tendon passes, as over 

 a reel, thus grasping the bone at a favourable angle ; 

 or, in other cases, it is found that cartilaginous or bony 

 thickenings exist in the tendon itself (so-called sesam- 

 oid bones), which act in the same way. The largest of 

 these sesamoid bones is that in the knee, which, in- 

 serted in the powerful tendon of the front muscle of 

 the upper thigh, gives a more favourable direction to 

 the attachment of this tendon than there would other- 

 wise be. 



Sometimes the tendon of a muscle passes over an 

 actual reel, so that the direction in which the muscle- 

 fibres contract is entirely different from that in which 

 their force of tension acts. 



3. The last important consequence of the attach- 



