122 



THE HUMAN MOTOR 



FIG 



considerable movement mast be operated by a muscle capable 

 of sustaining considerable deformation ; hence long muscles are 

 best adapted for such movements. / 



On the contrary, the number of fibres being very great in 

 " short " and " thick" muscles, they can develop considerable 

 efforts. A glance at the muscular system (fig. 105. p, 130) shows that 

 not only are muscles short or long, but their shapes are very varied : 

 some being composed of parallel fibres, like the biceps, others of 

 radiating fibres like the temporal. These 

 shapes are rational functional adaptations ; 

 if the muscle is attached to two bones at a 

 distance from each other, susceptible to a 

 small reciprocal displacement, then the fibres 

 are lengthened by thin tendons, there being 

 an economy of muscular substance. Such is 

 the case of the flexors of the fingers. If the 

 muscle has to overcome a powerful resistance, 

 its fibres are " oblique " (see 69) so as to 

 present a large section ; and they have the 

 appearance of grains of corn, for a third of 

 their length, the other two-thirds being thin tendons, resulting 

 in a considerable total section, and an economy of matter (fig. 

 124) . This is what takes place in the Masseter, which has short 

 and parallel fibres, and which must be powerful in order to effect 

 mastication. 



The adaptation of each muscle to its function has been noted 

 by numerous authors, f 1 ) An observation of Marey 's is particularly 

 interesting : the calf of the negro is thin and has long fibres"; 

 that of the white man is fat, short, and prolonged by the long 

 Achilles tendon. Now, in these two cases, the muscle must have 

 the power to lift the weight of the man. Marey found that the 

 calf muscle, being weaker in the negro, is attached to a longer 

 lever, the salient of the os calcis (heel) being 40% further behind 

 the axis of rotation than in the case of the European. This 

 clever physiologist experimented by shortening the os calcis of. a 

 rabbit, and at the end of a year he saw the calf increase, that is 

 to say, increase in strength to adapt itself to its function. 



Functional variations also are found in the cartilages, which 

 adapt themselves to the work of the locomotor organs. ( 2 ) 



The motive elements of the body are so arranged as to occupy 

 the minimum of space. This will explain why the muscle is nearly 

 parallel to the bone which it moves instead of approximately 



( 1 ) Specially W. Roux, Marey, G. Weiss. Also consult W. Roux, Die 

 Entwickelungs-Mechanik, Leipzig, 1905 ; Vortrage u. Aufsatze uber Entwick 

 1905 ; Marey (Arch, de Physiol, 1889). 



( 2 ) Retterer (Comptes Rendus Biologie, 1908, 1909, 1911, passim}. 



