94 PHYSIOLOGY OF MUSCL1> AM' NKKVES. 



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

 sume a >tate nf act is it y at the same time, then the action 



Will be the roultant <f the separate tensions of cadi, 

 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, afford id 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 

 nioveable bone lever, but is u-iially at an acute angle. 

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

 does not take effect, but only a component, \\hicli is at 

 ri^ht angles to tin- arm of the lever. Now, it H notice- 

 able that in many cases the bones ha\e 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 large>t of 

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

 serted iu the powerful tendon of the front muscle of 

 the upper thigh, gives a more favourable direction to 

 the attachment of this tendon than there \\oiild other- 

 \\ ise be. 



Sometimes the tendon of a mux-le passes over an 

 actual reel, -o that the direction in \\ hich t he muscle- 

 tibres contract i- cut iivly diffei cut from that in which 

 their force of ten-ion acts. 



3. The last important ( "ii-i ijnence of the attach- 



