ANIMAL AND VEGETABLE 175 



a single force which acts in the direction in which the 

 movement is really accomplished, and the whole action of 

 the muscle is the sum of these separate components, each 

 derived from a single fibre. In order to calculate the force 

 which one of these muscles can exert, as well as the height 

 of elevation proper to it, it would be necessary to determine 

 the number of the fibres, the angle which each of these 

 makes, with the direction finally taken by the compound 

 action, as well as the length of the fibres these not being 

 always equal. . . . The direction in which the action takes 

 effect does not, however, depend only on 'the structure of 

 the muscle, but chiefly on the nature of its attachment to 

 the bone. Owing to the form of the bones and their 

 sockets, the points of connection by which the bones are 

 held together, the bones are capable of moving only within 

 certain limits, and usually only in certain directions. For 

 instance, let us watch a true hinge-socket, such as that of 

 the elbow, which is capable only of bending and stretching. 

 As, in this case, the nature of the socket is such that motion 

 is only possible in one plane, the muscles which do not lie 

 in this plane can only bring into action a portion of their 

 power of tension, and this may be found if the tension 

 exercised by the muscle is analysed in accordance with the 

 law of the parallelogram of forces, so as to find such of the 

 component forces as lie within the plane." (Rosenthal, 1895.) 



Here it may be useful to give a brief description of what 

 is meant by the parallelogram of forces, my authority 

 being Dr. M'Gregor-Robertson. 



Let O, in the figure on next page, be a particle under the 

 influence of two forces, one, OB, urging it in the direction 

 of B, and the other, OA, urging it in the direction of A. 

 It is evident that the particle cannot proceed along either 

 path, but will choose a path which is a compromise between 

 the two. It will move upwards. Let a third force, 

 by the weight, be applied to O, and let this 



