174 STUDIES IN ELECTRO-PHYSIOLOGY: 



acts exactly as do all the others, and the whole action of 

 the muscle is simply the sum of the separate actions of all 

 the fibres. As a rule, however, the structure of muscles is 

 not so simple. According to the form and the arrange- 

 ment of the fibres, anatomists distinguish short, long, and 

 flat muscles. The last mentioned generally exhibit devia- 

 tions from the ordinary parallel arrangement of the fibres. 

 Either the fibres proceed at one end from a broad tendon, 

 and are directed towards one point from which a short 

 round tendon then effects their attachment to the bones 

 (fan-shaped muscles), or the fibres are attached at an angle 

 to a long tendon, from which they all branch off in one 

 direction (semi-pennate muscles), or in two directions like 

 the plumes of a feather (pennate muscles). In the radiate 

 or fan-shaped muscles the pull of the separate parts takes 

 effect in different directions. Each of these parts may act 

 separately, or all may work together ; and in the latter case 

 they combine their forces, as is invariably 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 (the deltoid 

 muscle) may be examined. Contractions 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, the 

 action of all the separable forces of tension constitutes 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 contracts 

 each fibre exerts a force of tension in the direction of its 

 contraction. All these numerous forces, however, produce 



