334 THE HUMAN MOTOR 



269. (A) Statics of the Human Body.f 1 ) The human body is 

 obviously at all times under the influence of Gravitation. Its 

 centre of gravity cannot, however, be deduced from its shape, 

 but can only be found experimentally. 



If a plank, on which a man is lying is arranged to project over 

 the edge of a table ; by gradually advancing the plank, a position 

 of equilibrium can be 'found and the centre of gravity will be in 

 a vertical plane cutting the above edge. If we determine the 

 position of equilibrium for two further positions, the required 

 centre of gravity will be at the point of intersection of the three 

 planes. (Borelli, 1679). 



It will be obvious that the position of the centre of gravity of 

 the body must depend on the relative disposition of its constituent 

 masses, i.e., on the attitude of the body. Thus Otto Fischer 

 found the centres of gravity of the various members of the body 

 in known relative positions and reduced these to a single centre. 

 He used for his experiments corpses in a state of rigidity. 



When a man stands in an upright position the segments of his 

 body, the double curvature of the vertebral column, and his pyra- 

 midical shape result in a state of instability, so that the muscles 

 have to be continuously (though unconsciously) exercised to main- 

 tain equilibrium. The instability is due both to the small area 

 of the base of support (21) and to the fact that the centre of 

 gravity is comparatively high. 



According to Braune and Fischer, when the body is at rest in 

 the upright position the centre of gravity will be found to lie at 

 the level of the top of the third sacral vertebrae, or of the um- 

 bilicus. From an anatomical point of view the upright position 

 is either symmetrical or asymmetrical. In the first of the 

 symmetrical poses (fig. 233) the body is straight, the heels are in 

 contact, and the line of gravity passes through their centre (this 

 is the " erect attitude " which Braune and Fischer call Normal- 

 Stellung) . 



In the second symmetrical pose (fig. 234) the line of gravity 

 falls in front of the heel (this is the " easy attitude " or the 

 Bequeme-Haltung). Fig. 235 shows the asymmetrical pose, other- 



(!) Numerous writers have dealt with this subject, amongst whom may 

 be mentioned Giraud-Teulon (Principes de M&canique Animate, Paris, 1858) ; 

 Pettigrew (Animal Locomotion, London, 1873) ; Marey (La Machine Animal*, 

 Paris, 1873 ; Engl. Tr., London, 1874 ; Le Mouvement, 1894, Paris ; Engl. 

 Tr., London, 1895) ; Du Bois-Raymond (Specielle Muskelphysiologie, oder 

 Bewegungslehre, Berlin, 1903} ; Otto Fischer (Theoretische Grundlagen fiir 

 eine Mechanik der Lebenden Korper, Leipzig, 1906 ; Beitrage zuv Muskel- 

 statik und Muskeldynamik, in Abhandl d. Sachs. Gesellsch. d. Wiss., Math.- 

 Phys. Cl., vols. xxii. and xxiii ; 1895-7). Fischer's work is very good 

 mathematically . 



