478 MANUAL OF PHYSIOLOGY. 



ticular surface of the tibia proceeds forwards. The effect of this 

 is, that in extension of the leg the ligaments are made tense, and 

 the bones are firmly locked together. Owing to the inequality 

 between the size of the internal and external coudyles the axis 

 of rotation is not at right angles to the axis of the femur, but is 

 at such an angle that extreme extension causes also a slight 

 amount of outward motion of the leg. 



6. In the ball and socket joints the name of which implies 

 their mechanism the most varied movements occur. (Hip and 

 shoulder.) 



STANDING. 



In order that an elongated rigid body may stand upright it is 

 only necessary that a line drawn vertically through its centre of 

 gravity should pass within its basis of support, and if the latter 

 be sufficiently wide the object will remain permanently in that 

 position. The human body is, however, in the first place not 

 rigid, and in the second place the basis of support is too small 

 to insure a satisfactory degree of steadiness. The act of standing 

 must, therefore, be accomplished by the action of certain muscles, 

 which are employed in preventing the different joints from col- 

 lapsing, and in so balancing the various parts of the body as to 

 keep the whole frame from toppling over. 



In order to economize muscular energy while standing, we 

 must lock as many of our joints as possible, and thus depend 

 rather on the passive ligaments than upon muscle action for the 

 rigidity of the body. With this object we are taught to place 

 the heels together, turn out the toes, bring the legs parallel by 

 approximating them, and extending the knees to the utmost, to 

 straighten and to throw back the trunk so as to render tense the 

 anterior hip ligaments, to direct the face straight forwards so as 

 to balance the head evenly, and to let the arms fall by the sides. 



In this position, as a soldier stands at attention, the knee and 

 hip-joints remain fixed, without any effort on the part of the 

 muscles, but it is far from being the most comfortable attitude 

 one can assume for prolonged standing, and hence the position 

 known best by the order " stand at ease " is adopted if more com- 



