502 SYNERGETIC AND ANTAGONISTIC MUSCLES. 



a = (p M F) : F C. If the foot be altered to the position R S, the force of the muscles of 

 the calf would then be o^ - (ft M F) : F C. 



In muscles also, which, like the coraco-brachialis, are stretched over the angle of 

 a hinge, the same result obtains. 



In fig. 347, III., H E is the humerus, E, the elbow-joint, E R, the radius, B R, the coraco- 

 brachialis. Its moment in this position is = A, a E. When the radius is raised to E R 

 then it is = A, a E. We must notice, however, that B R x < BR. Hence, the absolute 

 muscular force must be less in the Hexed position, because every muscle, as it becomes shorter, 

 lifts less weight. What is lost in power is gained by the elongation of the lever-arm. 



5. Many muscles have a double action ; when contracted in the ordinary way 

 they execute a combined movement, e.g., the biceps is a flexor and supinator of 

 the fore-arm. If one of these movements be prevented by the action of other 

 muscles, the muscle takes no part in the execution of the other movement. 



If the fore-arm be strongly pronated and flexed in this position, the biceps takes no part 

 therein ; or, when the elbow-joint is rigidly supinated, only the supinator brevis acts, not the 

 biceps. The muscles of mastication are another example. The masseter elevates the lower jaw, 

 and at the same time pulls it forward. If the depressed jaw, however, be strongly pulled back- 

 wards when the jaw is raised, the masseter is not concerned. The temporal muscle raises the 

 jaw, and at the same time pulls it backwards. If the depressed jaw be raised after being pushed 

 forward, then the temporal is not concerned in its elevation. 



6. Muscles acting on two or more joints are those which, in their course from 

 their origin to their insertion, pass over two or more joints. Either the tendons 

 may deviate from a straight course, e.g., the extensors and flexors of the fingers 

 and toes, as when the latter are flexed ; or the direction is always straight, e.g., the 

 gastrocnemius. The muscles of this group present the following points of interest 

 (a) The phenomenon of so-called "active insufficiency." If the position of the 

 joints over which the muscle passes be so altered that its origin and insertion 

 come too near each other, the muscle may require to contract so much before 

 it can act on the bones attached to it, that it cannot contract actively any 

 further than to the extent of the shortening from which it begins to be active ; e.g., 

 when the knee-joint is bent, the gastrocnemius can no longer produce plantar flexion 

 of the foot, but the traction on the tendo Achilles is produced by the soleus. (6) 

 " Passive insufficiency " is shown by many-jointed muscles under the following 

 circumstances : In certain positions of the joint, a muscle may be so stretched 

 that it may act like a rigid strap, and thus limit or prevent the action of other 

 muscles, e.g., the gastrocnemius is too short to permit complete dorsal flexion of 

 the foot when the knee is extended. The long flexors of the leg, arising from the 

 tuber ischii, are too short to permit complete extension of the knee-joint when the 

 hip-joint is flexed at an acute angle. The extensor tendons of the fingers are too 

 short to permit of complete flexion of the joints of the fingers when the hand is 

 completely flexed. 



7. Synergetic muscles are those which together subserve a certain kind of 

 movement, e.g., the flexors of the leg, the muscles of the calf, and others. The 

 abdominal muscles act along with the diaphragm in diminishing the abdomen 

 during straining, while the muscles of inspiration or expiration, even the different 

 origins of one muscle, or the two bellies of a biventral muscle, may be regarded 

 from the same point of view 7 . 



Antagonistic muscles are those which, during their action, have exactly the 

 opposite effect of other muscles, e.g., flexors and extensors pronators and supina- 

 tors adductors and abductors elevators and depressors sphincters and dilators 

 inspiratory and expiratory. 



When it is necessary to bring the full power of our muscles into action, we 

 quite involuntarily bring them beforehand into a condition of the greatest tension, 

 as a muscle in this condition is in the most favourable position for doing 

 work ( 300, I., 3). Conversely, when we execute delicate movements requiring 



