ii MECHANICS OF LOCOMOTOE APPAEATUS 109 



The centre of gravity of the trunk may be determined on the 

 dead subject in the same manner after exarticulating all the 

 limbs. It lies in the plane between the lower extremity of 

 the sternum or the ensiform cartilage and the tenth dorsal 

 vertebra, and in a vertical transverse plane that passes somewhat 

 behind the axes of rotation of the heads of the femurs. We shall 

 presently see the importance of this fact. 



The position of the centre of gravity for the whole body is 

 important in determining the positions of more or less stable 

 equilibrium of the body. Braune and Fischer defined the normal 

 erect posture (Normal- Stellung) as that in which the axes of 

 rotation of the principal articulations fall in the same vertical 

 transverse plane as the line of gravity (Fig. 68). From this 

 they distinguish the military or " stand-at-ease " position (Bequeme 

 Haltung) in which the line of gravity falls 4 cm. in front of 



FIG. 70. The base of support and the line of gravity in different postures. At A the base of 

 support is represented by the area abed, and g is the point through which the line of gravity 

 passes ("attention attitude"). At B the base of support comprises abed, g being the point 

 through which the line of gravity passes (" normal position "). 



the line of junction of the articular heads of the femurs (Fig. 69). 

 In the first posture the line of gravity falls near the posterior 

 margin of the base of support ; in the second it falls considerably 

 more forward (Fig. 70). Obviously this last posture represents a 

 more stable condition of equilibrium. 



In each different posture assumed by the body resting on the 

 soles of both feet there is a new displacement of the centre of gravity 

 (Fig. 71). If the individual carries a weight he is constrained to 

 modify his position because the system is thrown out of equilibrium, 

 unless the centre of gravity lies within the common base of 

 support. If the load is placed on the back he must lean forward, 

 if the weight is placed in front, backward. If a weight is held up 

 with the right arm the body inclines to the left ; if with the left 

 arm, to the right. Heavier weights can be borne on the head, as 

 then the normal posture of the trunk may be but little changed, 

 and the line of gravity little displaced, but the centre of gravity 

 is raised, which renders the equilibrium less stable, though the 

 base of support is unchanged. 



