128 THE HUMAN MOTOR 



impulses or which it is the resultant, escape perception. The 

 nervous flux acts in a periodic manner, and not continuously. 

 The rapid succession of impulses in voluntary contraction can be 

 observed, and can even produce a sound. If one finger is placed 

 in the concha of the ear, and the biceps strongly contracted, a 

 sound is heard ; the impulses number about 50 per second.) 1 ) In 

 the contraction of the massetei ( 2 ) (jaws) there aie from 60 to 

 65 impulses ; and in the flexor muscles of the fingers from 8 

 to 11 according to the finger. ( 3 ) 



Voluntary contraction, whatever may be its rapidity, has 

 therefore the character of a " physiological tetanus." Thus a 

 series of excitations leave the nervous centres, which fuse the 

 separate contractions of the muscular fibres into a single con- 

 traction, said to be voluntary. Tieves has shown, in addition, 

 that the rhythm of the impulses is not modified, whether the muscle 

 sustains a small or a large load ( 4 ). Fatigue alone can change it. 



Hence static effort leads to a real vibratory movement, that is 

 to say, to internal work. There must be an expenditure of work 

 to produce an effort ; for example : 



1. The body of a pump in which slides a weighted piston, is 

 connected to a water-pipe in which there is pressure ; the piston 

 will not descend because of the stationary flux or the force exerted 

 by the water ( 5 ). It is thus that the pressure of a jet of water 

 can keep a door closed. 



2. We have seen that the elastic work of traction is ( 49) : 



T=1 ES/*_F/ 

 L ~ 2' 



the differential of the work in respect to the deformation will be: 



_ _ 

 dl~ L 



91. The contraction of the muscle, and its tetanic character 

 when voluntary, alternates, like the strokes of the piston of a 

 steam engine. The number of alternations can vary within 



(!) H. Piper (Pflttg. Arch., 1909, vol. cxxiv., p. 591 ; ibid., cxxvii., p. 474, 

 and Arch. f. Anat. u. Phys., 1910, p. 207). 



(*) P. Hoffmann (Pfliig. Arch., vol. cxxiv., p. 341). Helmholtz had found 

 earlier, from 35 to 40 impulses instead of 60. But it must be admitted 

 that the number varies according to the individual and the stature. 



( 8 ) Canney and Tunstall (Journal of Physiol., vol. vi., p. xvii, 1885) ; 

 S haefjr (ibid., vol. vii., p. I'll, 1886). 



( 4 ) Z. Treves (Arch. Ital. Riol., vol. xxxiii., p. 87, 1899) ; Simonelli and 

 Coop (Ann. di Neurol., vol. xviii., p. 310, Naples, 1900). 



( 6 ) Gariel, Traili de Physique Biologique d'Arsonval, Gariel, etc., vol. i., 

 p. 994, 1901. 



( 8 ) Lebert (Comptes Rendus A cad. Sciences, 1904, p. 1481), 



