92 PHYSIOLOGY OF MUSCLES AND NERYES. 



parts may act separately, or all may work together ; and 

 in the latter case they combine their forces, as is inva- 

 riably the case with forces acting in different directions, 

 in accordance with the so-called parallelogram of forces. 

 As an example of this sort of muscle the elevator of the 

 upper arm^which was before alluded to in the second 

 chapter, and which on account of its triangular shape is 

 called the deltoid muscle — may be examined. Contrac- 

 tions of the separate parts really occur in this. When 

 only the front section of the muscle contracts, the arm is 

 raised and advanced in the shoulder socket ; when only 

 the posterior part of the muscle contracts, the arm is 

 raised backward. When, however, all the fibres of the 

 muscle act in unison, th-e action of all the separable forces 

 of tension constitute a diagonal which results in the 

 lifting of the arm in the plane of its usual position. 



In some semi-pennate and pennate muscles the line of 

 union of the two points of attachment does not coincide 

 with the direction of the fibres. When the muscle con- 

 tracts each fibre exerts a force of tension in the direction 

 of its contraction. All these numerous forces, however, 

 produce 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. This task if only carried out in the case of a single 

 muscle would be a very great test of patience Foriu- 



