124 THE HUMAN MOTOR 



must also be remembered that, in the human body, the muscular 

 power generally acts upon levers of the third order. If a stress 

 is applied at the centre of gravity G of a mobile limb (fig. 126), 

 the useful component of the power will be OO' = P sin a, that is 

 to say, the effort of flexion varies as the sin of the angle of the 

 inclination of the muscle to the mobile limb. The arm of the 

 lever Al' of the power is smaller than that Al of the resistance ; 

 but the amplitude of the movement of this resistance is increased 



Al 



in the relation of and produces a large number of useful 

 Al 



actions. The motor function of the muscle must be considered 

 in reference to the nature of the movement. " Isolated " 

 muscular action, as Duchenne wrote, "does not exist in nature" (*); 

 there is co-operation of all the motive forces, often apparently 

 antagonistic ; the " biceps " bends the forearm; the " triceps " 

 produces its "extension"; in fact, they co-ordinate their move- 

 ment, and combine their efforts so well that the real grouping of 

 the muscles is functional and physiological, and not anatomical. 

 The precise and gradual association of the muscular movements 

 is the work of the nervous system ; it is automatic, the effect of 

 habit, and rapid, because it is in the charge of near centres in the 

 spinal cord ; but normally these centres only begin the move- 

 ment and the cerebral centres, further away, achieve its perfec- 

 tion. 



88. Muscular Force : Absolute Force. It has been seen that 

 the iorce of the muscle increases in proportion to its contraction ; 

 if it is loaded so as to prevent its shortening, the " absolute " 

 effort can be measured. ( 2 ) On a living subject a particular 

 muscle is chosen with a known section, the kind of lever to v^hich 

 it is attached is noted, and the value of the maximum static 

 effort is calculated. Thus a man of 70 kilogrammes, loaded with 

 a weight of 70 kilogrammes, cannot lift himself when he is 

 seated. ( 3 ) The resistance is 140 kilogrammes and 



P x Ac = Q X AB ; whence P = Q^ (fig. 127). 



Ac 



Hermann, ( 4 ) calculated P by determining exactly the relation 



, he obtained a force per square millimetre of the muscles of 

 Ac 



the calf of as much as 62-4 grammes. For the flexors of the arm,( 6 ) 



( x ) Duchenne de Boulogne, Physiologic des Mouvements, p. 811. Paris. 

 1867. 



( 2 ) So called by E. Weber, author of numerous works on muscular 

 elasticity and with his brother Wilhelm, of a treatise which became 

 famous (?) : Mecanique de la Locomotion chez I'Homme, trans. Jourdan, 

 Paris, 1843. 



() De la Hire (Mim. Acad. Roy. d.' Sciences, 1699, p. 153). 



(*) Hermann (Arch./, Physiol., vol. Ixxiii., p. 429, 1898). 



( 6 ) Hencke and Knorz (Zeitsch. /. Rat. Med., vol. xxiv., 1866). 



