ATTACHMENT OF MUSCLES TO BONES. 93 



nately no such tedious calculations are requisite for our 

 purpose. The force may be directly determined by ex- 

 periment in the case of many muscles, by the method 

 already described in Chapter IV. § 6 ; the height of 

 ^elev^ation possible under the conditions present in the 

 body may be yet more easily found ; and as* regards the 

 work which the muscle is able to perform, it makes no 

 ditference whether the fibres are all parallel and act in 

 their own direction, or if they form any angle with the 

 direction of work.^ 



2. 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 direc- 

 tions. For instance, let us watch a true hinge-socket, 

 such as that of the elbow, which admits only of bending 

 and stretching (c/. ch. ii. § 4). 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. 



It is different in the case of the more free ball- 

 sockets, which permit movement of the bone in any 

 direction within certain limits. When a socket of this 

 sort is surrounded by many muscles, each of the latter, 

 if it acts alone, sets the bone "in motion in the direction 

 * * See Notes and Additions, No 2. 



